A TEXT-BOOK OF ENTOMOLOGY
[Illustration]
A
TEXT-BOOK OF ENTOMOLOGY
INCLUDING
THE ANATOMY, PHYSIOLOGY, EMBRYOLOGY AND METAMORPHOSES
OF
INSECTS
_FOR USE IN AGRICULTURAL AND TECHNICAL SCHOOLS AND COLLEGES_
AS WELL AS BY THE WORKING ENTOMOLOGIST
BY
ALPHEUS S. PACKARD, M.D., PH.D.
PROFESSOR OF ZOÖLOGY AND GEOLOGY, BROWN UNIVERSITY AUTHOR OF “GUIDE TO
STUDY OF INSECTS,” “ENTOMOLOGY FOR BEGINNERS,” ETC.
New York
THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., LTD.
1898
_All rights reserved_
COPYRIGHT, 1898,
BY THE MACMILLAN COMPANY.
Norwood Press
J. S. Cushing & Co.—Berwick & Smith
Norwood Mass. U.S.A.
PREFACE
[Illustration]
In preparing this book the author had in mind the wants both of the
student and the teacher. For the student’s use the more difficult
portions, particularly that on the embryology, may be omitted. The work
has grown in part out of the writer’s experience in class work.
In instructing small classes in the anatomy and metamorphoses of
insects, it was strongly felt that the mere dissection and drawing of a
few types, comprising some of our common insects, were by no means
sufficient for broad, thorough work. Plainly enough the laboratory work
is all important, being rigidly disciplinary in its methods, and
affording the foundation for any farther work. But to this should be
added frequent explanations or formal lectures, and the student should
be required to do collateral reading in some general work on structural
and developmental entomology. With this aim in view, the present work
has been prepared.
It might be said in explanation of the plan of this book, that the
students having previously taken a lecture course in the zoölogy of the
invertebrates, were first instructed in the facts and conclusions
bearing on the relations of insects to other Arthropoda, and more
especially the anatomy of Peripatus, of the Myriopoda, and of
Scolopendrella. Then the structure of Campodea, Machilis, and Lepisma
was described, after which a few types of winged insects, beginning with
the locust and ending with the bee, were drawn and dissected; the nymph
of the locust, and the larva and pupa of a moth and of a wasp and bee
being drawn and examined. Had time permitted, an outline of the
embryology and of the internal changes in flies during their
metamorphoses would have been added.
This book gives, of course with much greater fulness and detail for
reference and collateral reading, what we roughly outlined in our class
work. The aim has been to afford a broad foundation for future more
special work by any one who may want to carry on the study of some group
of insects, or to extend in any special direction our present knowledge
of insect morphology and growth.
Many of our entomologists begin their studies without any previous
knowledge of the structure of animals, taking it up as an amusement.
They may be mere collectors and satisfied simply to know the name of
their captures, but it is hoped that with this book in their hands they
may be led to desire farther information regarding what has already been
done on the structure and mode of growth of the common insects. For
practical details as to how to dissect, to make microscopic slides, and
to mount and preserve insects generally, they are referred to the
author’s “Entomology for Beginners.”
It may also be acknowledged that even in our best and latest general
treatises on zoölogy, or comparative anatomy, or morphology, the portion
related to insects is scarcely so thoroughly done as those parts devoted
to other phyla, that of Lang, however, his invaluable Comparative
Anatomy, being an exception. On this account, therefore, it is hoped
that this hiatus in our literature may be in a degree filled.
The author has made free use of the excellent article “Insecta” of
Newport, of Lang’s comprehensive summary in his most useful Text-book of
Comparative Anatomy, of Graber’s excellent Die Insecten, of Miall and
Denny’s The Structure and Life-History of the Cockroach, and of Sharp’s
Insecta. Kolbe’s Einführung has been most helpful. But besides these
helps, liberal use has been made of the very numerous memoirs and
monographic articles which adorn our entomological literature. The
account of the embryology of insects is based on Korschelt and Heider’s
elaborate work, Lehrbuch der Vergleichenden Entwicklungsgeschichte der
Wirbellosen Thiere, the illustrations of this portion being mainly taken
from it, through the Messrs. Swan Sonnenschein & Co., London.
Professor H. S. Pratt has kindly read over the manuscript and also the
proofs of the portion on embryology and metamorphoses, and the author is
happy to acknowledge the essential service he has rendered.
The bibliographical lists are arranged by dates, so as to give an idea
of the historical development of each subject. The aim has been to make
these lists tolerably complete and to include the earliest, almost
forgotten works and articles as well as the most recent.
Much care has been taken to give due credit either to the original
sources from which the illustrations are copied, or to the artist; about
ninety of the simpler figures were drawn by the author, many of them for
this work. For the use of certain figures acknowledgments are due to the
Boston Society of Natural History, to the Division of Entomology, U. S.
Department of Agriculture, through the kind offices of Mr. L. O. Howard,
and to the Illinois State Laboratory of Natural History, through
Professor S. A. Forbes and Mr. C. A. Hart. Professor W. M. Wheeler, of
the University of Chicago, has kindly loaned for reproduction several of
his original drawings published in the Journal of Morphology. A number
are reproduced from figures in the reports of the United States
Entomological Commission.
PROVIDENCE, R. I.,
March 4, 1898.
TABLE OF CONTENTS
[Illustration]
PART I. MORPHOLOGY AND PHYSIOLOGY
PAGE
POSITION OF INSECTS IN THE ANIMAL KINGDOM 1
RELATIONS OF INSECTS TO OTHER ARTHROPODA 2
The Crustacea 4
The Merostomata 5
The Trilobita 5
The Arachnida 6
Relations of Peripatus to insects 9
Relation of Myriopods to insects 11
Relations of the Symphyla to insects 18
Diagnostic or essential characters of Symphyla 22
INSECTA (HEXAPODA) 26
Diagnostic characters of insects 26
1. EXTERNAL ANATOMY
_a._ Regions of the body 27
_b._ The integument (exoskeleton) 28
Chitin 29
_c._ Mechanical origin and structure of the segments (somites,
arthromeres, etc.) 30
_d._ Mechanical origin of the limbs and of their jointed
structure 35
THE HEAD AND ITS APPENDAGES 42
_a._ The head 42
The labrum 42
The epipharynx and labrum-epipharynx 43
Attachment of the head to the trunk 46
The basal or gular region of the head 46
The occiput 48
The tentorium 49
Number of segments in the head 50
The composition of the head in the Hymenoptera 55
_b._ Appendages of the head 57
The antennæ 57
The mandibles 59
The first maxillæ 62
The second maxillæ 68
The hypopharynx 70
Does the hypopharynx represent a distinct segment? 82
THE THORAX AND ITS APPENDAGES 86
_a._ The thorax: its external anatomy 86
The patagia 89
The tegulæ 89
The apodemes 92
The acetabula 94
_b._ The legs: their structure and functions 95
Tenent hairs 99
Why do insects have but six legs? 100
Loss of limbs by disuse 101
_c._ Locomotion (walking, climbing, and swimming) 103
Mechanics of walking 103
Locomotion on smooth surfaces 111
Climbing 116
The mode of swimming of insects 116
_d._ The wings and their structure 120
The veins 121
The squamæ 123
The halteres 124
The thyridium 124
The tegmina and hemelytra 124
The elytra 124
_e._ Development and mode of origin of the wings 126
Embryonic development of the wings 126
Evagination of the wing outside of the body 132
Extension of the wing; drawing out of the tracheoles 133
_f._ The primitive origin of the wings 137
The development and structure of the tracheæ and veins of
the wing 144
_g._ Mechanism of flight 148
Theory of insect flight 150
Graber’s views as to the mechanism of the wings, flight,
etc. 153
THE ABDOMEN AND ITS APPENDAGES 162
The median segment 163
The cercopoda 164
The ovipositor and sting 167
The styles and genital claspers (Rhabdopoda) 176
Velum penis 181
The suranal plate 181
The podical plates or paranal lobes 182
The infra-anal lobe 183
The egg-guide 183
THE ARMATURE OF INSECTS: SETÆ, HAIRS, SCALES, TUBERCLES, ETC. 187
The cuticula 187
Setæ 188
Glandular hairs and spines 190
Scales 193
Development of the scales 195
Spinules, hair-scales, hair-fields, and androconia 197
THE COLORS OF INSECTS 201
Optical colors 201
Natural colors 203
Chemical and physical nature of the pigment 206
Ontogenetic and phylogenetic development of colors 207
2. INTERNAL ANATOMY
THE MUSCULAR SYSTEM 211
Musculature of a caterpillar 213
Musculature of a beetle 213
Minute structure of the muscles 215
Muscular power of insects 217
THE NERVOUS SYSTEM 222
_a._ The nervous system as a whole 222
_b._ The brain 226
The optic or procerebral segment 231
Procerebral lobes 232
The mushroom or stalked bodies 233
Structure of the mushroom bodies 234
The central body 237
The antennal or olfactory lobes (Deutocerebrum) 237
The œsophageal lobes (Tritocerebrum) 237
_c._ Histological elements of the brain 238
_d._ The visceral (sympathetic or stomatogastric) system 238
_e._ The supraspinal cord 240
_f._ Modifications of the brain in different orders of insects 240
_g._ Functions of the nerve-centres and nerves 243
THE SENSORY ORGANS 249
_a._ The eyes and insect vision 249
The simple or single-lensed eye (ocellus) 249
The compound or facetted eye (ommateum) 250
The facet or cornea 250
The crystalline lens or cone 251
The pigment 253
The basilar membrane 253
The optic tract 253
Origin of the facetted eye 255
Mode of vision by single eyes or ocelli 255
Mode of vision by facetted eyes 256
The principal use of the facetted eye to perceive the
movements of animals 259
How far can insects see? 260
Relation of sight to the color of eyes 260
The color sense of insects 260
_b._ The organs of smell 264
Historical sketch of our knowledge of the organs of smell 264
Physiological experiments 268
Relation of insects to smelling substances before and
after the loss of their antennæ 269
Experiments on the use of the antennæ in seeking for food 270
Experiments testing the influence of the antennæ of the
males in seeking the females 270
Structure of the organs of smell in insects 271
_c._ The organs of taste 281
Structure of the taste organs 282
Distribution in different orders of insects 282
Experimental proof 286
_d._ The organs of hearing 287
The ears or tympanal and chordotonal sense-organs of
Orthoptera and other insects 288
Antennal auditory hairs 292
Special sense-organs in the wings and halteres 293
_e._ The sounds of insects 293
THE DIGESTIVE CANAL AND ITS APPENDAGES 297
_a._ The digestive canal 302
The œsophagus 303
The crop or ingluvies 303
The “sucking stomach” or food-reservoir 305
The fore-stomach or proventriculus 306
The œsophageal valve 311
Proventricular valvule 313
The peritrophic membrane 313
The mid-intestine 314
Histology of the mid-intestine 316
The hind-intestine 316
Large intestine 316
The ileum 317
The gastro-ileal folds 317
The colon 317
The rectum 318
The vent (anus) 319
Histology of the digestive canal 320
_b._ Digestion in insects 324
The mechanism of secretion 326
Absorbent cells 328
THE GLANDULAR AND EXCRETORY APPENDAGES OF THE DIGESTIVE CANAL 331
_a._ The salivary glands 331
_b._ The silk or spinning glands, and the spinning apparatus 339
The process of spinning 340
How the thread is drawn out 343
Appendages of the silk-gland (Filippi’s glands) 345
_c._ The cæcal appendages 347
_d._ The excretory system (urinary or Malpighian tubes) 348
Primitive number of tubes 353
_e._ Poison-glands 357
_f._ Adhesive or cement-glands 360
_g._ The wax-glands 361
_h._ “Honey-dew” or wax-glands of Aphids 364
_i._ Dermal glands in general 365
DEFENSIVE OR REPUGNATORIAL SCENT-GLANDS 368
Eversible coxal glands 369
Fœtid glands of Orthoptera 369
Anal glands of beetles 372
The blood as a repellent fluid 374
Eversible glands of caddis-worms and caterpillars 375
The osmeterium in Papilio larvæ 377
Dorsal and lateral eversible metameric sacs in other larvæ 377
Distribution of repugnatorial or alluring scent-glands in
insects 382
THE ALLURING OR SCENT-GLANDS 391
THE ORGANS OF CIRCULATION 397
_a._ The heart 397
The propulsatory apparatus 401
The supraspinal vessel 403
The aorta 404
The pericardial cells 405
Pulsatile organs of the legs 405
_b._ The blood 407
The leucocytes 407
_c._ The circulation of the blood 409
Effects of poisons on the pulsations 412
THE BLOOD TISSUE 419
_a._ The fat-body 419
_b._ The pericardial fat-body or pericardial cells 420
Leucocytes or phagocytes in connection with the
pericardial cells 421
_c._ The œnocytes 423
_d._ The phosphorescent organs 424
Physiology of the phosphorescence 426
THE RESPIRATORY SYSTEM 430
_a._ The tracheæ 431
Distribution of the tracheæ 432
_b._ The spiracles or stigmata 437
The position and number of pairs of stigmata 439
The closing apparatus of the stigma 441
_c._ Morphology and homologies of the tracheal system 442
_d._ The spiral threads or tænidia 444
_e._ Origin of the tracheæ and of the “spiral thread” 447
Internal, hair-like bodies 451
_f._ The mechanism of respiration and the respiratory
movements of insects 451
_g._ The air-sacs 456
The use of the air-sacs 457
_h._ The closed or partly closed tracheal system 459
_i._ The rectal, tracheal gills, and rectal respiration of
larval Odonata and other insects 463
_j._ Tracheal gills of the larvæ of insects 466
Blood-gills 475
_k._ Tracheal gills of adult insects 476
THE ORGANS OF REPRODUCTION 485
_a._ The male organs of reproduction 494
The testes 495
The seminal ducts 496
The ejaculatory duct 497
The accessory glands 497
The spermatozoa 497
Formation of the spermatozoön 498
_b._ The female organs of reproduction 500
The ovaries and the ovarian tubes 500
Origin of incipient eggs in the germ of the testes 504
The bursa copulatrix 505
The spermatheca 506
The colleterial glands 506
The vagina or uterus 507
Signs of copulation in insects 507
PART II. EMBRYOLOGY OF INSECTS
_a._ The egg 515
Mode of deposition 518
Vitality of eggs 520
Appearance and structure of the ripe egg 520
The egg-shell and yolk-membrane 520
The micropyle 522
Internal structure of the egg 524
_b._ Maturation or ripening of the egg 525
_c._ Fertilization of the egg 525
_d._ Division and formation of the blastoderm 526
_e._ Formation of the first rudiments of the embryo and of the
embryonic membranes 531
Formation of the embryonic membranes 532
The gastrula stage 535
Division of the embryo or primitive band into
body-segments 536
Differences between the invaginated and overgrown
primitive band 538
Revolution of the embryo where the primitive band is
invaginated 540
_f._ Formation of the external form of the body 542
Origin of the body-segments 542
The procephalic lobes 544
Fore-intestine (stomodæum) and hind-intestine
(proctodæum), labrum 547
Completion of the head 548
_g._ The appendages 548
The cephalic appendages 548
The thoracic appendages 550
The abdominal appendages 550
Appendages of the first abdominal segment (pleuropodia) 551
Are the abdominal legs of Lepidoptera and phytophagous
Hymenoptera true limbs? 552
The tracheæ 553
_h._ Nervous system 554
Completion of the definite form of the body 555
_i._ Dorsal closure and involution of the embryonic membranes 556
_j._ Formation of the germ-layers 558
_k._ Farther development of the mesoderm; formation of the
body-cavity 563
_l._ Formation of organs 566
The nervous system 566
Development of the brain 567
Development of the eyes 567
Intestinal canal and glands 569
The salivary glands 570
The urinary tubes 572
The heart 572
The blood-corpuscles 574
Musculature; connective tissue; fat-body 574
The reproductive organs 575
Development of the male germinal glands 579
_m._ Length of embryonic life 582
_n._ The process of hatching 583
The hatching spines 585
PART III. THE METAMORPHOSES OF INSECTS
_a._ The nymph as distinguished from the larval stage 593
_b._ Stages or stadia of metamorphosis 594
_c._ Ametabolous and metabolous stages 594
THE LARVA 599
_a._ The Campodea-form type of larva 600
_b._ The eruciform type of larva 602
_c._ Growth and increase in size of the larva 608
_d._ The process of moulting 609
The number of moults in insects of different orders 615
Reproduction of lost limbs 619
Formation of the cocoon 619
Sanitary conditions observed by the honey-bee larva, and
admission of air within the cocoon 623
THE PUPA STATE 625
_a._ The pupa considered in reference to its adaptation to its
surroundings and its relation to phylogeny 631
_b._ Mode of escape of the pupa from its cocoon 632
_c._ The cremaster 636
Mode of formation of the cremaster and suspension of the
chrysalis in butterflies 637
FORMATION OF THE PUPA AND IMAGO IN THE HOLOMETABOLOUS INSECTS (THE
DIPTERA EXCEPTED) 640
_a._ The Lepidoptera 642
The changes in the head and mouth-parts 646
The change in the internal organs 647
The wings 654
Development of the feet and of the cephalic appendages 654
Embryonic cells and the phagocytes 655
Formation of the femur and of the tibia; transformation of
the tarsus 656
The antennæ 657
Maxillæ and labial palpi 658
Process of pupation 660
_b._ The Hymenoptera 661
Ocular or oculo-cephalic buds 665
The antennal buds 665
The buds of the buccal appendages 665
The buds of the ovipositor 665
DEVELOPMENT OF THE IMAGO IN THE DIPTERA 666
_a._ Development of the outer body-form 668
Formation of the imago in Corethra 668
Formation of the imago in Culex 670
Formation of the imago in Chironomus 671
Formation of the imago in Muscidæ 673
_b._ Development of the internal organs of the imago 678
The hypodermis 678
The muscles 680
The digestive canal 681
The tracheal system 683
The nervous system 684
The fat-body 685
Definitive fate of the leucocytes 685
The post-embryonic changes and imaginal buds in the
Pupipara (Melophagus) 686
_c._ General summary 687
HYPERMETAMORPHISM 688
SUMMARY OF THE FACTS AND SUGGESTIONS AS TO THE CAUSES OF
METAMORPHISM 705
Theoretical conclusions; causes of metamorphosis 708
TEXT-BOOK OF ENTOMOLOGY
[Illustration]
PART I.—MORPHOLOGY AND PHYSIOLOGY
[Illustration]
POSITION OF INSECTS IN THE ANIMAL KINGDOM
Although the insects form but a single class of the animal kingdom, they
are yet so numerous in orders, families, genera, and species, their
habits and transformations are so full of instruction to the biologist,
and they affect human interests in such a variety of ways, that they
have always attracted more attention from students than any other class
of animals, the number of entomologists greatly surpassing that of
ornithologists, ichthyologists, or the special students of any other
class, while the literature has assumed immense proportions.
Insects form about four-fifths of the animal kingdom. There are about
250,000 species already named and contained in our museums, while the
number of living and fossil species in all is estimated to amount to
between one and two millions.
In their structure insects are perhaps more complicated than any other
animals. This is partly due to the serial arrangement of the segments
and the consequent segmental repetition of organs, especially of the
external appendages, and of the muscles, the tracheæ, and the nerves.
The brain is nearly or quite as complicated as that of the higher
vertebrates, while the sense-organs, especially those of touch, sight,
and smell are, as a rule, far more numerous and only less complex than
those of vertebrates. Moreover, in their psychical development, certain
insects are equal, or even superior, to any other animals, except birds
and mammals.
The animal kingdom is primarily divided into two grand divisions, the
one-celled (_Protozoa_) and many-celled animals (_Metazoa_). In the
latter group the cells and tissues forming the body are arranged in
three fundamental cell-layers; viz. the _ectoderm_ or outer layer, the
_mesoderm_, and _endoderm_. The series of branches, or phyla, comprised
under the term Metazoa are the Porifera, Cœlenterata, Vermes,
Echinodermata, Mollusca, Arthropoda, and Vertebrata. Their approximate
relationships may be provisionally expressed by the following
TABULAR VIEW OF THE EIGHT BRANCHES OR PHYLA OF THE ANIMAL KINGDOM.
VIII. _Vertebrata._
Ascidians and Fishes
to Man.
|
| VII. _Arthropoda._
| Trilobites, Crustacea, Arachnida,
| Insects, etc.
| |
| | VI. _Mollusca._
| | Clams, Snails, Cuttles.
| | |
| | | V. _Echinodermata._
| | | Crinoids, Star-fish, Sea-urchins, etc.
| | | |
| +----------+----------+----------+
| |
+---------------------------+------------+
|
IV. _Vermes._
Flat and Round Worms, Polyzoa, Brachiopods, Annelids.
|
III. _Cœlenterata._ | II. _Porifera._
Hydra, Jelly-fishes. | Sponges.
| | |
+------------------+------------------+
|
METAZOA.
Many-celled animals with 3 cell-layers.
|
I. PROTOZOA.
Single-celled animals.
RELATIONS OF INSECTS TO OTHER ARTHROPODA
The insects by general consent stand at the head of the Arthropoda.
Their bodies are quite as much complicated or specialized, and indeed,
when we consider the winged forms, more so, than any other class of the
branch, and besides this they have wings, fitting them for an aërial
life. It is with little doubt that to their power of flight, and thus of
escaping the attacks of their creeping arthropod enemies, insects owe,
so to speak, their success in life; _i.e._ their numerical superiority
in individuals, species, and genera. It is also apparently their power
of moving or swimming swiftly from one place to another which has led to
the numerical superiority in species of fishes to other Vertebrata.
Among terrestrial vertebrates, the birds, by virtue of their ability to
fly, greatly surpass in number of species the reptiles and mammals.
The Arthropoda are in general characterized by having the body composed
of segments (somites or arthromeres) bearing jointed appendages. They
differ from the worms in having segmented appendages, _i.e._ antennæ,
jaws, and legs, instead of the soft unjointed outgrowths of the annelid
worms. Moreover, their bodies are composed of a more or less definite
number of segments or rings, grouped either into a head-thorax
(cephalothorax) and hind-body, as in Crustacea, or into a head
differentiated from the rest of the body (trunk), the latter not being
divided into a distinct thorax and abdomen, as in Myriopoda; or into
three usually quite distinct regions—the head, thorax, and hind-body or
abdomen, as in insects. In certain aberrant, modified forms, as the
Tardigrada, or the Pantopoda, and the mites, the body is not
differentiated into such definite regions.
In their internal organs arthropods agree in their general relations
with the higher worms, hence most zoölogists agree that they have
directly originated from the annelid worms.
The position and general shape of the digestive canal, of the nervous
and circulatory systems, are the same in Arthropoda as in annelid
(oligochete) worms, so much so that it is generally thought that the
Arthropoda are the direct descendants of the worms. It is becoming
evident, however, that there was no common ancestor of the Arthropoda as
a whole, and that the group is a polyphyletic one. Hence, though a
convenient group, it is a somewhat artificial one, and may eventually be
dismembered into at least three or four phyla or branches.
The following diagram may serve to show in a tentative way the relations
of the classes of Arthropoda to each other, and also may be regarded as
a provisional genealogical tree of the branch.
9. _Insecta._
| 7. _Chilopoda._
4. _Arachnida._ | |
| | 6. _Diplopoda._ |
| | | |
3. _Merostomata._ | 8. _Symphyla._ | |
| | | | |
1. _Crustacea._ | | | |6_a_. _Pauropoda._|
| |2. _Trilobita._| | | | |
| | | | | +-------+ |
| | | | | | |
| +-------+-------+ +--------+------------------+
| | |
| | +--------+
| | |
| | 5. _Peripatus._
| |4_a_. _Pantopoda._ |
| | | |
| | |4_b_. _Tardigrada._|
| | | | |
+----------------+-------+--------+----------+
|
_Different Annelida._
Trochosphæra.
We will now rapidly review the leading features of the classes of
Arthropoda.
=The Crustacea.=—These Arthropoda are in many most important
characteristics unlike the insects; they have two pairs of antennæ, five
pairs of buccal appendages, and they are branchiate Arthropoda. They
have evidently originated entirely independently, and by a direct line
of descent from some unknown annelid ancestor which was either a
many-segmented worm, with parapodia, or the two groups together with the
Rotifera may have originated from a common appendigerous Trochosphæra.
Their segments in the higher forms are definite in number (23 or 24) and
arranged into two regions, a head-thorax (cephalothorax) and hind-body
(abdomen). Nearly all the segments, both of the cephalothorax and
abdomen, bear a pair of jointed limbs, and to them at their base are, in
the higher forms, appended the gills (branchiæ). The limbs are in the
more specialized forms (shrimps and crabs) differentiated into
eye-stalks, two pairs of antennæ, a pair of palpus-bearing jaws
(mandibles), two pairs of maxillæ and three pairs of maxillipeds; these
appendages being biramose, and the latter bearing gills attached to
their basal joints. The legs are further differentiated into ambulatory
thoracic legs and into swimming or abdominal legs, and in the latter the
first pair of the male is modified into copulatory organs (gonopoda).
The male and female reproductive organs as a rule are in separate
individuals, hermaphrodites being very unusual, and the glands may be
paired or single. The sexual outlets are generally paired, and, as in
the male lobster and other Macrura, open in the basal joint of the last
pair of legs, and in the female in the third from the last; while
originally in all Crustacea the sexual organs were most probably paired
(Fig. 3, _B_).
They are, except a few land Isopoda, aquatic, mostly marine, and when
they have a metamorphosis, pass through a six-legged larval stage,
called the Nauplius, the shrimps and crabs passing through an additional
stage, the Zoëa. Crustacea also differ much from insects in the highly
modified nature of the nephridia, which are usually represented by the
green gland of the lobster, or the shell-glands of the Phyllopoda, which
open out in one of the head-segments; also in the possession of a pair
of large digestive glands, the so-called liver.
Intermediate in some respects between the Crustacea and insects, but
more primitive, in respect to what are perhaps the most weighty
characters, than the Crustacea, are the Trilobita, the Merostomata
(Limulus), and, finally, the Arachnida, these being allied groups. In
the Trilobita and Merostomata (Limulus), the head-appendages are more
like feet than jaws, while they have in most respects a similar mode of
embryonic development, the larval forms being also similar.
[Illustration:
FIG. 1.—Restoration of under side of a trilobite (_Triarthrus becki_),
the trunk limbs bearing small triangular respiratory lobes or
gills.—After Beecher.
]
=The Merostomata.=—The only living form, Limulus, is undoubtedly a very
primitive type, as the genital glands and ducts are double, opening wide
apart on the basal pair of abdominal legs (Fig. 3). Moreover, their
head-appendages, which are single, with spines on the basal joint, are
very primitive and morphologically nearer in shape to those of the worms
(Syllidæ, etc.) than even those of the Crustacea. Besides, their four
pairs of coxal glands, with an external opening at the base of the fifth
pair of head-appendages, and which probably are modified nephridia
(Crustacea having but a single pair in any one form, either opening out
on the second antennal, green gland, or second maxillary, shell-gland,
segment), indicate a closer approximation to the polynephrous worms.
Limulus has other archaic features, especially as regards the structure
of the simple and compound eyes and the simple nature of the brain.
=The Trilobita.=—These archaic forms are still more generalized and
primitive than the Merostomata and Crustacea, and probably were the
first Arthropoda to be evolved from some unknown annelid worm. They had
jointed biramose limbs of nearly uniform shape and size on each segment
of the body, which were not, as in Crustacea, differentiated into
antennæ, jaws (mandibles), maxillæ, maxillipeds, and two kinds of legs
(thoracic and abdominal), showing that they are a much more primitive
type, and nearer to the annelids than any other Arthropoda. Their gills,
as shown by the researches of Walcott and of Beecher, were attached to
nearly if not every pair of limbs behind the antennæ (Figs. 1, 2). The
fact that in Trilobita the first pair of limbs is antenniform does not
prove that they are Crustacea, since Eurypterus has a similar pair of
appendages.
[Illustration:
FIG. 2.—Restored section of Calymene: _C_, carapace; _en_, endopodite;
_en′_, exopodite; with the gills on the epipodal or respiratory part
of the appendage.—After Walcott.
]
The limbs in trilobites, as well as the abdominal ones of merostomes,
and all those of Crustacea, except the first antennæ, are biramose,
consisting of an outer (exopodite) and an inner division (endopodite).
In this respect the terrestrial air-breathing tracheate forms,
Arachnida, Myriopoda, and Insecta, differ from the branchiate forms, as
their legs are single or undivided, being adapted for supporting the
body during locomotion upon the solid earth. It is to be observed that
when, as in Limulus, the body is supported by cephalic ambulatory limbs,
they are single, while the abdominal limbs, used as they are in
swimming, are biramose, much as in Crustacea.
=The Arachnida.=—The scorpions and spiders are much less closely allied
to the myriopods and insects than formerly supposed. Their embryology
shows that they have descended from forms related to Limulus, possibly
having had an origin in common with that animal, or having, as some
authors claim, directly diverged from some primitive eurypteroid
merostome. But they differ in essential respects, and not only in the
nature and grouping of their appendages; the first pair instead of
antenniform being like mandibles, and the second pair like the maxillæ,
with the palps, of insects, the four succeeding segments (thoracic)
bearing each a pair of legs. They also have a brain quite unlike that of
Limulus, the nervous cord behind the brain, however, being somewhat
similar, though that of Limulus differs in being enveloped by an
arterial coat. Arachnida respire by tracheæ, besides book-lungs, which,
however, are possibly derivatives of the book-gills of Limulus, while
they perform the office of excretion by means of the malpighian tubes,
and like Limulus possess two large digestive glands (“liver”). Their
embryos have, on at least six abdominal segments, rudiments of limbs,
three pairs of which form the spinnerets, showing their origin from
Limulus-like or eurypteroid forms; their coxal glands are retained from
their eurypteroid ancestors. The Arachnida probably descended from
marine merostomes, and not from an independent annelid ancestry, hence
we have represented them in the diagram on p. 3 as branching off from
the merostomatous phylum, rather than from an independent one.
[Illustration:
FIG. 3.—Paired genital openings of different classes of arthropods.
_A_, the most primitive, of _Limulus polyphemus_: _gen. p_,
generative papillæ; _d_, duct; _vd_, vas deferens; _t_, tendinous
stigmata; _stig_, stigmata; _e_, external branchial muscle; _ant_,
anterior lamellar muscle.—After Benham, with a few changes. _B_,
lobster (_Homarus vulgaris_), ♀: _oe_, genital aperture on 3d pair
of legs; _ov_, ovary; _u_, unpaired portion of the same; _od_,
oviduct. _C_, ♀, scorpion: _ov_, ovary, with a single external
opening. _D_, ♂: _t_, testis; _vd_, vasa deferentia; _sb_, seminal
vesicle; _a_, glandular appendage; _p_, penis.—After Blanchard. _E_,
a myriopod (_Glomeris marginata_, ♀): _os_, ovarian sac, laid open;
_od_, paired oviducts. _F_, ♂: _t_, testis; _gvd_, common vas
deferens; _pa_, paired ducts.—After Favre, from Lang. _G_, _Lepisma
saccharina_, young ♂: _vd_, vas deferens, _ed_, ejaculatory duct;
_ga_, external appendages.—After Nassonow. _H_, Ephemera, ♂, showing
the double outlets.—After Palmén.
]
The characters in which arachnids approach insects, such as tracheæ and
malpighian tubes (none occur, as a rule, in marine or branchiate
arthropods), may be comparatively recent structures acquired during a
change from a marine to a terrestrial life, and not primitive heirlooms.
Arachnida also show their later origin than merostomes by the fact that
their sexual glands are in most cases single, and though with rare
exceptions the ducts are paired, these finally unite and open externally
by a common single genital aperture in the median line of the body, at
the base of the abdomen (Fig. 3, _C_, _D_). In this respect Limulus,
with its pair of genital male or female openings, situated each at the
end of a papilla, placed widely apart at the base of the first abdominal
limbs, is decidedly more archaic. Unlike Crustacea and insects,
Arachnida do not, except in the mites (Acarina), which is a very much
modified group, undergo a metamorphosis.
We see, then, that the insects, with the Myriopoda, are somewhat
isolated from the other Arthropoda. The Myriopoda have a single pair of
antennæ, and as they have other characters in common with insects, Lang
has united the two groups in a single class _Antennata_; but, as we
shall see, this seems somewhat premature and unnecessary. Yet the two
groups have perhaps had a common parentage, and may prove to belong to a
distinct, common phylum.
Not only by their structure and embryology, as well as their
metamorphosis, do the myriopods and insects stand apart from the
Arachnida and other arthropods, but it seems probable that they have had
a different ancestry, the arthropods being apparently polyphyletic.
There are two animals which appear to connect the insects with the
worms, and which indicate a separate line of descent from the worms
independent of that of the other classes. These are the singular
Peripatus, which serves as a connecting link between arthropods and
worms, and Scolopendrella (Symphyla). These two animals are guide-posts,
pointing out, though vaguely to be sure, the way probably trod by the
forms, now extinct, which led up to the insects.
=Relations of Peripatus to Insects.=—We will first recount the
characteristics of this monotypic class. Peripatus (Fig. 4) stands
alone, with no forms intermediate between itself and the worms on the
one hand, and the true Arthropoda on the other. Originally supposed to
be a worm, it is now referred to a class by itself, the Malacopoda of
Blainville, or Protracheata of Haeckel. It lives in the tropics, in damp
places under decaying wood. In general appearance it somewhat resembles
a caterpillar, but the head is soft and worm-like, though it bears a
pair of antenna-like tentacles. It may be said rather to superficially
resemble a leech with clawed legs, the skin and its wrinkles being like
those of a leech. There is a pair of horny jaws in the mouth, but these
are more like the pharyngeal teeth of worms than the jaws of arthropods.
The numerous legs end each in a pair of claws. The ladder-like nervous
system is unlike that of annelid worms or arthropods, but rather recalls
that of certain molluscs (Chiton, etc.), as well as that of certain flat
and nemertine worms. Its annelid features are the large number of
segmentally arranged true nephridia, and the nature of the integument.
Its arthropodan features, which appear to take it out of the group of
worms, are the presence of tracheæ, of true salivary and slime glands,
of a pair of coxal glands (Fig. 4, _C_, _cd_) as well as the claws at
the end of the legs. The tracheæ, which are by no means the only
arthropodan features, are evidently modified dermal glands. The heart is
arthropodan, being a dorsal tube lying in a pericardial sinus, with many
openings. This assemblage of characters is not to be found in any marine
or terrestrial worm.
The tracheæ (Fig. 4, _D_, _tr_) are unbranched fine tubes, without a
“spiral thread,” and are arranged in tufts, in _P. edwardsii_ opening by
simple orifices or pores (“stigmata”) scattered irregularly over the
surface of the body; but in another species (_P. capensis_) some of the
stigmata are arranged more definitely in longitudinal rows,—on each side
two, one dorsally and one ventrally. “The stigmata in a longitudinal row
are, however, more numerous than the pairs of legs.” (Lang.)
The salivary glands, opening by a short common duct into the under side
of the mouth, in the same general position as in insects, are evidently,
as the embryology of the animal proves, transformed nephridia, and being
of the arthropodan type explain the origin and morphology of those of
insects. It is so with the slime glands; these, with the coxal glands,
being transformed and very large dermal glands. Those of insects arose
in the same manner, and are evidently their homologues, while those of
Peripatus were probably originally derived from the setiparous glands in
the appendages (parapodia) of annelid worms.
[Illustration:
FIG. 4.—_A_, _Peripatus novæ zealandiæ_.—After Sedgwick, from Lang.
_B_, _Peripatus capensis_, side view, enlarged about twice the
natural size.—After Moseley, from Balfour. _C_, Anatomy of
_Peripatus capensis_. The enteric canal behind the pharynx has been
removed. _g_, brain; _a_, antenna; _op_, oral or slime papillæ;
_sd_, slime gland; _sr_, slime reservoir, which at the same time
acts as a duct to the gland; _so_{4}_, _so_{5}_, _so_{6}_, _so_{9}_,
nephridia of the 4th, 5th, 6th, and 9th pairs of limbs; _cd_,
elongated coxal gland of the last pair of feet; _go_, genital
aperture; _an_, anus; _ph_, pharynx; _n_, longitudinal trunk of the
nervous system.—After Balfour, from Lang. _D_, Portion of the body
of _Peripatus capensis_ opened to show the scattered tufts of
tracheæ (_tr_); _v_, _v_, ventral nerve cords.—After Moseley.
]
The genital glands and ducts are paired, but it is to be observed that
the outlets are single and situated at the end of the body. In the male
the ejaculatory duct is single; in its base a spermatophore is formed.
It will be seen, then, that Peripatus is not only a composite type, and
a connecting link between worms and tracheate arthropods, but that it
may reasonably be regarded, if not itself the ancestor, as resembling
the probable progenitor of myriopods and insects, though of course there
is a very wide gap between Peripatus and the other antennate,
air-breathing Arthropoda.
[Illustration:
FIG. 4.—_E_, _Peripatus edwardsii_, head from the under side: _a_,
base of antenna; _op_, oral papilla; the figure also shows the
papillæ around the mouth, and the four jaws.—After Balfour, from
Lang. _F_, Anterior end of _Peripatus capensis_, ventral side, laid
open: _a_, antenna; _z_, tongue; _k_, jaw; _sd_, salivary gland;
_gs_, union of the two salivary glands; _ph_, pharynx; _œ_,
œsophagus; _l_, lip papillæ around the mouth; _op_, oral or slime
papilla; _sld_, duct or reservoir of the slime gland.—After Balfour,
from Lang.
]
=Relation of Myriopods to Insects.=—The Myriopoda are the nearest allies
of the insects. They have a distinct head, with one pair of antennæ. The
eyes are simple, with the exception of a single genus (Cermatia), in
which they are aggregated or compound. The trunk or body behind the head
is, as a rule, long and slender, and composed of a large but variable
number of segments, of equal size and shape, bearing jointed legs, which
invariably end in a single claw.
The mouth-parts of the myriopods are so different in shape and general
function from those of insects, that this character, together with the
equally segmented nature of the portion of the body behind the head (the
trunk), forbids our merging them, as some have been inclined to do, with
the insects. There are two sub-classes of myriopods, differing in such
important respects that by Pocock[1] and by Kingsley they are regarded
as independent classes, each equivalent to the insects.
Of these the most primitive are the Diplopoda (Chilognatha), represented
by the galley-worms (Julus, etc.).
[Illustration:
FIG. 5.—Mandible of Julus: _l_, lacinia; _g_, galea; _p_, dens
mandibularis; _ma_, “mala”; _lt_, lamina tritoria; _st_, stipes;
_c_, cardo; _m_, muscle.—After Latzel.
]
In the typical Diplopoda the head consists of three segments, a preoral
or antennal, and two postoral, there being two pairs of jaw-like
appendages, which, though in a broad morphological sense homologues of
the mandibles and first maxillæ of insects, are quite unlike them in
details.
[Illustration:
FIG. 6.—Under lip or deutomala of _Scoterpes copei_: _hyp_, hypostoma
or mentum; _lam. lab_, lamina labialis; _stip. e_, stipes exterior;
with the malella exterior (_mal. e_) and malella interior (_mal.
i_); the stipes interior, with the malulella; and the labiella
(hypopharynx of Vom Rath) with its stilus (_stil._).
]
As we have previously stated,[2] the so-called “mandibles” of diplopods
are entirely different from those of insects, since they appear to be 2–
or 3–jointed, the terminal joint being 2–lobed, thus resembling the
maxillæ rather than the mandibles of insects, which consist of but a
single piece or joint, probably the homologue of the galea or molar
joint of the diplopod protomala. The mandible of the Julidæ (Fig. 5,
_Julus molybdinus_), Lysiopetalidæ, and Polydesmidæ consists of three
joints; viz. a basal piece or cardo, a stipes, and the _mala
mandibularis_, which supports two lobes analogous to the galea and
lacinia of the maxilla of an insect. There is an approach, as we shall
see, in the mandible of Copris, to that of the Julidæ, but in insects in
general the lacinia is wanting, and the jaw consists of but a single
piece.
The deutomalæ (gnathochilarium), or second pair of diplopod jaws, are
analogous to the labium or second maxillæ of insects, forming a
flattened, plate-like under-lip, constituting the floor of the mouth
(Fig. 6). This pair of appendages needs farther study, especially in the
late embryo, before it can be fully understood. So far as known, judging
by Metschnikoff’s work on the embryology of the diplopods, these
myriopods seem to have in the embryo but two pairs of post-antennal
mouth-parts, which he designated as the “mandibles” and “labium.”
Meinert, however, regards as a third pair of mouth-parts or “labium”
what in our Fig. 7 is called the internal stipes (_stip. i._), behind
which is a triangular plate, _lamina labialis_ (_lam. lab_), which he
regards as the sternite of the same segment.
[Illustration:
FIG. 7.—Deutomala of Julus, the lettering as in Fig. 6.
]
[Illustration:
FIG. 8.—Head of _Scolopendra_, seen from beneath, showing the
“mandible” (protomala) with its cardo (_card._) and stipes (_st._),
also the labrum and epilabrum.
]
The hypopharynx, our “labiella,” (Fig. 6), with the supporting rods or
_stili linguales_ (_sti. l_), of Meinert, are of nearly the same shape
as in some insects.
Of the clypeus of insects there is apparently no homologue in myriopods,
though in certain diplopods there is an interantennal clypeal region.
The labium of insects is represented by a short, broad piece, which,
however, unlike that of insects, is immovable, and is flanked by a
separate piece called the _epilabrum_ (Fig. 8). Vom Rath has observed an
epipharynx, which has the same general relations as in insects.
[Illustration:
FIG. 9.—Larva of Julus: _a_, the 3d abdominal segment, with the new
limbs just budding out; _b_, new segments arising between the
penultimate and the last segment.—After Newport.
]
The embryology of myriopods is in many respects like that of insects.
The larva of diplopods hatches with but few segments, and with but three
pairs of limbs; but these are not, as in insects, appended to
consecutive segments, but in one species the third, and in another,
_Julus multistriatus?_ (Fig. 10), the second, segment from the head is
footless, while Vom Rath represents the first segment of an European
Blaniulus as footless, the feet being situated consecutively on segments
2 to 4. The new segments arise at “the growing point” situated between
the last and penultimate segment, growing out in groups of sixes
(Newport) or in our _Julus multistriatus?_ in fives (Fig. 10). In adult
life diplopods (Julus) have a single pair of limbs on the three first
segments, or those corresponding to the thoracic segments of insects,
the succeeding segments having two pairs to each segment.
[Illustration:
FIG. 10.—Freshly hatched larva of _Julus multistriatus?_ 3 mm. long:
_a_, 5 pairs of rudimentary legs, one pair to a segment.
]
Sinclair (Heathcote) regards each double segment in the diplopods as
not two original segments fused together, nor a single segment
bearing two pairs of legs, but as “two complete segments perfect in
all particulars, but united by a large dorsal plate which was
originally two plates which have been fused together.” (Myriopods,
1895, p. 71.) That the segments were primitively separate is shown,
he adds, by the double nature of the circulatory system, the nerve
cord, and the first traces of segmentation in the mesoblast. Kenyon
believes that from the conditions in pauropods, Lithobius, etc.,
there are indications of alternate plates (not segments) having
disappeared, and of the remaining plates overgrowing the segments
behind them, so as to give rise to the anomalous double segments.[3]
[Illustration:
FIG. 11.—Sixth pair of legs of _Polyzonium germanicum_, ♀: _cs_,
ventral sacs; _cox_, coxa; _st_, sternal plate; _sp_,
spiracle.—After Haase.
]
Diplopods are also provided with eversible coxal sacs, in position like
those of Symphyla and Synaptera; Meinert, Latzel, and also Haase having
detected them in several species of Chordeumidæ, Lysiopetalidæ, and
Polyzonidæ (Fig. 11). In _Lysiopetalum anceps_ these blood-gills occur
in both sexes between the coxæ of the third to sixteenth pair of limbs.
In the Diplopods the blood-gills appear to be more or less permanently
everted, while in Scolopendrella they are usually retracted within the
body (Fig. 15, _cg_).
Diplopods also differ externally from insects in the genital armature, a
complicated apparatus of male claspers and hooks apparently arising from
the sternum of the sixth segment and being the modified seventh pair of
legs. In myriopods there are no pleural pieces or “pleurites,” so
characteristic of winged insects.
Perhaps the most fundamental difference between diplopods and insects is
the fact that the paired genital openings of the former are situated not
far behind the head between the second and third pair of legs. Both the
oviducts and male ejaculatory ducts are paired, with separate openings.
The genital glands lie beneath, while in chilopods they lie above the
intestine; this, as Korschelt and Heider state, being a more primitive
relation, since in Peripatus they also lie above the digestive canal.
The nervous system of diplopods is not only remarkable for the lack of
the tendency towards a fusion of the ganglia observable in insects, but
for the fact that the double segments are each provided with two
ganglia. The brain also is very small in proportion to the ventral cord,
the nervous system being in its general appearance somewhat as in
caterpillars.
The arrangement of the tracheæ and stigmata is much as in insects, but
in the Diplopoda the tracheary system is more primitive than in
chilopods, a pair of stigmata and a pair of tracheal bundles occurring
in each segment, while the bundles are not connected by anastomosing
branches, branched tracheæ only occurring in the Glomeridæ. The tracheæ
themselves are without spiral threads (tænidia). It is noteworthy that
the tracheæ arise much later than in insects, not appearing until the
animal is hatched; in this respect the myriopods approximate Peripatus.
In the Chilopoda also the parts of the head, except the epicranium, are
not homologous with those of insects, neither are the mouth-parts, of
which there are five pairs.
The structure of the head of centipedes is shown in part in Fig. 12,
compare also Fig. 8. It will be seen that it differs much from that of
the diplopods, though the mandibles (protomalæ) are homologous; they are
divided into a cardo and stipes, thus being at least two-jointed.
The second pair of postoral appendages is in centipedes very different
from the gnathochilarium of diplopods. As seen in Fig. 12 2, they are
separate, cylindrical, fleshy, five-jointed appendages, the maxillary
appendages of Newport, which are “connected transversely at their base
with a pair of soft appendages” (_c_), the lingua of Newport. The third
and fourth pair are foot-jaws, and we have called them _malipedes_, as
they have of course no homology with the maxillipedes of Crustacea. The
second pair of these malipedes, forming the last pair of
mouth-appendages, is the poison-fangs (4), which are intermediate
between the malipedes and the feet; Meinert does not allow that these
are mouth-appendages.
[Illustration:
FIG. 12.—Structure of a chilopod. _A_, _Lithobius americanus_, natural
size. _B_, under side of head and first two body-segments and legs,
enlarged: _ant_, antenna; 1, jaws; 2, first accessory jaw; _c_,
lingua; 3, second accessory jaw and palpus; 4, poison-jaw. (Kingsley
del.) _C_, side view of head (after Newport): _ep_, epicranium; _l_,
frontal plate; _sc_, scute; 1, first leg; _sp_, spiracle.
]
The embryology of Geophilus by Metschnikoff shows plainly the four pairs
of post-antennal appendages. The embryo Geophilus is hatched in the form
of the adult, having, unlike the diplopods, no metamorphosis, its
embryological history being condensed or abbreviated. But in examining
Metschnikoff’s figures certain primitive diplopod features are revealed.
The body of the embryo shortly before hatching is cylindrical; the
sternal region is much narrower than in the adult, hence the insertions
of the feet are nearer together, while _the first six pairs of
appendages begin to grow out before the hinder ones_. Thus the first six
pairs of appendages of the embryo Geophilus correspond to the antennæ,
two pairs of jaws, and three pairs of legs of the larval Julus. These
features appear to indicate that the chilopods may be an offshoot from
the diplopod stem. The acquisition of a second pair of legs to a segment
in diplopods, as in the phyllopod Crustacea, is clearly enough a
secondary character, as shown by the figures of Newport in his memoir on
the development of the Myriopoda (Pl. IV.). Thus the tendency in the
Myriopoda, both diplopods and chilopods, is towards the multiplication
of segments and the elongation of the body, while in insects the
polypodous embryo has the three terminal segments of the abdomen well
formed, these being, however, before hatching, partly atrophied, so that
the body of insects after birth tends to become shortened or condensed.
This indicates the descent of insects from ancestors with elongated
polypodous hind-bodies like Scolopendrella. Korschelt and Heider suggest
that the stem-form of myriopods was a homonomously jointed form like
Peripatus, consisting of a rather large number of segments, but we
might, with Haase, consider that the great number of segments which we
now find indicates a late acquisition of this form.
The genital opening in chilopods is single, and situated in the
penultimate segment of the body, as in insects. While recognizing the
close relationship of the Myriopoda with the insects, it still seems
advisable not to unite them into a single group (as Oudemans, Lang, and
others would do), but to regard them as forming an equivalent class. On
the other hand, when we take into account the form and structure of the
head, antennæ, and especially the shape of the first pair of
mouth-appendages, being at least two-jointed in both groups, we think
these characters, with the homonomously segmented body behind the head,
outweigh the difference in the position of the genital outlet, important
as that may seem. It should also be taken into account that while
insects are derived from polypodous ancestors, no one supposes, with the
exception of one or two authors, that these ancestors are the Myriopoda,
the latter having evidently descended from a six-legged ancestor, quite
different from that of the Campodea ancestor of insects.[4]
In regard to the sexual openings of worms, though their position is in
general in the anterior part of the body, it is still very variable,
though, in general, paired. In the oligochete worms the genital zone,
with the external openings, is formed by the segments lying between the
9th and 14th rings, though in some the genital organs are situated still
nearer the head. The myriopods, which evolved from the worms earlier
than insects, appear to have in their most primitive forms (the
Diplopoda) retained this vermian position of the genital outlets. In the
later forms, the chilopods, the genital openings have been carried back
to near the end of the body, as in insects. From observations made by
three different observers on the freshly hatched larva of the Julidæ, it
appears that the ancestral diplopods were six-footed, or oligopod, the
larva of Pauropus (Fig. 13) approaching nearest to our idea of the
ancestral myriopod, which might provisionally be named Protopauropus.
=Relations of the Symphyla to Insects.=—Opinions respecting the position
of the Symphyla, represented by Scolopendrella (Fig. 14), are very
discordant. By most writers since Newport, Scolopendrella has been
placed among the myriopods. The first author, however, to examine its
internal anatomy was Menge (1851), who discovered among other structures
(tracheæ, etc.) the silk-glands situated in the last two segments, and
which open at the end of each cercus. He regarded the form as “the type
of a genus or family intermediate between the hexapod Lepismidæ and the
Scolopendridæ.”
[Illustration:
FIG. 13.—_Pauropus huxleyi_, much enlarged. _A_, enlarged view of
head, antennæ, and first pair of legs (original). _B_, young.—After
Lubbock. _C_, longitudinal section of _Pauropus huxleyi_, ♂: _a_,
brain; _b_, salivary gland; _k_, mid-intestine; _g_, rectum; _h_,
ventral nerve-cord; _c_, bud-like remnants of coxæ; _d_, penis; _e_,
vesicula seminalis; _f_, ductus glandularis; _i_^1, divisions of
testes.—After Kenyon.
]
In 1873[5] the writer referred to this form as follows: “It may be
regarded as a connecting link between the Thysanura and Myriopoda, and
shows the intimate relation of the myriopods and the hexapods, perhaps
not sufficiently appreciated by many zoölogists.”
In 1880 Ryder regarded it as “the last survival of the form from which
insects may be supposed to have descended,” and referred it to “the new
ordinal group Symphyla, in reference to the singular combination of
myriopodous, insectean, and thysanurous characters which it
presents.[6]”
[Illustration:
FIG. 14.—_Scolopendrella immaculata_, from above,—after Lang; also
from beneath, the genital opening on the 4th trunk-segment: _sac_,
eversible or coxal sac; _an_, anus; _c_, cereopod; _v_, vestigial
leg.—After Haase, from Peytoureau. _A_ _B_ _C_, head and buccal
appendages of _Scolopendrella immaculata_: _A_, head seen from
above; _cl_, clypeus. _B_, head from beneath; _l_, first pair of
legs; _mx_, 1st maxilla; _mx_^1, 2d maxilla; _t_, “labial plates” of
Latzel, labium of Muhr. _C_, 1st maxilla; _l_, lacinia; _g_, galea;
_p_, rudiment of the palpus.—After Latzel. _D_, end of the body:
_p__{11}, eleventh, _p__{12}, twelfth undeveloped pair of legs;
_p__{13}, modified, vestigial legs, bearing tactile organs (_so_);
_sg_, cercopod, with duct of spinning gland, _dg_; _cd_, eversible
or coxal gland; _h_{8}s_, coxal spur of the 11th pair of legs.—After
Latzel from Lang.
]
Wood-Mason considered it to be a myriopod, and “the descendant of a
group of myriopods from which the Campodeæ, Thysanura, and Collembola
may have sprung.” We are indebted to Grassi for the first extended work
on the morphology of Scolopendrella (1885). In 1886 he added to our
knowledge facts regarding the internal anatomy, and gives a detailed
comparison with the Thysanura, besides pointing out the resemblances of
Scolopendrella to Pauropus, diplopods, chilopods, as well as Peripatus.
[Illustration:
FIG. 15.—Section of _Scolopendrella immaculata_: _œ_, œsophagus; _oe.
v_, œsophageal valve entering the mid-intestine (“stomach”); _i_,
intestine; _r_, rectum; _br_, brain; _ns_, abdominal chain of
ganglia; _ovd_, oviduct; _ov_, ovary; _s. gl_, silk-gland, and _op_,
its outer opening in cercus, _ur. t_, urinary tube; _cg_, coxal
glands or blood-gills.—Author _del._
]
In 1888 Grassi expressed his view as to the position of the Symphyla,
stating that it should not be included in the Thysanura, since it
evidently has myriopod characters; these being the supraspinal vessel,
the ventral position of the genital glands; the situation of the genital
opening in the fourth segment of the trunk, its ganglionic chain being
like that of diplopods, its having limbs on all the segments, etc. On
the other hand, Grassi has with much detail indicated the points of
resemblance to the Thysanura. The principal ones are the thin
integument, the want of sympathetic ganglia, the presence of a pair of
cephalic stigmata, like that said to occur in certain Collembola, and in
the embryo of Apis; two endoskeletal processes situated near the ventral
fascia of the head; the epicranial suture also occurring in Thysanura,
Collembola, Orthoptera, and other winged insects, and being absent in
diplopods and chilopods. He also adds that the digestive canal both in
Symphyla and Thysanura is divided into three portions; the malpighian
tubes in Thysanura present very different conditions (there being none
in Japyx), among which may be comprised those of Scolopendrella. In both
groups there is a single pair of salivary glands. The cellular
epithelium of the mid-intestine of Scolopendrella is of a single form as
in Campodea and Japyx. The fat-body, dorsal vessel, with its valves and
ostia, are alike in the two groups, as are the appendages of the end of
the abdomen, the anal cerci (cercopoda) of Scolopendrella being the
homologues of the multiarticulate appendages of Lepisma, etc., and of
the forceps of Japyx. In those of Scolopendrella, we have found the
large duct leading from the voluminous silk-gland, a single large sac
extending forwards into the third segment from the end of the body (Fig.
15, _s. gl_). Other points of resemblance, all of which he enumerates,
are the slight differences in the number of trunk-segments, the presence
in the two groups of the abdominal “false-legs” (parapodia), the dorsal
plate, and the mouth-parts. As regards the latter, Grassi affirms that
there is a perfect parallelism between those of Scolopendrella and
Thysanura. To this point we will return again in treating more
especially of those of the Symphyla. Finally, Grassi concludes that
there is “a great resemblance between the Thysanura and Scolopendrella.”
He, however, believed that the Symphyla are the forerunners of the
myriopods, and not of the insects, his genealogical tree representing
the symphylan and thysanuran phyla as originating from the same point,
this point also being, rather strangely, the point of origin of the
arachnidan phylum.
Haase (1889) regarded Scolopendrella as a myriopod, and Pocock (1893)
assigned the Symphyla to an independent class, regarding Scolopendrella
as “the living form that comes nearest to the hypothetical ancestor of
the two great divisions of tracheates.” Schmidt’s work (1895) on the
morphology of this genus is more extended and richly illustrated than
Grassi’s, his method of research being more modern. He also regards this
form as one of the lower myriopods.
In conclusion, it appears to us that, on the whole, if we throw out the
single characteristic of the anteriorly situated genital opening, the
ovarian tubes being directed toward the end of the body (Fig. 15, _ovd_,
_ov_), there is not sufficient reason for placing the Symphyla among the
Myriopoda, either below or near the diplopods. This is the only valid
reason for not regarding Scolopendrella as the representative of a group
from which the insects have descended, and which partly fills the wide
abyss between Peripatus and insects. With the view of Pocock, that both
insects and myriopods have descended from Scolopendrella, we do not
agree, because this form has so many insectean features, and a single
unpaired genital opening. For the same reason we should not agree with
Schmidt in interpolating the Symphyla between the Pauropoda and
Diplopoda. In these last two progoneate groups the genital openings are
paired, hence they are much more primitive types than Scolopendrella, in
which there is but a single opening. It seems most probable that the
Symphyla, though progoneate, are more recent forms than the progoneate
myriopods, which have retained the primitive feature of double sexual
outlets. It is more probable that the Symphyla were the descendants of
these polypodous forms. Certainly Scolopendrella is the only extant
arthropod which, with the sole exception of the anteriorly situated
genital opening, fulfils the conditions required of an ancestor of
Thysanura, and through them of the winged insects. No one has been so
bold as to suggest the derivation of insects from either diplopods or
chilopods, while their origin from a form similar to Scolopendrella
seems not improbable. Yet Uzel has very recently discovered that
Campodea develops in some respects like Geophilus, the primitive band
sinking in its middle into the yolk, with other features as in
chilopods.[7] The retention of a double sexual opening in the diplopods
is paralleled by the case of Limulus with its double or paired sexual
outlets, opening in a pair of papillæ, as compared with what are
regarded as the generalized or more primitive Crustacea, which have an
unpaired sexual opening.
The following summary of the structural features of the Symphyla, as
represented by Scolopendrella, is based mainly on the works of Grassi,
Haase, and Schmidt, with observations of my own.
=Diagnostic or essential characters of Symphyla.=—_Head shaped as in
Thysanura (Cinura), with the Y-shaped tergal suture, which occurs
commonly in insects (Thysanura, Collembola, Dermaptera, Orthoptera,
Platyptera, Neuroptera, etc.), but is wanting in Myriopoda (Diplopoda
and Chilopoda); antennæ[8] unlike those of Myriopoda in being very long,
slender, and moniliform. Clypeus distinct. Labrum emarginate, with six
converging teeth. Mandibles 2–jointed, consisting of a vestigial stipes
and distal or molar joint, the latter with eight teeth. First maxillæ
with an outer and inner mala situated on a well-developed stipes; with a
minute, 1–jointed palpus. Second pair of maxillæ: each forming two
oblong flat pieces, median sutures distinct, with no palpi; these pieces
are toothed in front, and appear to be homologous with the two median
pieces of the gnathochilarium of Diplopoda. Hypopharynx? Epipharynx?_
_Trunk with from fifteen to sixteen dorsal, more or less free subequal
scutes, the first the smallest. Pedigerous segments twelve; also twelve
pairs of 5–jointed legs, which are of nearly equal length, the first
pair 4–, the others 5–jointed, all ending in two claws, as in Synaptera
and winged insects. A pair of 1–jointed anal cerci homologous with those
of Thysanura and Orthoptera, into each of which opens a large abdominal
silk-gland. Abdominal segments with movable styles or “pseudopods”
(“Parapodia” of Latzel and of Schmidt), like those of Campodea and
Machilis, and situated on the base of the coxal joint in front of the
ventral sac. Within the body near the base of each abdominal style is an
eversible coxal sac or blood-gill (Fig. 15, cg). The single genital
opening is on the fourth trunk-segment in both sexes (Fig. 15, indicated
by the arrow). The malpighian tubes (ur. t) are two in number, opening
into the digestive canal at the anterior end of the hind intestine; they
extend in front to the third or second segment from the head. They are
broad and straight at their origin, becoming towards the end very
slender and convoluted._
_The three divisions of the digestive tract are as in insects, the
epithelium of the mid-gut being histologically as in Campodea and Japyx;
rectal glands are present. A pair of very large salivary glands are
situated in the first to the fourth trunk-segments, consisting of a
glandular portion with its duct, which unite into a common duct opening
on the under side of the head, probably in the labium._
_But a single pair of stigmata is present, and these are situated in the
front of the head, beneath the insertion of the antennæ and within the
stipes of the mandibles; the tracheæ are very fine, without spiral
threads (tænidia), and mostly contained within the head, two fine
branches extending on each side into the second trunk-segment._
_After birth the body increases in length by the addition of new
segments at the growing point._
In respect to the nervous system, there are no diagnostic characters;
there are, however, not as many as two pairs of ganglia to a segment.
The brain is well developed, sending a pair of slender nerves to the
small eyes. The ganglia of the segment bearing the first pair of legs is
fused with the subœsophageal ganglion. Grassi was unable to detect a
true sympathetic system, but he suspects the existence of a very small
frontal ganglion.
The slender dorsal vessel, provided with ostia and valvules, pulsates
along the entire length of the trunk; an aorta passes into the head.
The internal genital organs of both sexes are paired, and extend along
the greater part of the trunk; in either sex they may be compared to two
long, slender, straight cords extending from the fourth to the tenth
pair of legs. The two oviducts do not unite before reaching the sexual
opening (Fig. 15, _ovd_).
The male sexual organs are more complicated than the feminine. The
paired testicular tubes lie in trunk-segments 6 to 12, on each side, and
partly under the intestinal canal, communicating with each other by a
cross-anastomosis situated under the intestine, and which, like the
testes, is filled with sperm. Of the paired seminal ducts (vas deferens)
in trunk-segment 4, each unites again into a thick tube, sending a blind
tube forward into the third segment. Under the place of union of the two
vasa deferentia arise the paired ductus ejaculatorii, which open beneath
in the uterus masculinus. The anterior blind ends of the vasa deferentia
form a sort of small paired vesiculæ seminales in which a great quantity
of ripe sperm is stored. The uterus masculinus is in its structure
homologous with the evaginable penis of Pauropus, Polyxenus, and some
diplopods, and the sexual opening has without doubt become secondarily
unpaired. The sexual opening is rather long and is closed by two
longitudinal folds. “In several respects the male sexual organs of
Scolopendrella are like those of Pauropus; in the last-named form we
have indeed an unpaired testis, but also in Scolopendrella we see the
beginning of such a singleness; namely, the presence of an anastomosis
uniting the two tubes, their communication by means of a transverse
connecting canal and a glandular structure in the epithelium forming
them. The male sexual organs of Pauropus differ only through a still
greater complication.” (Schmidt.)
Scolopendrella in habits resembles chilopods, being found in company
with Geophilus burrowing deep in light sand under leaves, or living at
the surface of the ground under sticks or stones. It is very agile in
its movements, and is probably carnivorous. It was considered by Haase
to be eyeless, but the presence of two ocelli has been demonstrated both
by Grassi and by Schmidt. Whether the pigment and corneous facet are
present is not certain. The embryology is entirely unknown (although
Henshaw reports finding a hexapodous young one), and it need not be said
that a knowledge of it is a very great desideratum. It is most probable
that the young is hexapodous, since the first pair of limbs are
4–jointed, all the rest 5–jointed; while Newport, and also Ryder,
observed specimens with nine, ten, eleven, and twelve pairs, and
Wood-Mason confirms their observations, “which prove that a pair of legs
is added at each moult,” and he concludes that the addition of new
segments “therefore takes place in this animal by the intercalation of
two at each moult between the antepenultimate and penultimate sterna, as
in the Chilognatha, and as also in some of the Chilopoda.”
There is but one family, Scolopendrellidæ, and a single genus,
Scolopendrella, which seems to be, like other archaic types,
cosmopolitan in its distribution.
Our commonest species is _S. immaculata_ Newport, which occurs from
Massachusetts to Cordova, Mexico, and in Europe from England to the
Mediterranean and Russia; Mr. O. F. Cook tells me he has found a
species in Liberia, West Africa. The other species are _S.
notacantha_ Gervais, Europe and Eastern United States; _S. nivea_
Scopoli (_S. gratiæ_ Ryder), Europe and United States; _S. latipes_
Scudder, Massachusetts.
LITERATURE ON SCOLOPENDRELLA
=Newport, George.= Monograph of the class Myriopoda, order Chilopoda.
(Trans. Linn. Soc. xix, pp. 349–439, 1 Pl., 1845.)
=Menge, A.= Myriapoden der Umgegend von Danzig. (Neuste Schriften der
naturforsch. Gesell. Danzig. iv, 1851.)
=Ryder, John H.= Scolopendrella as the type of a new order of
articulates (Symphyla). (Amer. Nat., May, 1880, xiv, pp. 375, 376.)
—— The structure, affinities, and species of Scolopendrella. (Proc.
Acad. Nat. Soc. Phil., pp. 79–86, 1881, 2 Figs.)
=Packard, A. S.= Scolopendrella and its position in nature. (Amer.
Nat., 1881, pp. 698–704, Fig.)
=Muhr, Jos.= Die Mundtheile von Scolopendrella und Polyzonium. Prag,
1882, 1 Pl.
=Mason, J. Wood.= Morphological notes bearing on the origin of
insects. (Trans. Ent. Soc. London, 1879, pp. 145–167, Figs.)
—— Notes on the structure, post-embryonic development, and systematic
position of Scolopendrella. (Ann. and Mag. Nat. Hist., July, 1883,
pp. 53–63.)
=Latzel, Robert.= Die Myriapoden der osterreichisch-ungarischen
Monarchie, ii, Wien, 1884, pp. 1–39, Pls.
=Haase, Erich.= Die Abdominalanhänge der Insekten mit Berücksichtigung
der Myriapoden. (Morph. Jahrbuch, xv, pp. 331–435, 2 Pls., 1889.)
=Schmidt, Peter.= Beiträge zur Kenntnis der niederen Myriapoden.
(Zeits. f. wissen. Zool., lix, pp. 436–510, 2 Pls., 1895.)
INSECTA (HEXAPODA)
We are now prepared to discuss the fundamental or essential characters
of the insects, including the wingless subclass (Synaptera), and the
winged (Pterygota).
=Diagnostic characters of insects.=—_Body consisting of not more than
twenty-one segments, which are usually heteronomous or of unequal size
and shape, arranged in three usually well-defined regions_; i.e. _a
head, thorax, and hind-body or abdomen. Head small and flattened or
rounded, composed of not less than six segments, and bearing, besides
the eyes, at least four pairs of jointed appendages_; i.e. _one pair of
antennæ, and three pairs of masticatory appendages, the distal or molar
portion of which is primarily divided into three divisions, supported on
a stipes and cardo, and in certain orders modified into piercing or
sucking structures. The head is composed of an epicranium, bearing a
distinct clypeus and labrum, with the epipharynx. Mandibles 1–jointed,
without a palpus and very generally with no, or uncertain, traces of a
lacinia and a stipes. Two pairs of maxillæ; the first pair separate,
usually 3–lobed, comprising a lacinia, galea, and palpifer, with a
palpus which is never more than 6–jointed. The second pair united to
form the labium or under lip, composed of two laciniæ fused together; in
the generalized forms with a rudimentary galea; bearing a pair of palpi,
never more than 4–jointed; with paraglossæ sometimes present._
(_A third pair of mouth-appendages situated between the antennæ and
mandibles in the embryo of Anurida, and Apis, and adult Campodea._)
_The epipharynx forming the roof of the mouth, and bearing gustatory
organs. Hypopharynx usually well developed, lying on the under side of
the mouth, just above the labium, and receiving the end of the salivary
duct._
_Eyes of two kinds: a pair of compound, and from two to three simple
eyes (ocelli)._
_The thorax consisting of three segments, the two latter segments in the
winged orders highly differentiated into numerous tergal and lateral
pieces and a single sternum; in the Synaptera the segments are
undivided. (In the higher Hymenoptera the basal abdominal segment
coalesced with the thorax.) Three pairs of legs, each foot ending in a
pair of claws. Two pairs of wings (except in the Synaptera), a pair to
each of the two hinder thoracic segments; the wings occasionally reduced
or wanting in certain adaptive forms, which, however, had winged
ancestors._
_Abdomen consisting at the most of from ten to twelve segments. No
functional abdominal legs except in the Thysanura, and in the larvæ of
Lepidoptera. A pair of 1– or many-jointed cercopods on the tenth
segment; and in certain forms a pair of styles on the ninth segment. In
certain orders an ovipositor or sting formed of three pairs of styliform
processes; in Collembola a single pair of processes forming the elater._
_The genital openings opisthogoneate, usually single, but paired in
Thysanura (Lepisma), Dermaptera, and Plectoptera (Ephemeridæ)._
_The digestive canal in the winged orders is highly differentiated, the
fore-intestine being divided into an œsophagus and proventriculus, the
hind-intestine into an ileum, colon, and rectum, with rectal glands._
_The nervous system consists of a well-developed brain, in the more
specialized orders highly complicated; no more than thirteen pairs of
ganglia, which may be more or less fused in the more specialized orders.
Three frontal ganglia, and a well-developed, sympathetic system
present._
_Stigmata confined (except possibly in Sminthurus) to the thorax and
abdomen, not more than ten pairs in all, and usually but nine pairs.
Tracheal system as a rule highly differentiated; invariably with
tænidia._
_Dorsal vessel with ostia and valvules; no arteries except the cephalic
aorta; no veins. After birth there is in the more specialized pterygote
orders a reduction in the number of terminal segments of the abdomen._
_Development either direct (Synaptera), or with an incomplete (with
nymph and winged or imaginal stages), or complete metamorphosis; in the
latter case with a larval, pupal, and imago stage._
The insects may be divided into two sub-classes,—the _Synaptera_, and
the winged orders, _Pterygota_, of Gegenbaur (1877), since the
differences between the two groups appear on the whole to be of more
than ordinal rank.
1. EXTERNAL ANATOMY
_a._ The regions of the body
The insects differ from other arthropods in that the body is divided
into three distinct regions,—the head, thorax, and abdomen, the latter
regions in certain generalized forms not always very distinctly
differentiated. The body behind the head may also conveniently be called
the trunk, and the segments composing it the trunk-segments.
In insects the head is larger in proportion to the trunk than in other
classes, notably the Crustacea; the thorax is usually slightly or
somewhat larger than the head, while the hind-body or abdomen is much
the larger region, as it consists of ten to eleven, and perhaps in the
Dermaptera and Orthoptera twelve, segments, and contains the mid- and
hind-intestine, as well as the reproductive organs.
When we compare the body of an insect with that of a worm, in which the
rings are distinctly developed, we see that in insects ring distinctions
have given way to regional distinctions. The segments lose their
individuality. It is comparatively easy to trace the segments in the
hind-body of an insect, as in this region they are least modified; so
with the thorax; but in the head of the adult insect it is impossible to
discover the primitive segments, as they are fused together into a sort
of capsule, and have almost entirely lost their individuality.
In general it may be said that the head contains or bears the organs of
sense and of prehension and mastication of the food; the thorax the
organs of locomotion; and the abdomen those of reproduction.
When we compare the body of a wasp or bee with that of a worm, we see
that there is a decided transfer of parts headward; this process of
cephalization so marked in the Crustacea likewise obtains in insects.
Also the two hinder regions of the body are, in a much greater degree
than in worms, governed by the brain, the principal seat of the
intelligence, which, so to speak, dominates and unifies the functions of
the body, both digestive, locomotive, and reproductive, as also those of
the muscles moving the different segments and regions of the body. To a
large extent arthropodan morphology and class distinctions are based on
the regional arrangement of the somites themselves. Thus in the process
of grouping of the segments into the three regions, some increase in
size, while others undergo a greater or less degree of reduction; one
segment being developed at the expense of one or more adjoining ones.
This principle was first pointed out by Audouin, and is called Audouin’s
law. It is owing to the greater development of certain segments and the
reduction of others, both of the body-segments and of the segments of
the limbs, that we have the wonderful diversity of form in the species
and genera, and higher groups of insects, as well as those of other
arthropods.
_b._ The integument (exoskeleton)
The skin or integument of insects consists, primarily, as in worms and
all arthropods, of an epithelial layer of cells called the _hypodermis_.
This layer secretes the cuticle, which is of varying thickness and
flexibility, and is usually very dense, impermeable, and light, compared
with the crust of the Crustacea, where the cuticle becomes heavy and
solid by the deposition of the carbonate and phosphate of lime. This is
due to the presence of a substance called by Odier chitin.[9] The
cuticle is thin, delicate, and flexible between the joints; it is
likewise so in such diaphanous aquatic larvæ as that of Corethra, and in
the gills of aquatic insects, also in the walls of the tracheæ and of
the salivary ducts. The cuticle thus forms a more or less solid crust
which is broken into joints and pieces (sclerites), forming supports for
the attachments of the muscles and serving to protect the soft parts
within.
=Chitin.=—If we allow an insect to soak for a long time in acids, or
boil it in liquid potassa or caustic potash, the integument is not
affected. The muscles and the other soft parts are dissolved, leaving
the cuticle clear and transparent. This insolubility of the cuticle is
due to the presence of chitin, the insoluble residue left after such
treatment. It also resists boiling in acids, in any alkalies, alcohol or
ether. The chemical formula is C_{15}H_{26}N_{2}O_{10}.[10]
“Chitin forms less than one-half by weight of the integument, but it
is so coherent and uniformly distributed that when isolated by
chemical reagents, and even when cautiously calcined, it retains its
original organized form. The color which it frequently exhibits is
not due to any essential ingredient; it may be diminished or even
destroyed by various bleaching processes.” (Miall and Denny.)
“The chemical stability of chitin is so remarkable that we might
expect it to accumulate like the inorganic constituents of animal
skeletons, and form permanent deposits. Schlossberger (Ann. d. chem.
u. pharm., bd. 98) has, however, shown that it changes slowly under
the action of water. Chitin kept for a year under water partially
dissolved, turned into a slimy mass, and gave off a peculiar smell.
This looks as if it were liable to putrefaction. The minute
proportion of nitrogen in its composition may explain the complete
disappearance of chitin in nature.” (Miall and Denny, The Cockroach,
p. 29.)
Chitin, or a substance closely similar to it, occurs in worms and in
their tubes, especially in the pharyngeal teeth of annelids and in
their setæ. The shell of Lingula and the pen of cuttle-fish contain
true chitin (Krukenberg). The integument of Limulus, of trilobites,
and of Arachnida, as well as Myriopoda, appears to consist of
chitin.[11]
The chitin is rapidly deposited at the end of embryonic life, also
during the larval and pupal stages. As is well known, insects after
moulting are white, but in a few hours turn dark, and those which live
in total darkness are white, showing that light has a direct effect in
causing the dark color of the integument.
Moseley analyzed one pound weight of Blatta, and found plenty of iron
with a remarkable quantity of manganese.
Schneider regarded chitin as a hardening of the protoplasm rather than a
secretion, and the cuticle is looked upon as an exudation. It is
structureless, not consisting of cells, and consists of fine irregular
laminæ. “A cross-section of the chitinous layer or ‘cuticle’ examined
with a high power shows extremely close and fine lines perpendicular to
the laminæ.” In the cockroach the free surface of the cuticle is divided
into polygonal, raised spaces or areas which correspond each to a
chitinous cell of the hypodermis. (Miall and Denny.)
Numerous pore-canals pass through the cuticle of all the external parts
of the body. The larger canals nearly always form the way for the
passage of secretions from dermal cells, or connect with the cavities of
hairs or setæ; when very fine and not connected with hairs or scales,
they are either empty or filled with air, and may possibly serve for
respiration.
Vosseler distinguishes in the cuticle two layers of different physical
and chemical characters. Besides the external chitinous layer there is
an inner layer which entirely agrees with cellulose. (Zool.
Centralblatt, ii, 1895, p. 117.)
The reparative nature of chitin is seen in the fact that Verhoeff finds
that a wound on an adult Carabus, and presumably on other insects, is
speedily closed, not merely by a clot of blood, but by a new growth of
chitin.
_c._ Mechanical origin and structure of the segments (somites,
arthromeres, metameres, zonites)
The segments are merely thickenings of the skin connected by folds or
duplications of the integument, and not actually separate or individual
rings or segments. This is shown by longitudinal (sagittal) sections
through the body, and also by soaking or boiling the entire insect in
caustic potash, when it is seen that the integument is continuous and
not actually subdivided into separate somites or arthromeres, since they
are seen to be connected by a thin intersegmental membrane (Fig. 16).
But this segmentation or metamerism of the integument is, however, the
external indication of the segmentation of the arthropodan body most
probably inherited from the worms, being a disposition of the soft parts
which is characteristic of the vermian type. This segmentation of the
integument is correlated with the serial repetition of the ganglia of
the nervous system, of the ostia of the dorsal vessel, the primitive
disposition of the segmental and reproductive organs, of the soft,
muscular dissepiments which correspond to the suture between the
segments, and with the metameric arrangement of the muscles controlling
the movements of the segments on each other, and which internal
segmentation or metamerism is indicated very early in embryonic life by
the mesoblastic somites.
[Illustration:
FIG. 16.—Diagram of the anterior part of an insect, showing the
membranous intersegmental folds, _g_.—After Graber.
]
In the unjointed worms, as Graber states, the body forms a single but
flexible lever. In the earthworm the muscular tube or body-wall is
enclosed by a stiffer cuticle, divided into segments; hence the worm can
move in all required directions, but only by sections, as seen in Fig.
16, which represents the thickened integument divided into segments, and
folded inward between each segment, this thin portion of the skin being
the intersegmental fold. Each segment corresponds to a special zone of
the subdivided muscular tube (_m_), the fascia extending longitudinally.
The figure shows the mode of attachment of the fascia of the muscle-tube
to the segment. The anterior edge is inserted on the stiff, unyielding,
inner surface of each segment: the hinder edge of the muscle is attached
to the thin, flexible, intersegmental fold, which thus acts as a tendon
on which the muscle can exert its force. (Graber.)
[Illustration:
FIG. 17.—Diagram of the integument and arrangement of the segmental
muscles: _A_, relaxed; _m_, muscle; _g_, membranous articulation;
_r_, chitinous ring. _B_, the same contracted on both sides. _C_, on
one side.—After Graber.
]
“Fig. 17 makes this still clearer. The muscles (_m_) extend between two
segments immediately succeeding each other. Supposing the anterior one
(_A_) to be stationary, what do we then see when the muscle contracts?
Does it also become shorter? The intersegmental fold is drawn forwards,
and hence the entire hinder segment moves forward and is shoved into the
front one, and so on with the others, as at _B_. Afterwards, if the
strain of the muscle is relieved by the diminishing action of the
tensely stretched, intersegmental membrane, it again returns to a state
of rest.” (Graber.)
[Illustration:
FIG. 18.—Diagrams to demonstrate the mechanism of the motion of the
segmented body in the Arthropoda: One larger segment (_cf_) and 4
smaller. The exoskeleton is indicated by black lines, the
interarticular membranes by dotted lines. The hinges between
consecutive segments are marked _at_, tergal (dorsal) skeleton; _s_,
sternal (ventral) skeleton; _d_, dorsal longitudinal muscles =
extensors (and flexors in an upward direction); _v_, ventral
longitudinal muscles = flexors. In _B_, the row of segments is
stretched; in _A_, by the contraction of the muscles (_d_) bent
upward; in _C_, downward; _tg_, tergal; _sg_, sternal interarticular
membranes.—After Lang.
]
While we look upon the dermal tube of worms as a single but flexible
lever, the body of the arthropods, as Graber states, is a linear system
of stiff levers. We have here a series of stiff, solid rings, or hooks,
united by the intersegmental membrane into a whole. When the muscles,
extending from one ring to the next behind contract, and so on through
the entire series, the rings approximate each other.
The ectoskeletal segments bend to one side by the contraction of the
muscles on one side, the point of the outer segmental fold opposite the
fixed point becoming converted into the turning-point (_C_).
The usual result of the arrangement of the locomotive system is the
simple curving of the body (_C_), and then the alternate bending of the
body to right and left, which produces the serpentine movements
characteristic of the earthworms, the centipede, and many insect larvæ.
The most striking example of the wonderful variety of movements which
can be made by an insect are those of the Syrphus larva. When feeding
amid a herd of aphides, it is seen to now raise the front part of the
body erect and stiff, then to bend it down, or rapidly turn it to either
side, or move it in a complete circle. (Graber, pp. 23–26.)
The arrangement and mode of working of the muscles, says Lang, is
illustrated by Fig. 18, which shows us five segments, one larger (_ct_)
and four smaller, in vertical projection. The thicker portion of the
integument is marked by strong outlines, the delicate and flexible
interarticular membranes (_tg_, _sg_) in dotted lines. The hinges
between two consecutive segments are marked _a_. A dorsal muscle (_d_)
is attached to the larger segment (_ct_), and runs through the smaller
segments, being inserted in the dorsal portion of the crust (_t_) of
each by means of a bundle of fibres. A ventral muscle (_v_) does the
same on the sternal side (_s_).
“The skeletal segments,” adds Lang, “may be compared to a double-armed
lever, whose fulcrum lies in the hinges. If the dorsal muscle contracts,
it draws the dorsal arm of the lever (the tergal portion of the
skeleton) in the direction of the pull towards the larger segments; the
tergal interarticular membranes become folded, the ventral stretched,
and the four segments bend upward (Fig. 18, _A_). If the ventral muscle
contracts, while at the same time the dorsal slackens, the row of
segments will be bent downwards (Fig. 18, _C_).”
L. B. Sharp suggests, that in the Crustacea the rings formed by “the
regularity and stress of muscular action” would be hardened by the
deposition of lime at the most prominent portion, _i.e._ between what we
have called the intersegmental folds. (American Naturalist, 1893, p.
89.) Cope also states that “with the beginning of induration of the
integument, segmentation would immediately appear, for the movements of
the body and limbs would interrupt the deposit at such points as would
experience the greatest flexure. The muscular system would initiate the
process, since flexure depends on its contractions, and its presence in
animals prior to the induration of the integuments in the order of
phylogeny, furnishes the conditions required.” (The Primary Factors of
Organic Evolution, p. 268, 1895.)
It is apparent that the jointed or metameric structure of the bodies of
insects and other arthropods is an inheritance from the segmented worms.
In the worms the body is a continuous dermo-muscular tube, while in
arthropods this tube is divided into regions, and the cuticle is thicker
and more resistant. To go back to the incipient stages in the process of
segmentation of the body, we conceive that the worms probably arose from
a creeping gastrula-like form, the gastræa. The act of creeping
gradually induced an elongated shape of the body. The movement of such
an organism in a forward direction would gradually evolve a fore and
aft, dorsal and ventral, and bilateral symmetry. As soon as this was
attained, as the effect of creeping over rough irregular surfaces there
would result mechanical lateral strains intermittently acting during the
serpentine movements of the worm. The integument would, we can readily
suppose, tend to bend or yield, or become permanently wrinkled, at more
or less regular intervals. The arrangement of the muscles would
gradually conform to this habit of creeping, and finally the nervous
system and other organs more directly connected with the creeping
movements of the organism would tend to be correlated in their
arrangement with that of the segments. In this way the homonomous
segments of the annelid body probably became developed, and their
relations and shapes were eventually fixed by inheritance. After this
stage was reached, and limbs began to appear, the segments would tend to
become heteronomous, and to be grouped into regions.
[Illustration:
FIG. 19.—_Dujardinia rotifera_, with jointed tentacles and caudal
appendages.—With some changes, after Quatrefages.
]
The origin of the joints or segments in the limbs of arthropods was
probably due to the mechanical strains to which what were at first soft
fleshy outgrowths along the sides of the body became subjected. Indeed,
certain annelid worms of the family Syllidæ have segmented tentacles and
parapodia, as in Dujardinia (Fig. 19). We do not know enough about the
habits of these worms to understand how this metamerism may have arisen,
but it is possibly due to the act of pushing or repeated efforts to
support the body while creeping over the bottom among broken shells,
over coarse gravel, or among seaweeds.
It is obvious, however, that the jointed structure of the limbs of
arthropods, if we are to attempt any explanation at all of the origin of
such structure, was primarily due mainly to lateral strains and impacts
resulting from the primitive endeavors of the ancestral arthropods to
raise and to support the body while thus raised, and then to push or
drag it forward by means of the soft, partially jointed, lateral limbs
which were armed with bristles, hooks, or finally claws.
On the other hand, by adaptation, or as the result of parasitism and
consequent lack of active motion, the original number of segments may by
disuse be diminished. Thus in adult wasps and bees, the last three or
four abdominal segments may be nearly lost, though the larval number is
ten. During metamorphosis the body is made over, and the number, shape,
and structure of the segments greatly modified. In the female of the
Stylopidæ the thorax loses all traces of segments, and is fused with the
head, and the abdominal segments are faintly marked, losing their
chitin.
While the maxillæ have several joints, the mandibles are 1–jointed, but
there are traces of two joints in Campodea, certain beetles, etc. In the
antenna there is a great elasticity in respect to the number of joints,
which vary from one or two to a hundred or more. It is likewise so in
the thoracic legs, where the number of tarsal joints varies from one to
five; also in the cercopoda, the number of joints varying from one or
two to twelve or more.
_d._ Mechanical origin of the limbs and of their jointed structure
We have already hinted at the mode of origin of the limbs of arthropods.
Like the body or trunk, the limbs are chitinous dermo-muscular tubes,
with a dense solid cuticle, and internal muscles, and were it not for
their division at more or less regular intervals into segments, forming
distinct sets of levers, set up by the strains in these tubular
supports, there would be no power of varied motion.
Even certain worms, as already stated, have their tentacles and
parapodia, or certain appendages of their parapodia, more or less
jointed, but there are no indications of claws or of any other hard
chitinous armature at the extremity, and the skin is thin and soft.
In the most simple though not the most primitive arthropods, such as the
Tardigrades, whose body is not segmented, there are four pairs of short
unjointed legs, ending each in two claws, which have probably arisen in
response to the stimulus of pushing or dragging efforts.
The legs of Peripatus are unjointed, and have a thin cuticle, but end in
a pair of claws, which have evidently arisen as a supporting armature,
the result of the act of moving or pulling the body over the uneven
surface of the ground.
[Illustration:
FIG. 20.—A prothoracic leg of Chironomus larva; and pupa.
]
[Illustration:
FIG. 21.—_A_, larva of _Ephydra californica_: _a_, _b_, _c_, pupa.
]
There is good reason to suppose that such limbs arose from dynamical
causes, similar to those exciting the formation of secondary adaptations
such as are to be seen in the prop or supporting legs of certain
dipterous larvæ, as the single pair of Chironomus (Fig. 20) and
Simulium, or the series of unjointed soft tubercles of Ephydra (Fig.
21), etc., which are armed with hooks and claws, and are thus adapted
for dragging the insect through or over vegetation or along the ground.
Now by frequent continuous use of such unjointed structures, the cuticle
would tend to become hard, owing to the deposit of a greater amount of
chitin between the folds of the skin, until finally the body being
elongated and homonomously segmented, the movements of walking or
running would be regular and even, and we would have homonomously
jointed legs like those of the trilobites, or of the most generalized
Crustacea and of Myriopoda.
In the most primitive arthropods,—and such we take it were on the whole
the trilobites, rather than the Crustacea,—the limbs were of nearly the
same shape, being long and slender and evenly jointed from and including
the antennæ, to the last pair of limbs of the abdominal region. In these
forms there appear to be, so far as we now know, no differentiation into
mandibles, maxillæ, maxillipedes, and thoracic legs, or into gonopoda.
The same lack of diversity of structure and function of the
head-appendages has survived, with little change, in Limulus. In the
trilobites (Fig. 1) none of the limbs have yet been found to end in
claws or forceps; being in this respect nearly as primitive as in the
worms. Secondary adaptations have arisen in Limulus, the cephalic
appendages being forcipated, adapted as supports to the body and for
pushing it onward through the sand or mud, while the abdominal legs are
broad and flat, adapted for swimming and bearing the broad gill-leaves.
It is thus quite evident that we have three stages in the evolution of
the arthropodan limb; _i.e._ 1, the syllid stage, of simple, jointed,
soft, yielding appendages not used as true supports (Fig. 19); 2, the
trilobite stage, where they are more solid, evenly jointed, but not
ending in claws; and by their comparatively great numbers (as in the
trilobite, Triarthrus) fully supporting the body on the bottom of the
sea. In Limulus they are much fewer in number, thicker, and acting as
firm supports, the cephalic limbs of use in creeping, and ending in
solid claws. 3, The third stage is the long slender swimming
head-appendages of the nauplius stage of Crustacea.
As regards the evolution of limbs of terrestrial arthropods, we have the
following stages: 1, the soft unjointed limbs of Tardigrades, ending in
two claws, and those of Peripatus, and the pseudo- or prop-legs of
certain dipterous larvæ; 2, finally the evolution of the long, solid,
jointed limbs of Pauropus and other primitive myriopods, the legs
forming solid, firm supports elevating the body, and enabling the insect
to drag itself over the ground or to walk or run. When the body is
elongated and many-segmented, the legs are necessarily numerous; but
when it is short, the legs become few in number, _i.e._ six, in the
hexapodous young of myriopods and in insects, or eight in Arachnida.
Whenever the legs are used for walking, _i.e._ to raise and support the
body, they end in a solid point or in a pair of forceps or claws. On the
other hand, as in phyllopods, where the legs are used mainly for
swimming, they are unarmed and are soft and membranous, or, as in the
limbs of the nauplius or zoëa stage of crustaceans, end in a simple soft
point, which often bears tactile setæ.
The tarsal joints are more numerous in order to give greater flexibility
to the limb in seizing and grasping objects, both to drag the body
forwards and to support it.
Unlike those of the Crustacea, the limbs of insects are not primitively
biramose, but single, the three-lobed first maxillæ, and secondarily
bilobed second maxillæ being the result of adaptation. Embryology on the
whole proves the truth of this assumption; the maxillæ of both pairs are
at first single buds, afterwards becoming lobed. All the appendages of
the body, including the ovipositor or sting, are modified limbs, as
shown by their embryological development.
It is noticeable that in the crab, where the body is raised by the limbs
above the bottom, it is much shorter and more cephalized than in the
shrimps. Also in the simply walking and running spiders, the hind-body
is shorter than in scorpions, while in the running and flying insects,
such as the Cicindelidæ, and in the swiftly flying flies and bees, there
is a tendency to a shortening of the body, especially of the abdomen.
The long body of the dragon-fly is an impediment to flight, but
compensated for by the action of the large wings.
The arthropodan limb is a compound leverage system. It is, says Graber,
a lateral outgrowth of the trunk, which repeats in miniature that of the
main trunk, its single series of joints or segments forming a jointed
dermo-muscular tube. Yet the lateral appendages of an insect differ from
the main trunk in two ways: (1) they taper to the end which bears the
two claws, and (2) their segments are in the living animal arranged not
in a straight line, but at different angles to each other. The basal
joint turning on the trunk acts as the first of a whole series of
levers. The second joint, however, is connected with the musculature of
the first or basal joint, and thus each succeeding joint is moved on the
one preceding. Each lever, from the first to the last, is both an active
and a passive instrument. (Graber.)
While, however, as Graber states, the limbs possess their own sets of
muscles and can move by the turning of the basal joint, the labor is
very much facilitated, as is readily seen, by the trunk, though the
latter has to a great extent delegated its locomotive function to the
appendages, which again divide its labor among the separate joints.
Graber then calls attention to the analogy of the mechanics of
locomotion of insects to those of vertebrates. An insect’s and a
vertebrate’s legs are constructed on the same general mechanical
principles, the limbs of each forming a series of levers.
[Illustration:
FIG. 22.—Diagram of the knee-joint of a vertebrate (_A_) and an
insect’s limb (_B_): _a_, upper; _b_, lower, shank, united at _A_ by
a capsular joint, at _B_ by a folding joint; _d_, extensor or
lifting muscle; _d_^1, flexor or lowering muscle of the lower joint.
The dotted line indicates in _A_ the contour of the leg.—After
Graber.
]
Fig. 22, _A_, represents diagrammatically the knee joint of a
vertebrate, and _B_ that of an insect; _a_, the femur or thigh, and _b_,
the tibia or shank. In the vertebrate the internally situated bones are
brought into close union and bend by means of a hinge-joint; so also in
the chitinous-skinned insect.
The stiff dermal tube of the insect acts as a lever by means of the thin
intersegmental membrane (_c_) pushed in or telescoped in to the thigh
joint, a special joint-capsule being superfluous. The muscles are in
general the same in both types; they form a circle. In both the shank is
extended by the contraction of the upper muscles (_d_) and is bent by
the contraction of the lower (_d^1_). The intersegmental membrane of the
insect’s limb is in a degree a two-armed lever, whose pivot (_f_) lies
in the middle. The internal invagination of the intersegmental fold
(_B_, _g-h_) affords the necessary support to the muscles acting like
the tendon in the vertebrate. (Graber.)
[Illustration:
FIG. 23.—Primitive band or germ of a Sphinx moth, with the segments
indicated, and their rudimentary appendages: _c_, upper lip; _at_,
antennæ; _md_, mandibles; _mx_, _mx′_, first and second maxillæ;
_l_, _l′_, _l″_, legs; _al_, abdominal legs.—After Kowalevsky.
]
Graber also calls attention to the fact that this insect limb differs in
one important respect from that of land vertebrates. The leverage system
in the last is divided at the end into five parallel divisions or
digits. In arthropods, on the contrary, all the joints succeed one
another in a linear series.
In insects, as well as in other arthropods, modifications of the limbs
usually take the form of a simple reduction in the number of segments.
Thus while the normal number of tarsal joints is five, we have trimerous
and dimerous Coleoptera, and in certain Scarabæidæ the anterior tarsi
are lost.
Savigny was the first, in 1816, in his great work, “Théorie des organes
de la bouche des Crustacés et des Insectes,” to demonstrate that not
only were the buccal appendages of biting insects homologous with those
of bugs, moths, flies, etc., but that they were homologous with the
thoracic legs, and that thus a unity of structure prevails throughout
the appendages of the body of all arthropods. Oken also observed that
“the maxillæ are only repeated feet.”
What was modestly put forth as a theory by the French morphologist has
been abundantly proved by the embryology of insects of different orders
to be a fact. As shown in Fig. 23 the antennæ and buccal appendages
arise as paired tubercles exactly as the thoracic legs. The abdominal
region also bears similar embryonic or temporary limbs, all of which in
those insects without an ovipositor disappear, except the cercopoda,
after birth.
LITERATURE ON THE EXTERNAL ANATOMY
General
=Swammerdam, Johann.= Biblia naturæ. (In Dutch, German, and English.)
1737–1738, fol., London, 1758, Pls.
=Réaumur, Réné Antoine Ferchault, de.= Mémoires pour servir à
l’histoire des insectes. i-iv, 4º, Paris, 1734–1742.
=Lyonet, Pieter.= Traité anatomique de la chenille, qui ronge le bois
de saule, etc. 4º, pp. xxii, 616. À la Haye, 1732. Tab. 18.
—— Recherches sur l’anatomie et les metamorphoses de differentes
espèces d’insectes. Ouvrage posthume, publié par M. W. de Haan. pp.
580, tab. 54, 1832.
=Latreille, Pierre André.= Des rapports généraux de l’organization
extérieure des animaux invertébres articulés, et comparaison des
Annelides avec les Myriapodes. (Mémoires du Mus. d’Hist. Nat., 1820,
vi, pp. 116–144.)
—— De quelques appendices particuliers du thorax de divers insectes.
(Mémoires du Mus. d’Hist. Nat., 1821, vii, pp. 1–21).
—— Observations nouvelles sur l’organization extérieure et générale
des animaux articulés et à pieds articulés, et application de ces
connoissances à la nomenclature des principales parties des mêmes
animaux. (Mèmoires du Mus. d’Hist. Nat., viii, 1822, pp. 169–202.)
=Cuvier, George Leopold Christian Dagobert.= Rapport sur les
recherches anatomiques sur le thorax des animaux articulés et celui
des insectes en particulier par M. V. Audouin. 4º, pp. 15, tab. 1,
Paris, 1823.
=Audouin, Jean Victor.= Recherches anatomiques sur le thorax des
animaux articulés et celui des insectes hexapodes en particulier.
(Annales des Sciences naturelles, i, pp. 97–135, 416–432, 1824.)
=Kirby, William, and William Spence.= Introduction to entomology.
i-iv, 1816–1828, London.
=Straus-Durckheim, Hercule.= Considérations générales sur l’anatomie
comparée des animaux articulés. Paris, 1828, atlas of 19 plates.
=MacLeay, William Sharp.= Explanation of the comparative anatomy of
the thorax in winged insects, with a review of the present state of
the nomenclature of its parts. (Zoöl. Journal, v, pp. 145–179, 1830,
2 Pls.)
=Burmeister, Hermann.= A manual of entomology. Trans. by W. E.
Shuckard, 8º, pp. 654, London, 1836, 32 Pl.
=Westwood, John Obadiah.= An introduction to the modern classification
of insects, i, ii, 8º, pp. 462, 587, 158, 1 Pl. and 133 blocks of
figs., 1839–1840.
=Newport, George.= Art. Insecta in Todd’s Cyclopædia of Anatomy and
Phys. ii, pp. 853–994, 1839, Figs. 329–439.
=Erichson, Wilhelm Ferdinand.= Entomographien. Berlin, 1840.
=Brullé, Auguste.= Recherches sur les transformations des appendices
dans les articulés. (Annales des Sciences nat. Sér. 3, ii, pp.
271–374, tab. 1, 1844.)
=Winslow, A. P.: son.= Om byggnaden af thorax hos Insekterna.
Helsingborg, 1862, 1 Pl, pp. 24.
=Packard, Alpheus Spring.= Guide to the study of insects. 1869.
—— Systematic position of the Orthoptera in relation to other insects.
(Third report U. S. Ent. Commission, pp. 286–345, 1883, Pls.
xxiii-lxi.)
=Graber, Vitus.= Die Insekten. 12º, pp. 403, 603, München, 1877, many
Figs.
=Huxley, Thomas Henry.= A manual of the anatomy of invertebrated
animals. 12º, pp. 397–451, Figs., London, 1877.
=Hammond, Arthur.= Thorax of the blow-fly. (Journ. Linn. Soc., London,
xv. Zoöl., 1880, pp. 31.)
=Brauer, Friedrich.= Ueber das Segment médiaire Latreille’s. (Sitzb.
d. k. Akad. d. Wissensch. Wien, 1882, pp. 218–241, 3 tab.)
—— Systematisch-zoologische Studien. (Ibid., 1885, pp. 237–413.)
=Gosch, C. C. A.= On Latreille’s theory of “Le Segment médiaire.”
(Nat. Tidsskrift (3), xiii, pp. 475–531, 1883.)
=Miall, L. C., and Denny, Alfred.= The structure and life-history of
the cockroach (_Periplaneta orientalis_). An introduction to the
study of insects. 8º, pp. 224, London, 1886.
=Cheshire, Frank R.= Bees and bee-keeping. i, Scientific, London,
1886, Pls. and Figs.
=Lang, Arnold.= Text-book of comparative anatomy. i, pp. 426–508,
1891, many Figs.
=Kolbe, H. J.= Einführung in die Kenntniss der Insekten. 8º, pp. 709,
324 figs., Berlin, 1893.
=Sharp, David.= The Cambridge natural history. Insecta, i, 8º, pp.
83–584, 1895, Figs. 47–371.
Also the works of Bos, Chabrier, Cholodkowsky, Comstock, Dewitz, Eaton,
Erichson, Gerstaecker, Girard, Grassi, Hagen, Haase, Kellogg, Knoch,
Lacordaire, Latreille, Leuckart, Lendenfeld, Lowne, Lubbock, Mayer,
Meinert, F. Müller, Osten-Sacken, Pagenstecher, Reinhard, Schaum,
Schiödte, Scudder, J. B. Smith, Spinola, Stein, Weismann, Wood-Mason.
THE HEAD AND ITS APPENDAGES
_a._ The head
[Illustration:
FIG. 24.—Presumed larva of Nemoptera (_Necrophilus arenarius_),
Pyramids of Egypt.—After Roux, from Sharp.
]
While the head is originally composed of probably not less than six
segments, these are in the adult insect fused together into a capsule or
hard chitinous box, the _epicranium_, with no distinct traces of the
primitive segments. The head contains the brain and accessory ganglia,
the mouth or buccal cavity, also the air-sacs in many winged forms, and
gives support to the external organs of sense, the antennæ, and to the
buccal appendages, the larger part of the interior being filled with the
muscles moving these structures. The solid walls of the head serve as a
lever or support for the attachment of these muscles, especially those
of the mandibles. Thus there is a correlation between the large size of
the mandibles of the soldier white ants and ants, the head being
correspondingly large to accommodate the great mandibular muscles. The
other extreme is seen in the larva of Necrophilus (Fig. 24), with its
long slender neck and diminutive head.
=The clypeus.=—This is that part of the head situated in front of the
epicranium, and anterior to the eyes, forming the roof of the posterior
part of the mouth, and is, as embryology shows, probably a tergal
sclerite. It varies greatly in shape and size in the different orders of
insects. It is often divided into two parts, the _clypeus posterior_ and
_clypeus anterior_, or which may be designated as the _post-_ and
_ante-clypeus_ (Figs. 29, _B_).
=The labrum.=—The “upper lip” or labrum is an unpaired flap-like piece
hinged to the front edge of the clypeus, and may be seen to move up and
down when the insect moves its mandibles. It forms the roof of the
anterior part of the mouth (Figs. 69, 74), and its inner side is lined
with a soft membrane, usually provided with hairs and sense-papillæ or
cups, forming the epipharynx.
The labrum is more or less deeply bilobed, especially in caterpillars
and in adult Staphylinidæ, and has been thought by some writers
(Kowalevsky, Carrière, and also Chatin) to represent a pair of
appendages, but Heymons (1895) refutes this view, stating as his reason
that the labrum arises between the two halves of the nervous system
(protocerebrum), while all the true appendages arise on each side of the
nervous system. (See also Fig. 34.)
[Illustration:
FIG. 25.—Front view of the head of _C. spretus_: _E_, epicranium; _C_,
clypeus; _L_, labrum; _O_, _o_, ocelli; _e_, eye; _a_, antenna;
_md_, mandible; _mx_, portion of maxilla uncovered by the labrum;
_p_, maxillary palpus; _p′_, labial palpus.
]
In the fleas (Siphonaptera) both the clypeus and labrum are wanting.
While it apparently forms an anterior specialized portion of the
procephalic lobes, Viallanes regarded it as belonging to the third, or
his tritocerebral, segment, since the labral nerves arise from the
tritocerebral ganglia. But since in all the early as well as late stages
of embryonic life it appears to be situated in front of the mouth, it
would seem to belong to the first segment.
In the embryo of Blatta it first appears as a thick crescentic fold
being slightly divided anterior to the mouth, and in Doryphora it
appears as a heart-shaped or deeply bilobed prominence situated in front
of the mouth (Wheeler).
=The epipharynx and labrum-epipharynx.=—The epipharynx is the under
surface or pharyngeal lining of the clypeus and labrum, forming the
membranous roof of the mouth. As it contains the organs of taste and has
been generally overlooked by entomologists, we may dwell at some length
on its structure in different orders.
Réaumur was, so far as we have been able to ascertain, the first author
to describe and figure the epipharynx, which he observed in the honey
bee and bumble bee, and called _la langue_, remarking that it closes the
opening into the œsophagus, and that it is applied against the palate.
According to Kirby and Spence, De Geer described the epipharynx of the
wasp; and Latreille referred to it, calling it the _sous labre_.
The name _epipharynx_ was bestowed upon this organ by Savigny, who thus
speaks of that of the bees: “Ce pharynx est, à la vérité, non seulement
caché par la lèvre supérieure, mais encore exactement recouvert par un
organe particulier que Réaumur a déjà décrit. C’est une sorte
d’appendice membraneux qui est reçu entre les deux branches des
mâchoires. Cette partie ayant pour base le bord supérieur du pharynx,
peut prendre le nom d’_épipharynx_ ou d’_épiglosse_.”
He also describes that of Diptera. What Walter has lately proved to be
the epipharynx of Lepidoptera was regarded by Savigny and all subsequent
writers as the labrum.
The latest account of the function of this organ is that by Cheshire,
who states that the tube made by the maxillæ and labial palpi cannot act
as a suction pipe, because it is open above. “This opening is closed by
the front extension of the epipharynx, which closes down to the maxillæ,
fitting exactly into the space they leave uncovered, and thus the tube
is completed from their termination to the œsophagus.”
[Illustration:
FIG. 26.—Epipharynx of _Phaneroptera angustifolia_: _cl_, clypeus;
_lbr. e_, labrum-epipharynx; _t_ _c_, taste cups, both on the
clypeal and on the labral regions.
]
[Illustration:
FIG. 27.—Epipharynx of _Hadenœcus subterraneus_, cave cricket.
]
It is singular that this organ is not mentioned in Burmeister’s Manual
of entomology, in Lacordaire’s Introduction à l’entomologie, or by
Newport in his admirable article _Insecta_ in Todd’s Cyclopedia of
anatomy. Neither has Straus-Durckheim referred to or figured it in his
great work on the anatomy of _Melolontha vulgaris_.
In their excellent work on the cockroach, Miall and Denny state that
“The epipharynx, which is a prominent part in Coleoptera and Diptera, is
not recognizable in Orthoptera” (p. 45). We have, however, found it to
be always present in this order (Figs. 26, 27).
We are not aware that any modern writers have described or referred to
the epipharynx of the mandibulate orders of insects. Although Dr. G.
Joseph speaks of finding taste-organs on the palate of almost every
order of insects, especially plant-feeding forms, we are unable to find
any specific references, his detailed observations being apparently
unpublished.
The epipharynx is so intimately associated with the elongated labium of
certain Diptera, that, with Dr. Dimmock, we may refer to the double
organ as the labrum-epipharynx; and where, as in the lepidopterous
_Micropteryx semipurpurella_, described and figured by Walter, and the
Panorpidæ (Panorpa and Boreus), the labrum seems pieced out with a thin,
pale membranous fold which appears to be an extension of the epipharynx,
building up the dorsal end of the labrum, this term is a convenient one
to use.
In the lower orders of truly mandibulate insects, from the Thysanura to
the Coleoptera, excluding those which suck in liquid food, such as the
Diptera, Lepidoptera, and Hymenoptera, and the Mecoptera (Panorpidæ)
with their elongated head and feeble, small mandibles, the epipharynx
forms a simple membranous palatal lining of the clypeus and labrum. In
such insects there is no soft projecting or pendant portion, fitted to
close the throat or to complete a partially tubular arrangement of the
first and second maxillæ.
In all the mandibulate insects, then, the epipharynx forms simply the
under surface or pharyngeal lining of the clypeus and labrum, the
surface being uniformly moderately convex, and corresponding in extent
to that of the clypeus and labrum, posteriorly merging into the palatal
wall of the pharynx; the armature of peculiar gathering-hairs sometimes
spreading over its base, being continuous with those lining the mouth
and beginning of the œsophagus. The suture separating the labrum from
the clypeus does not involve the epipharynx, though since certain
gustatory fields lie under the front edge of the clypeus, as well as
labrum, one may in describing them refer to certain fields or groups of
cups or pits as occupying a labral or clypeal region or position.
The lack of traces of a suture in the epipharynx corresponding to the
labral suture above, suggests that the labrum does not represent a pair
of coalesced appendages, and that it, with the clypeus, simply forms the
solid cuticular roof of the mouth.
The only soft structures seen between the epipharynx and labrum, besides
the nerves of special sense, are the elevator muscles of the labrum, and
two tracheæ, one on each side.
The structure and armature of the epipharyngeal surface even besides the
taste-pits, taste-cups and rods, is very varied, the setæ assuming very
different shapes. There seem to be two primary forms of setæ, (1) the
normal forms which arise from a definite cell; and (2) soft, flattened,
often hooked hairs which are cylindrical towards the end, but arise from
a broad triangular base, without any cell-wall. These are like the
“gathering hairs” of Cheshire, situated on the bees’ and wasps’ tongue;
they also line the walls of the pharynx and extend toward the œsophagus.
They are also similar to the “hooked hairs” of Will. The first kind, or
normal setæ, are either simply defensive, often guarding the sense-cups
or sensory fields on which the sense-cups are situated, or they have a
nerve extending to them and are simply tactile in function.
The surface of the epipharynx, then, appears to be highly sensitive, and
to afford the principal seat of the gustatory organs, which are
described under the head of organs of taste.
LITERATURE ON THE EPIPHARYNX
=Réaumur.= Mémoires pour servir à l’histoire des insectes, v, 1740, p.
318, Pl. 28, Figs. 4, 7, 8, 9, 10, 11 l.
=Kirby and Spence.= Intr. to entomology, iii, 1828, p. 457.
=De Geer.= ii, 1778; v, 26, Fig. 11, M.
=Kirby and Spence.= Pl. xii, Fig. 2 K.
=Latreille.= Organisation extérieure des insectes, p. 184. (Quoted
from Kirby and Spence.)
=Savigny.= Mémoires sur les animaux sans vertèbres. Partie I^{re},
1816, p. 12.
=Walter, Alfred.= Beiträge zur Morphologie der Schmetterlinge. Erster
Theil. Zur Morphologie des Schmetterlingsmundtheile. (Jena. Zeits.,
xviii, 1885, p. 752.)
=Cheshire, F. R.= Bees and bee-keeping, i, London, 1886, p. 93.
=Joseph, Gustav.= Zur Morphologie des Geschmacksorganes bei Inseckten.
(Amtlicher Bericht der 50 Versammlung deutscher Naturforscher u.
Artzte in München. 1877, pp. 227, 228.)
=Dimmock, George.= The anatomy of the mouth-parts and of the sucking
apparatus of some Diptera, 1881. (Also in Psyche, iii, pp. 231–241,
Pl. 1, 1882.)
=Packard, A. S.= On the epipharynx of the Panorpidæ. (Psyche, 1889, v,
pp. 159–164.)
—— Notes on the epipharynx and the epipharyngeal organs of taste in
mandibulate insects. (Psyche, v, pp. 193–199, 222–228, 1889.)
=Attachment of the head to the trunk.=—The head is either firmly
supported by the broad prothoracic segment in Orthoptera, many beetles,
etc., into which it is more or less retracted, or it is free and
attached by a slender neck, easily turning on the trunk, as in
dragon-flies, flies, etc. In some insects there are several chitinous
plates, situated on an island in the membrane on the under side of the
neck; these are the “cervical sclerites” of Sharp, occurring “in
Hymenoptera, in many Coleoptera, and in Blattidæ.”
=The basal or gular region of the head.=—At the hinder part of the head
is the opening (occipital foramen) into the trunk. The cheek (gena) is
the side of the head, and to its inner wall is attached the mandibular
muscle; it thus forms the region behind the eye and over the base of the
mandibles. In the Termitidæ, where the head is broad and flat, it forms
a distinct piece on the under side of the head bounding the gulo-mental
region (Fig. 28). In the Neuroptera (Corydalus, Fig. 29, and Mantispa,
Fig. 30) it is less definitely outlined.
[Illustration:
FIG. 29.—Head of _Corydalus cornutus_, ♂: _A_, from above. _B_, from
beneath. _C_, from the side. _a. cly_, clypeus anterior; _p. cly_,
clypeus posterior; _lbr_, labrum; _md_, mandible; _mx_, base of
first maxilla; _mp_, its palpus; _m_, mentum; _sm_, submentum;
_plpr_, palpifer; _lig_, fused second maxillæ; _ant_, antenna;
_occ_, occiput.
]
[Illustration:
FIG. 28.—Head of _Termopsis angusticollis_, seen from beneath, showing
the gena and gula: _m_, mentum; _sm_, submentum; _labr_, under side
of the labrum; _x_, hypopharyngeal chitinous support.
]
All the gulo-mental region of the head appears to represent the base of
the second maxillæ, and the question hence arises whether the submentum
is not the homologue of the cardines of the first maxillæ fused, and the
mentum that of the stipites of the latter also fused together. If this
should prove to be the case, the homologies between the two pairs of
maxillæ will be still closer than before supposed. Where the gula is
differentiated, this represents the basal piece of the second maxillæ.
In Figs. 28, 29, 30, and 31, these three pieces are clearly shown to
belong to the second maxillary segment. It is evident that these pieces
or sclerites belong to the second maxillary or labial segment of the
head, as does the occiput, which may represent the tergo-pleural portion
of the segment. Miall and Denny also regarded the submentum as the basal
piece of the second maxillæ.
[Illustration:
FIG. 30.—Head of _Mantispa brunnea_, under side: _e_, eye; other
lettering as in Fig. 29.
]
[Illustration:
FIG. 31.—Head of _Limnephilus pudicus_, under side: _e_, eye; _l_,
ligula; _p_, palpifer; _lp_, labial palpi.
]
=The occiput= (Fig. 29, _B_, _C_), as stated beyond, is very rarely
present as a separate piece; in the adult insect we have only observed
it in Corydalus. The occipital region may be designated as that part of
the head adjoining and containing the occipital foramen. Newport
considers the occiput as that portion of the base of the head “which is
articulated with the anterior margin of the prothorax. It is perforated
by a large foramen, through which the organs of the head are connected
with those of the trunk. It is very distinct in Hydroüs and most
Coleoptera, and in some, the Staphylinidæ, Carabidæ, and Silphidæ is
constricted and extended backwards so as to form a complete neck.” (See
also p. 51.)
[Illustration:
FIG. 32.—Interior and upper and under surface of the head of _Hydroüs
piceus_: _d_, clypeus; _e_, labrum; _g_, maxilla; _h_, its palpus;
_i_, labium; _k_, labial palpus; _p_, sutura epicranii; _q_,
cotyloid cavity; _r_, torulus; _s_, _v_, laminæ squamosa; _t_,
laminaæ posteriores; _u_, tentorium; _w_, laminæ orbitales; _x_, os
transversum; _y_, articulating cavity for the mandible; _z_, os
hypopharyngeum.—After Newport.
]
=The tentorium.=—The walls of the head are supported or braced within by
two beams or endosternites passing inwards, and forming a solid
chitinous process or loop which extends in the cockroach downwards and
forwards from the lower edge of the occipital foramen. “In front it
gives off two long crura or props, which pass to the ginglymus, and are
reflected thence upon the inner surface of the clypeus, ascending as
high as the antennary socket, round which they form a kind of rim.”
(Miall and Denny.) The œsophagus passes upwards between its anterior
crura, the long flexor of the mandible lies on each side of the central
plate; the supraœsophageal ganglion rests on the plate above, and the
subœsophageal ganglion lies below it, the nerve cords which unite the
two passing through the circular aperture. (Miall and Denny.) In
Coleoptera (Hydroüs) it protects the nervous cord which passes under it.
(Newport, Fig. 32, _u_.)
[Illustration:
FIG. 33.—Posterior view of head of Anabrus; _t_, tentorium. Joutel
_del._
]
In Anabrus the tentorium is V-shaped, the two arms originating on each
side of the base of the clypeus next to the base of each mandible the
origin being indicated by two small foramina partly concealed externally
and passing inwards and backwards and uniting just before reaching the
posterior edge of the large occipital foramen (Fig. 33).
Palmén regards the tentorium as representing a pair of tracheæ (with
the cephalic spiracles) which have become modified for supports or
for muscular attachment, since he finds that in Ephemera the
tentorium breaks across the middle during exuviation, each half
being drawn out of the head like the chitinous lining of a tracheal
tube. This view is supported by Wheeler, who has shown that the
tentorium of Doryphora originates from five pairs of invaginations
of the longitudinal commissures, and which are anterior to those of
the second maxillary segment. “These invaginations grow inwards as
slender tubes, which anastomose in some places. Their lumina are
ultimately filled with chitin.” (Jour. Morph., iii, p. 368.)
This view has also been held by Carrière and Cholodkowsky, but
Heymons concludes from his embryological studies on Forficula and
Blattidæ (1895) that it is unfounded. That this is probably the case
is proved by the fact that the apodemes of the thoracic region are
evidently not modified tracheæ, since the stigmata and tracheæ are
present.
=Number of segments in the head.=—While it is taken for granted by many
entomologists that the head of insects represents a single segment,
despite the circumstance that it bears four pairs of appendages, the
more careful, philosophical observers have recognized the fact that it
is composed of more than a single segment. Burmeister recognized only
two segments in the head; Carus and Audouin recognized three; Macleay
and Newman four; Straus-Durckheim even so many as seven. Huxley supposed
that there are five segments bearing appendages, remarking, “if the eyes
be taken to represent the appendages of another somite, the insect head
will contain six somites.” (Manual of Anat. Invert. Animals, p. 398.)
These discordant views were based on the examination of the head in
adult insects; but if we confine ourselves to the imago alone, it is
impossible to arrive at a solution of the problem.
Newport took a step in the right direction when he wrote: “It is only by
comparing the distinctly indicated parts of the head in the perfect
insect with similar ones in the larva that we can hope to ascertain the
exact number of segments of which it is composed.” He then states that
in the head of _Hydroüs piceus_ are the remains of four segments, though
still in the next paragraph, when speaking of the head as a whole, he
considers it as the first segment, “while,” he adds, “the aggregation of
segments of which it is composed we shall designate individually
_subsegments_.”
That the head of insects is composed of four segments was shown on
embryological grounds by the writer (1871) and afterwards by Graber
(1879). The antennæ and mouth-parts are outgrowths budding out from the
four primitive segments of the head; the antennæ grow out from the under
side of the procephalic lobes, and these should therefore receive the
name of antennal lobes. In like manner the mandibles and first and
second maxillæ arise respectively from the three succeeding segments.
[Illustration:
FIG. 34.—Embryo of _Anurida maritima_: _tc. ap_, minute temporary
appendage of the tritocerebral segment, the premandibular appendage;
_at_, antenna; _md_, mandible; _mx_^1, first maxilla; _mx_^2, second
maxilla; _p_^1–_p_^3, thoracic; _ap_^1, _ap_^2, abdominal
appendages; _an_, anus—After Wheeler.
]
While the postoral segments and their appendages are readily seen to be
four in number, the question arises as to whether the eyes represent the
appendages of one or more preoral segments. In this case embryology thus
far has not afforded clear, indubitable evidence. We are therefore
obliged to rely on the number of neuromeres, or primitive ganglia. In
the postoral region of the head, as also in the trunk, a pair of
neuromeres correspond to each segment. (See also under Nervous System,
and under Embryology.) We therefore turn to the primitive number of
neuromeres constituting the procephalic lobes or brain.
From the researches of Patten, Viallanes, and of Wheeler, especially of
Viallanes, it appears that the brain or supraœsophageal ganglion is
divided into three primitive segments. (See Nervous System, Brain.) The
antennæ are innervated from the middle division or deutocerebrum. Hence
the ocular segment, _i.e._ that bearing the compound and simple eyes, is
supposed to represent the first segment of the head. This, however, does
not involve the conclusion that the eyes are the homologues of the
limbs, however it may be in the Crustacea.
The second head-segment is the antennal, the antennæ being the first
pair of true jointed appendages.
The third segment of the head is very obscurely indicated, and the facts
in proof of its existence are scanty and need farther elucidation.
Viallanes’ tritocerebral lobes or division of the brain is situated in a
segment found by Wheeler to be intercalated between the antennal and
mandibular segments. He also detected in _Anurida maritima_, the
rudiments of a pair of appendages, smaller than those next to it, and
which soon disappear (Fig. 34, _tc. ap_). He calls this segment the
intercalary.[12] Heymons (1895) designates it as the “Vorkiefersegment,”
and it may thus be termed the premandibular segment.
[Illustration:
FIG. 35.—Head of embryo of honey bee: _B_, a little later stage than
_A_. _pr.m_, premandibular segment; _cl_, clypeus; _ant_, antenna;
_md_, mandible; _mx_, first maxilla; _mx′_, second maxilla; _sp_,
spiracle.—After Bütschli.
]
As early as 1870 Bütschli observed in the embryo of the honey bee the
rudiments of what appeared to be a pair of appendages between the
antennæ and mandibles, but, judging by his figures, nearer to and more
like the mandibles than the rudimentary antennæ (Fig. 35); they seemed
to him “almost like a pair of inner antennæ.”
“I find,” he says, “in no other insects any indication of this peculiar
appendage, which at the time of its greatest development attains a
larger size than the antennæ, and which, afterwards becoming less
distinct, forms by fusion with that on the other side a sort of larval
lower lip. That this appendage does not belong to the category of
segmental appendages is indicated by the site of its origin on the upper
side of the primitive band.” (Zeitschr. wissen. Zool., xx, p. 538.)
Grassi has also observed it in Apis, and regards it as the germ of a
first, but deciduous, pair of jaws. In the embryo of Hylotoma Graber
(Figs. 134, 135) found what he calls three pairs of “preantennal
projections,” one of which he thinks corresponds to the “inner antennæ”
of Bütschli. This subject needs further investigation.
It thus appears that the procephalic lobes of the embryo of insects,
with the rudiments of the antennæ, constitute the primitive head, and
perhaps correspond to the annelidan head, while gradually the antennal
appendages were in the phylogenetic development of the class fused with
the two segments of the primary head. That the second maxillary segment,
the occiput, was the last to be added, and at first somewhat
corresponded in position to the poison-fangs of centipedes (Chilopods),
is shown by our observations on the embryology of Æschna (Fig. 36).
[Illustration:
FIG. 36.—Æschna nearly ready to hatch: 4, labium, between _T_ and _e_
the occipital tergite; 5–7, legs.
]
[Illustration:
FIG. 37.—Head of embryo Nematus, showing the labial segment: _occ_,
forming the occiput; _cl_, clypeus; _lb_, labrum; _md_, mandible;
_mdm_, muscle of same; _mx_, maxilla; _mx′_, second maxilla
(labium); _oe_, œsophagus.
]
The mandibular segment appears to form a large part of the post-antennal
region of the epicranium on account of the great mandibular muscle which
arises from so large an area of the anterior region of the head (Fig.
37).
Judging from the embryo of Nematus (Fig. 37), the first maxillary
segment is tergally aborted, there being no tergo-pleural portion
left.[13]
The second maxillary segment tergally appears to be represented by the
occipital region of the head.