THE OUTER SKELETON.
_SPECIAL REFERENCES._
KRUKENBERG. Vergleichend-Physiologische Vorträge. IV.--Vergl.
Physiologie der Thierischen Gerüstsubstanzen. (1885.) [Chemical
Relations of Chitin.]
GRABER. Ueber eine Art fibrilloiden Bindegewebes der Insectenhaut.
Arch. f. mikr. Anat. Bd. X. (1874.) [Minute Structure of
Integument.] Also,
VIALLANES. Recherches sur l’Histologie des Insectes. Ann. Sci.
Nat., Zool. VI^e Série, Tom. XIV. (1882).
AUDOUIN. Recherches anatomiques sur le thorax des Insectes, &c.
Ann. Sci. Nat. Tom. I. (1824.) [Theoretical Composition of Insect
Segments.] Also,
MILNE-EDWARDS. Leçons sur la Physiologie et l’Anatomie Comparée.
Tom. X. (1874.)
SAVIGNY. Mémoires sur les animaux sans vertèbres. Partie I^{e.}
Théorie des organes de la bouche des Crustacées et des Insectes.
(1816.) [Comparative Anatomy of the Mouth-parts.]
MUHR. Ueber die Mundtheile der Orthopteren. Prag. 1877.
[Mouth-parts of Orthoptera.]
_Chitin._
When the skin of an Insect is boiled successively in acids,
alkalies, alcohol, and ether, an insoluble residue known as Chitin
(C_{15}H_{26}N_{2}O_{10}) is obtained. It may be recognised and
sufficiently separated by its resistance to boiling liquor potassæ.
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
organised form. The colour which it frequently exhibits is not due to
any essential ingredient; it may be diminished or even destroyed by
various bleaching processes. The colouring-matter of the chitin of the
Cockroach, which is amber-yellow in thin sheets and blackish-brown in
dense masses, is particularly stable and difficult of removal. Its
composition does not appear to have been ascertained; it is white
when first secreted, but darkens on exposure to air. Fresh-moulted
Cockroaches are white, but gradually darken in three or four hours.
Lowne[30] observes that in the Blow-fly the pigment is “first to be met
with in the fat-bodies of the larvæ. These are perfectly white, but
when cut from the larva, and exposed to the air, they rapidly assume
an inky blackness.... When the perfect insect emerges from the pupa,
and respiration again commences, the integument is nearly white, or a
faint ashy colour prevails. This soon gives place to the characteristic
blue or violet tint, first immediately around those portions most
largely supplied with air vessels.” Professor Moseley[31] tells us
that, thinking it just within the limits of possibility that the brown
coloration of the Cockroach might be due to the presence of silver, he
analysed one pound weight of Blatta. He found no silver, but plenty
of iron, and a remarkable quantity of manganese. That light has some
action upon the colouring matter seems to be indicated by the fact that
in a newly-moulted Cockroach the dorsal surface darkens first.
[30] Anatomy of the Blow-fly, p. 11.
[31] Q. J. Micr. Sci., 1871, p. 394.
Chitin is not peculiar to Insects, nor even to Arthropoda. The pen of
cuttle-fishes and the shell of Lingula contain the same substance,[32]
which is also proved, or suspected, to occur in many other animals.
[32] Krukenberg. Vergl. Physiologische Vorträge, p. 200. Halliburton,
Q. J. Micr. Sci., 1885, p. 173.
The chemical stability of chitin is so remarkable that we might well
expect it to accumulate like the inorganic constituents of animal
skeletons, and form permanent deposits. Schlossberger[33] 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.
[33] Ann. d. Chem. u. Pharm., Bd. 98.
_The Chitinous Cuticle._
The chitinous exoskeleton is rather an exudation than a true tissue. It
is not made up of cells, but of many superposed laminæ, secreted by
an underlying epithelium, or “chitinogenous layer.” This consists of
a single layer of flattened cells, resting upon a basement membrane.
A cross-section of the chitinous layer, or “cuticle,” examined with a
high power shows extremely close and fine lines perpendicular to the
laminæ. The cells commonly form a mosaic pattern, as if altered in
shape by mutual pressure. The free surface of the integument of the
Cockroach is divided into polygonal, raised spaces. Here and there an
unusually long, flask-shaped, epithelial cell projects through the
cuticle, and forms for itself an elongate chitinous sheath, commonly
articulated at the base; such hollow sheaths form the hairs or setæ of
Insects--structures quite different histologically from the hairs of
Vertebrates.
[Illustration: Fig. 8.--Diagram of Insect integument, in section. _bm_,
basement membrane; _hyp_, hypodermis, or chitinogenous layer; _ct_,
_ct_′, chitinous cuticle; _s_, a seta.]
The polygonal areas of the cuticle correspond each to a chitinogenous
cell. Larger areas, around which the surrounding ones are radiately
grouped, are discerned at intervals, and these carry hairs, or give
attachment to muscular fibres.
Viallanes (loc. cit.) has added some interesting details to what was
previously known of Insect-hairs. There are, he points out, two kinds
of hairs, distinguished by their size and structure. The smaller
spring from the boundary between contiguous polygonal areas, and have
no sensory character. The larger ones occupy unusually large areas,
surmount chitinogenous cells of corresponding size, and receive a
special nervous supply. The nerve[34] expands at the base of the hair
into a spindle-shaped, nucleated mass (bipolar ganglion-cell), from
which issues a filament which traverses the axis of the hair, piercing
the chitinogenous cell, whose protoplasm surrounds it with a sheath
which is continued to the tip of the hair. Such sensory hairs are
abundant in parts which are endowed with special sensibility.
[34] Previously observed by Leydig in _Corethra_.
[Illustration: Fig. 9.--Nerve-ending in skin of Stratiomys larva. _h_,
hairs; _b_, their chitinous base; _c_, nucleus of generating cell; _g_,
ganglion cell. × 250. Copied from Viallanes.]
[Illustration: Fig. 10.--Diagram of sensory hair of Insect. _Cc_,
chitinous cuticle; _h_, hair; _c_, its generating cell; _g_, ganglion
cell; _bm_, basement-membrane.]
The chitinous cuticle is often folded in so as to form a deep pit,
which, looked at from the inside of the body, resembles a lever, or a
hook. Such inward-directed processes serve chiefly for the attachment
of muscles, and are termed _apodemes_ (_apodemata_). A simple example
is afforded by the two glove-tips which lie in the middle line of the
under-surface of the thorax (p. 58, and fig. 27). In other cases the
pit is closed from the first, and the apodeme is formed in the midst
of a group of chitinogenous cells distant from the superficial layer,
though continuous therewith. Many tendons of insertion are formed in
this way. The two forked processes in the floor of the thorax (p. 58,
and fig. 27) are unusually large and complex structures of the same
kind. In the tentorium of the head (p. 39, and fig. 17) a pair of
apodemes are supposed to unite and form an extensive platform which
supports the brain and gullet.
[Illustration: Fig. 11.--Nymph (in last larval stage) escaping from old
skin. × 2-1/2.]
Like other Arthropoda, Insects shed their chitinous cuticle from time
to time. A new cuticle, at first soft and colourless, is previously
secreted, and from it the old one gradually becomes detached. The setæ
probably serve the same purpose as the “casting-hairs” described by
Braun in the crayfish, and by Cartier in certain reptiles,[35] that is,
they mechanically loosen the old skin by pushing beneath it. In many
soft-bodied nymphs the new skin can be gathered up into a multitude of
fine wrinkles, which facilitate separation, but we have not found such
wrinkles in the Cockroach, except in the wings. The integument about
to be shed splits along the back of the Cockroach, from the head to
the end of the thorax,[36] and the animal draws its limbs out of their
discarded sheaths with much effort. It is remarkable that the long,
tapering, and many-jointed antennæ are drawn out from an entire sheath.
At the same time the chitinous lining of the tracheal tubes is cast,
while that of the alimentary canal is broken up and passed through the
body.
[35] A condensed and popular account of these researches will be
found in Semper’s Animal Life, p. 20.
[36] Prof. Huxley (Anat. Invert. Animals, p. 419) states that the
integument splits along the abdomen also, but this is a mistake.
[Illustration: Fig. 12.--Cast skin of older nymph (“pupa”). × 2-1/2.]
Prolonged boiling in caustic potash, though it dissolves the viscera,
does not disintegrate the exoskeleton. This shows that the segments
of the integument are not separate chitinous rings, but thickenings
of a continuous chitinous investment. Nevertheless, their constancy
in position and their conformity in structure often enable us to
trace homologies between different segments and different species as
certainly as between corresponding elements of the osseous vertebrate
skeleton.
_Parts of a Somite._
Audouin’s laborious researches into the exoskeleton of Insects[37]
resulted in a nomenclature which has been generally adopted. He divides
each somite (segment) into eight pieces, grouped in pairs--viz.,
_terga_ (dorsal plates), _sterna_ (ventral plates), _epimera_ (adjacent
to the terga), and _episterna_ (adjacent to the sterna).
[37] Audouin. Rech. anat. sur le thorax des Insectes, &c. (Ann. Sci.
Nat., Tom I., p. 97. 1824.)
While admitting the usefulness of these terms, we must warn the reader
not to suppose that this subdivision is either normal or primitive. The
eight-parted segment exists in no single larval or adult Arthropod.
Lower forms and younger stages take us further from such a type,
instead of nearer to it; and Audouin’s theoretical conception is most
fully realised in the thorax of an adult Insect with well-developed
legs and wings.
The morphologist would derive all the varieties of Arthropod segments
from the very simple and uniform chitinous cuticle found in Annelids
and many Insect-larvæ. This becomes differentiated by unequal
thickening and folding in, and a series of rings connected by flexible
membranes is produced. Locomotive and respiratory activity commonly
lead to the definition of terga and sterna, which are similarly
attached to each other by flexible membranes. A pair of limbs may
next be inserted between the terga and sterna, and the simple segment
thus composed occurs so extensively in the less modified regions and
in early stages that it may well be considered the typical Arthropod
somite.
Special needs may lead to the division of the sterna into lateral
halves, but this is purely an adaptive change. The third thoracic
sternum of the male Cockroach, and the second and third of the female
are thus divided, as is also the hinder part of the seventh abdominal
sternum of the female.
In an early stage every somite has its tergal region divided into
lateral halves, owing to the late completion of the body on this side.
Traces of this division may survive even in the imago. There is often
a conspicuous dorsal groove in the thoracic terga, and at the time of
moult the terga split along an accurately median line (see fig. 12).
Additional pieces may be developed between the terga and sterna, and
these have long been termed _pleural_.[38] There may be, for example,
single stigmatic plates, as in the abdomen of the Cockroach, pieces to
support the thoracic legs, and pieces to support the wings; but the
number and position of these plates depends upon their immediate use,
and their homologies become very uncertain when Insects of different
orders are compared. In general, the pleural elements of the segment
are late in development, variable, and highly adaptive.
[38] This application of the word to denote parts intermediate
between terga and sterna has become general since its adoption
by Audouin. It appears also in the older and deservedly obsolete
nomenclature of Kirby and Spence. Professor Huxley has unfortunately
disturbed the consistent use of this term by giving the name _pleura_
to the free edges of the terga in Crustacea.
_Somites of the Cockroach._
The exoskeleton of the Cockroach is divisible into about seventeen
segments, which are grouped into three regions, as follows:--
Head {Procephalic lobes
{Post-oral segments 3
Thorax 3[39]
Abdomen 11
--
17
--
[39] Where the thorax apparently consists of four somites, as in
some Hymenoptera, Hemiptera, Coleoptera, and Lepidoptera, the first
abdominal segment has become blended with it.
It is a strong argument in favour of this estimate that many Insects,
at the time when segmentation first appears, possess seventeen
segments.[40] The procephalic lobes, from which a great part of the
head, including the antennæ, is developed, are often counted as an
additional segment.[41]
[40] Balfour. Embryology, Vol. I., p. 337.
[41] _E.g._, by Graber. Insekten, Vol. II., p. 423.
The limbs, which in less specialised Arthropoda are carried with
great regularity on every segment of the body, are greatly reduced
in Insects. Those borne by the head are converted into sensory and
masticatory organs; those on the abdomen are either totally suppressed,
or extremely modified, and only the thoracic limbs remain capable of
aiding in locomotion.
The primitive structure of the Arthropod limb is adapted to
locomotion in water, and persists, with little modification, in
most Crustacea. Here we find in most of the appendages[42] a basal
stalk (_protopodite_), often two-jointed, an inner terminal branch
(_endopodite_), and an outer terminal branch (_exopodite_), each of
the latter commonly consisting of several joints. It does not appear
that the appendages of Insects conform to the biramous Crustacean type,
though the ends of the maxillæ are often divided into an outer and an
inner portion.
[42] See, for example, Huxley on the Crayfish.
We shall now proceed to describe, in some detail, the regions of the
body of the adult Cockroach.
_Head; Central Parts._
The head of the Cockroach, as seen from the front, is pear-shaped,
having a semi-circular outline above, and narrowing downwards. A
side-view shows that the front and back are flattish, while the top and
sides are regularly rounded. In the living animal the face is usually
inclined downwards, but it can be tilted till the lower end projects
considerably forward. The mouth, surrounded by gnathites or jaws,
opens below. On the hinder surface is the occipital foramen, by which
the head communicates with the thorax. A rather long neck allows the
head to be retracted beneath the pronotum (first dorsal shield of the
thorax) or protruded beyond it.
[Illustration: Fig. 13.--Front of Head. × 10.]
On the front of the head we observe the clypeus, which occupies a large
central tract, extending almost completely across the widest part of
the face. It is divided above by a sharply bent suture from the two
epicranial plates, which form the top of the head as well as a great
part of its back and sides. The labrum hangs like a flap from its lower
edge. A little above the articulation of the labrum the width of the
clypeus is suddenly reduced, as if a squarish piece had been cut out of
each lower corner. In the re-entrant angle so formed, the ginglymus, or
anterior articulation of the mandible, is situated.
The labrum is narrower than the clypeus, and of squarish shape,
the lower angles being rounded. It hangs downwards, with a slight
inclination backwards towards the mouth, whose front wall it forms.
On each side, about halfway between the lateral margin and the middle
line, the posterior surface of the labrum is strengthened by a vertical
chitinous slip set with large setæ. Each of these plates passes above
into a ring, from the upper and outer part of which a short lever
passes upwards, and gives attachment to a muscle (_levator menti_).
[Illustration: Fig. 14.--Top of Head. _ep_, epicranial plate; _oc_,
eye; _ge_, gena. × 10.]
The top and back of the head are defended by the two epicranial
plates, which meet along the middle line, but diverge widely as they
descend upon the posterior surface, thus enclosing a large opening,
the occipital foramen. Beyond the foramen, they pass still further
downwards, their inner edges receding in a sharp curve from the
vertical line, and end below in cavities for the articulation of the
mandibular condyles.[43]
[43] One of the few points in which we have to differ from the
admirable description of the Cockroach given in Huxley’s Comparative
Anatomy of Invertebrated Animals, relates to the articulation of the
mandible, which is there said to be carried by the gena.
[Illustration: Fig. 15.--Side of Head. _oc_, eye; _ge_, gena; _mn_,
mandible. × 10.]
The sides of the head are completed by the eyes and the genæ. The large
compound eye is bounded above by the epicranium; in front by a narrow
band which connects the epicranium with the clypeus; behind, by the
gena. The gena passes downwards between the eye and the epicranial
plate, then curves forwards beneath the eye, and just appears upon
the front of the face, being loosely connected at this point with the
clypeus. Its lower edge overlaps the base of the mandible, and encloses
the extensor mandibulæ.
The occipital foramen has the form of an heraldic shield. Its lateral
margin is strengthened by a rim continuous with the tentorium, or
internal skeleton of the head. Below, the foramen is completed
by the upper edge of the tentorial plate, which nearly coincides
with the upper edge of the submentum (basal piece of the second
pair of maxillæ); a cleft, however, divides the two, through which
nerve-commissures pass from the sub-œsophageal to the first thoracic
ganglion. Through the occipital foramen pass the œsophagus, the
salivary ducts, the aorta, and the tracheal tubes for the supply of air
to the head.
[Illustration: Fig. 16.--Back of Head. _ca_, cardo; _st_, stipes; _ga_,
galea; _la_, lacinia; _pa_, palp; _sm_, submentum; _m_, mentum; _pg_,
paraglossa. × 10.]
The internal skeleton of the head consists of a nearly transparent
chitinous septum, named _tentorium_ by Burmeister, which extends
downwards and forwards from the lower border 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. Each crus is twisted, so that the front surface
becomes first internal and then posterior as it passes towards the
clypeus. The form of the tentorium is in other respects readily
understood from the figure (fig. 17). Its lower surface is strengthened
by a median keel which gives attachment to muscles. The œsophagus
passes upwards between its anterior crura, the long flexor of the
mandible lies on each side of the central plate; the supra-œsophageal
ganglion rests on the plate above, and the sub-œsophageal ganglion
lies below it, the nerve-cords which unite the two passing through the
circular aperture. A similar internal chitinous skeleton occurs in the
heads of other Orthoptera, as well as in Neuroptera and Lepidoptera.
Palmén[44] thinks that it represents a pair of stigmata or spiracles,
which have thus become modified for muscular attachment, their
respiratory function being wholly lost. In Ephemera he finds that the
tentorium breaks across the middle when the skin is changed, and each
half is drawn out from the head like the chitinous lining of a tracheal
tube.
[44] Morphologie des Tracheen-systems, p. 103.
[Illustration: Fig. 17.--Fore-half of Head, with tentorium, seen from
behind. × 12.]
_Antennæ; Eyes._
A pair of antennæ spring from the front of the head. In the male of
the common Cockroach they are a little longer than the body; in the
female rather shorter. From seventy-five to ninety joints are usually
found, and the three basal joints are larger than the rest. Up to about
the thirtieth, the joints are about twice as wide as long; from this
point they become more elongate. The joints are connected by flexible
membranes, and provided with stiff, forward-directed bristles. The
ordinary position of the antennæ is forwards and outwards.
[Illustration: Fig. 18.--Base of Antenna of Male (to left) and Female
(to right). × 24.]
Each antenna is attached to a relatively large socket (fig. 15), which
lies between the epicranium and clypeus, to the front and inner side of
the compound eyes. A flexible membrane unites the antenna to the margin
of the socket, from the lower part of which a chitinous pin projects
upwards and supports the basal joint.
It is well known that in many Crustacea two pairs of antennæ are
developed, the foremost pair (antennules) bearing two complete
filaments. Some writers have suggested that both pairs may be present
in Insects, though not simultaneously, the Crustacean antennule being
found in the larva, and the Crustacean antenna in the adult. This view
was supported by the familiar fact that in many larvæ the antennæ are
placed further forward than in the adult. The three large joints at the
base of Orthopterous antennæ have been taken to correspond with those
of Crustacean antennules, and it has been inferred that in Insects
with incomplete metamorphosis, only antennules or larval antennæ are
developed.[45] This reasoning was never very cogent, and it has been
impaired by further inquiry. Weismann has shown that in _Corethra
plumicornis_, the adult antenna, though inserted much further back than
that of the larva, is developed within it,[46] and Graber has described
a still more striking case of the same thing in a White Butterfly.[47]
There is, therefore, no reason to suppose that Insects possess more
than one pair of antennæ, which is probably preoral, not corresponding
with either of the Crustacean pairs.
[45] Zaddach, Entw. des Phryganiden Eies, p. 86; Rolleston, Forms of
Animal Life, p. 75, &c.
[46] Zeits. f. wiss. Zool., Bd. XVI., pl. vii., fig. 33.
[47] Insekten, Vol. II., p. 508.
We have already noticed (p. 26) the superficial points in which the
antenna of the male Cockroach differs from that of the female.
The eyes of some Crustacea are carried upon jointed appendages, but
this is never the case in Insects, though the eye-bearing surface
may project from the head, as in _Diopsis_ and _Stylops_. Professor
Huxley[48] supposes that the head of an Insect may contain six somites,
the eyes representing one pair of appendages. The various positions
in which the eyes of Arthropoda may be developed weakens the argument
drawn from the stalk-eyed Crustacea. Claus and Fritz Müller go so far
on the other side as to deny the existence of an eye-segment even in
Crustacea.
[48] Anat. Invert. Animals, p. 398.
_Mouth-parts of the Cockroach._
Before entering upon a full description of the mouth-parts of the
Cockroach, which present some technical difficulties, the beginner in
Insect anatomy will find it useful to get a few points of nomenclature
fixed in his memory. Unfortunately, the terms employed by entomologists
are at times neither convenient nor philosophical.
There are three pairs of jaws, disposed behind the labrum, as in the
diagram:--
LABRUM.
1st pair of Jaws (MANDIBLES).
2nd " (MAXILLÆ).
3rd " (LABIUM, or 2nd pair of Maxillæ).
[Illustration: Fig. 19.--Diagram of Cockroach Jaws, in horizontal
section.]
The mandible is undivided in all, or nearly all, Insects. Each maxilla
may consist of
A _palp_ on the outer side,
A _galea_ (hood),
A _lacinia_ (blade), on the inner side.
The galea (hood) of the 3rd pair of jaws is sometimes called the
_paraglossa_.
A tongue-like process may be developed from the front wall of the mouth
(_epipharynx_), or from the back wall (_hypopharynx_ or _lingua_).[49]
Both epipharynx and hypopharynx project into the mouth, and, in some
Diptera, far beyond it.
[49] Professor Huxley has proposed to call the attached base
_hypopharynx_, and the free tip _lingua_.
The tip of the labium is sometimes produced into a long tongue, called
the _ligula_ (strap).
The mouths of Insects may be classed as:--
BITING.--Orthoptera, Neuroptera, Coleoptera (in some Coleoptera a
licking tongue is developed), most Hymenoptera.
LICKING AND SUCKING.--Some Hymenoptera--_e.g._, Honey Bee.
SUCKING.--(_a_) With lancets--Diptera, Hemiptera. (_b_) Without
lancets--Lepidoptera.
The reference of these to a common plan, and the determination of the
constituent parts, is mainly the work of Savigny. Mouth-parts were made
the basis of the classification of Insects by Fabricius (1745–1808).
[Illustration: Fig. 20.--The Jaws, separated. _Mn_, mandible, seen from
behind (to left) and front (to right); _Mx_' maxilla (first pair);
_Mx_" labium, or second pair of maxillæ. The other letters as before.
× 20.]
The mandibles of the Cockroach are powerful, single-jointed[50] jaws,
each of which is articulated by a convex “condyle” to the lower end
of the epicranial plate, and again by a concave “ginglymus” to the
clypeus. The opposable inner edges are armed with strong tooth-like
processes of dense chitin, which interlock when the mandibles close;
those towards the tip of the mandible are sharp, while others are
blunt, as if for crushing. Each mandible can be moved through an angle
of about 30°. A flexible chitinous flap extends from its inner border
to the labrum. The powerful flexor of the mandible arises within the
epicranial vault; its fibres converge to a chitinous tendon, which
passes outside the central plate of the tentorium, and at a lower
level through a fold on the lower border of the clypeus, being finally
inserted near the ginglymus. A short flexor arises from the crus of the
tentorium. The extensor muscle arises from the side of the head, passes
through the fold formed by the lower end of the gena, and is inserted
close to the outer side of the condyle of the mandible.
[50] Professor J. Wood-Mason points out that in _Machilis_ (one of
the Thysanura) the mandible shows signs of segmentation, while the
apical portion is deeply divided into an inner and an outer half.
Ripe embryos of _Panesthia (Blatta) javanica_ are said to exhibit
folds which indicate the consolidation of the mandible out of
separate joints, while the cutting and crushing portions of the edge
are divided by a “sutural mark,” which may correspond to the line of
junction of the divisions of a biramous appendage (Trans. Ent. Soc.,
1879, pt. 2, p. 145).
The anterior maxillæ lie behind the mandibles, and like them are
unconnected with each other. They retain much more of the primitive
structure of a gnathite than the mandibles, in which parts quite
distinct in the maxillæ are condensed or suppressed. The constituent
pieces are seen in fig. 20. There is a two-jointed basal piece,
consisting of the _cardo_ (_ca_) and the _stipes_ (_st_). The cardo
is a transverse plate bent upon itself, and enclosing muscles; it is
attached to the outward-directed pedicel of the occipital frame, and
carries the vertical stipes. To the side and lower end of the stipes
is attached the five-jointed palp (_pa_), a five-jointed limb used in
feeding and in exploration, while the lacinia (_la_) and galea (_ga_)
are articulated to its extremity. The lacinia is internal and posterior
to the galea; it is broad above, but narrows below to a bifid tooth of
dense chitin; its inner surface is beset with a cluster of strong setæ.
The galea is more flexible, and forms an irregular three-cornered
prism with an obliquely truncated end, upon which are many fine hairs.
A flexible and nearly transparent flap connects the inner edges of the
stipes and cardo, and joins both to the labium. The muscles which move
the bases of the maxillæ spring from the crura, central plate, and keel
of the tentorium.
On the posterior surface of the head, below the occipital foramen,
we find a long vertical flap, the labium, which extends downwards to
the opening of the mouth. It represents a second pair of maxillæ,
fused together in their basal half, but retaining elsewhere sufficient
resemblance to the less modified anterior pair to permit of the
identification of their component parts. The upper edge is applied to
the occipital frame, but is neither continuous with that structure nor
articulated thereto. By stripping off the labium upwards it may be seen
that it is really continuous with the chitinous integument of the neck.
The broad shield-like base is incompletely divided by a transverse
hinge into an upper and larger piece, the _submentum_, and a distal
piece, the _mentum_. To the mentum are appended representatives of the
galeæ (here named _paraglossæ_) and laciniæ, while a three-jointed
palp with an additional basal joint (distinguished as the _palpiger_)
completes the resemblance to the maxillæ of the first pair.[51]
In front of the labium, and lying in the cavity of the mouth is a
chitinous fold of the oral integument, the _lingua_, which lies like
a tongue in the floor of the mouth. The common duct of the salivary
glands enters the lingua, and opens on its hinder surface. The lingua
is supported by the chitinous skeleton represented in the figures of
the salivary glands. (Chap. vii., _infra_.)
[51] The homology of the labium with the first pair of maxillæ is in
no other Insects so distinct as in the Orthoptera.
The epipharynx, which is a prominent part in Coleoptera and Diptera, is
not recognisable in Orthoptera.
_Functions of the Antennæ and Mouth-parts._
We must now shortly consider the functions of the parts just described.
The antennæ have long been regarded as sense-organs, and even the
casual observer can hardly fail to remark that they are habitually
used by the Insect to gain information concerning its immediate
surroundings. Long antennæ, such as those of the Cockroach, are
certainly organs of touch, but it has been much disputed whether they
may not also be the seat of some special sense, and if so, what that
sense may be. Several authors have found reason to suppose that in the
Insect-antenna resides the sense of hearing, but no evidence worth the
name is forthcoming in favour of this view. Much better support can be
found for the belief that the antenna is an olfactory organ,[52] and
some experiments which seem conclusive on this point will be cited in a
later chapter.
[52] Rosenthal, Ueb. d. Geruchsinn der Insekten. Arch. f. Phys. Reil
u. Autenrieth, Bd. X. (1811). Hauser, Zeits. f. wiss. Zool., Bd.
XXXIV. (1880).
In the Cockroach the mandibles and maxillæ are the only important
instruments of mastication. The labium is indirectly concerned
as completing the mouth behind and supporting the lingua, which
is possibly of importance in the ordinary operations of feeding.
Plateau[53] has carefully described the mode of mastication as observed
in a Carabus, and his account seems to hold good of biting Insects in
general. The mandibles and maxillæ act, as he tells us, alternately,
one set closing as the others part. The maxillæ actually push the
morsel into the buccal cavity. When the mandibles separate, the head
is slightly advanced, so that the whole action has some superficial
resemblance to that of a grazing quadruped.
[53] Mém. Acad. Roy. de Belgique, Tom. XLI. (1874). Prof. Plateau’s
writings will often be referred to in these pages. We owe to him the
most important researches into the physiology of Invertebrates which
have appeared for many years.
The palps of the maxillæ and labium have been variously regarded as
sensory and masticatory instruments. Not a few authors believe that
they are useful in both ways. The question has lately been investigated
experimentally by Plateau,[54] who finds that removal of both maxillary
and labial palps does not interfere either with mastication or the
choice of food. He observes that in the various Coleoptera and
Orthoptera submitted to experiment the palps are passive while food is
being passed into the mouth.
[54] Exp. sur le Rôle des Palpes chez les Arthropodes Maxillés. Pt.
I. Bull. Soc. Zool. de France, Tom. X. (1885).
Plateau’s experiments are conclusive as to the subordinate value of the
palps in feeding. The observation of live Cockroaches has satisfied
us that the palps are constantly used when the Insect is active,
whether feeding or not, to explore the surface upon which it moves.
We have seen no ground for attributing to the palps special powers
of perceiving odours or flavours, nor have we observed that they aid
directly in filling the mouth with food.
It is worthy of note that Leydig has described and figured in the
larva of _Hydroporus_ (?), and Hauser in _Dytiscus_, _Carabus_, &c., a
peculiar organ, apparently sensory, which is lodged in the maxillary
and labial palps. It consists of whitish spots, sometimes visible to
the naked eye, characterised by unusual thinness of the chitinous
cuticle and by the aggregation beneath it of a crowd of extremely
minute sensory rods. Of this organ no satisfactory explanation has yet
been given.[55]
[55] Leydig, Taf. z. vergl. Anat., pl. x., fig. 3. Hauser, Zeits.
f. wiss. Zool., Bd. XXXIV., p. 386. Jobert has figured the sensory
organs of the maxillary palps of the Mole-cricket (Ann. Sci. Nat.,
1872), and Forel similar organs in Ants (Bull. Soc. Vaudoise, 1885).
_Comparison of Mouth-parts in different Insects._
The jaws of the Cockroach form an excellent standard of comparison
for those of other Insects, and we shall attempt to illustrate the
chief variations by referring them to this type.[56] Mouth-parts
are so extensively used in the classification of Insects that every
entomologist ought to have a rational as well as a technical knowledge
of their comparative structure. No part of Insect anatomy affords
more striking examples of adaptive modification. In form, size, and
mode of application the jaws vary extremely. It would be hard to find
feeding-organs more unlike, at first sight, than the stylets of a Gnat
and the proboscis of a Moth, yet the study of a few well-selected types
will satisfy the observer that both are capable of derivation from a
common plan. Nor is this common plan at all vague. It is accurately
pictured in the jaws of the Cockroach and other Orthoptera. These
correspond so entirely with the primitive arrangement, inferred by a
process of abstraction from the most dissimilar Insects, as to furnish
a strong argument for the descent of all higher Insects from forms not
unlike Orthoptera in the structure of their mouth-parts.
[56] The reader who desires to follow this subject further is
recommended to study chap. vi. of Graber’s Insekten, which we have
found very useful.
[Illustration: Fig. 21.--Embryo of Aphis. Copied from Mecznikow, Zeits.
f. wiss. Zool., Bd. XVI., taf. xxx., fig. 30. References in text.
× 220.]
Though the jaws of the Cockroach are eminently primitive with respect
to those of most other Insects, they are themselves derived from a far
simpler arrangement, which is demonstrable in all embryonic Insects.
Fig. 21 shows an Aphis within the egg. The rudiments of the antennæ
(_At_), mandibles (_Mn_), and maxillæ (_Mx^1_, _Mx^2_) form simple
blunt projections, similar to each other and to the future thoracic
legs (_L^1_, _L^2_, _L^3_). We see, therefore, that all the appendages
of an Insect are similar in an early stage of growth; and we may add
that a Centipede, a Scorpion, or a Spider would present very nearly
the same appearance in the same stage. A Crustacean in the egg would
not resemble an Insect or its own parent so closely.[57] Aquatic life
favours metamorphosis, and most Crustacea do not begin life with their
full quota of legs, but acquire them as they are wanted.
[57] Freshwater Crustacea, however, are sometimes similar to their
parents at the time of hatching.
Paired appendages of perfectly simple form are therefore the first
stage through which all Insect-jaws must pass. Our second stage is
a little more complex, and not nearly so universal as the first. A
caterpillar (fig. 22) has its own special wants, and these are met
by the unequal development of its jaws. The mandibles are already as
complete as those of the Cockroach, which they closely resemble, but
the maxillæ are stunted cylinders formed mainly of simple rings, and
very like the antennæ. They show, however, the beginnings of three
processes (palp, galea, and lacinia), which are usually conspicuous
in well-developed maxillæ. The second pair of maxillæ (_Lm_) are
coalesced, as usual, and form the spinneret. The mouth-parts of the
Caterpillar do not therefore in all respects represent a universal
stage of development, but show important adaptive modifications. The
mandibles are rapidly pushed forward, and attain their full development
in the larva; the first pair of maxillæ are temporarily arrested
in their growth, and persist for a long time in a condition which
Orthopterous embryos quickly pass through; the maxillæ of the second
pair are not only arrested in their growth, but converted to a special
use, which seems to stop all further progress. The labial palps,
indeed, which are not at all developed in the caterpillar, survive, and
become important parts in the moth; but the greater part of the labium
disappears when the time for spinning the cocoon is over.
[Illustration: Fig. 22.--Head of larva of Goat Moth, seen from behind.
Copied from Lyonnet.]
We come next to the Orthopterous mouth, which is well illustrated
by the Cockroach. This is retained with little modification in all
the biting Insects (Coleoptera and Neuroptera). The mandibles may
become long and pointed, as in _Staphylinus_ and other predatory
forms; in some larvæ of strong carnivorous propensities (Ant-lion,
_Dytiscus_,[58] _Chrysopa_) they are perforate at the tip, and through
them the juices of the prey are sucked into the mouth, which has no
other opening. The labium undergoes marked adaptive change, without
great deviation from the common plan, in the “mask” of the larva of
the Dragon-fly. This well-known implement has a rough likeness, in the
arrangement and use of its parts, to a man’s fore-limb. The submentum
forms the arm, the mentum the fore-arm. Both these are simple, straight
pieces, connected by an elbow-joint. The hand is wider, and carries
a pair of opposable claws, the paraglossæ. In some Coleoptera the
labium is reduced to a stiff spine, while in the Stag-beetle it is
flexible and hairy, and foreshadows the licking tongue of the Bee. The
maxillæ become long and hairy in flower-haunting Beetles, and even
the mandibles are flexible and hairy in the Scarabæus-beetles. Fritz
Müller has found a singular resemblance to the proboscis of a Moth in
a species of _Nemognatha_, where the maxillæ are transformed into two
sharp grooved bristles 12 mm. long, which, when opposed, form a tube,
but are incapable of rolling up.[59]
[58] In _Dytiscus_ the mandibles are perforate at the base, and not
at the tip. See Burgess in Proc. Bost. Soc. Nat. Hist., Vol. XXI.,
p. 223.
[59] Ein Käfer mit Schmetterlingsrüssel, Kosmos, Bd. VI. We take this
reference from Hermann Müller’s Fertilisation of Flowers.
In the Honey Bee (fig. 23) nearly all the mouth-parts of the Cockroach
are to be made out, though some are small and others extremely produced
in length. The mandibles (_Mn_) are not much altered, and are still
used for biting, as well as for kneading wax and other domestic work.
The mandibular teeth have proved inconvenient, and are gone. The
lacinia of the maxilla (_Mx_′) forms a broad and flexible blade, used
for piercing succulent tissues, but the galea has disappeared, and
there is only a vestige of the maxillary palp (_Mxp_). In the second
pair of maxillæ the palp (_Lp_) is prominent; its base forms a blade,
while the tip is still useful as an organ of touch. The paraglossæ
(_Pa_) can be made out, but the laciniæ are fused to form the long,
hairy tongue. This ends in a spoon-shaped lobe (not unlike the “finger”
of an elephant’s trunk), which is used both for licking and for sucking
honey.
The proboscis of the Bee is therefore more like a case of instruments
than a single organ. The mandibles form a strong pair of blunt
scissors. The maxillæ are used for piercing, for stiffening and
protecting the base of the tongue, and when closed they form an
imperfect tube outside the tongue, which, according to Hermann Müller,
is probably suctorial. The labial palps are protective and sensory.
Lastly, the central part, or tongue, is a split tube used for suction;
it is very long, so as to penetrate deep flower-cups, and hairy, so
that pollen may stick to it. When the proboscis is not in use it can
be slid into the mentum (_M_), while it and the mentum together can be
drawn out of the way downwards and backwards.[60]
[60] An interesting account of the structure and mode of action of
the Bee’s tongue is to be found in Hermann Müller’s Fertilisation of
Flowers, where also the evolution of the parts is traced through a
series of graduated types.
[Illustration: Fig. 23.--Mouth-parts of Honey Bee.]
[Illustration: Fig. 23A.--Diagram of Mouth-parts of Honey Bee.]
[Illustration: Fig. 24.--Mouth-parts of Burnet Moth.]
[Illustration: Fig. 24A.--Diagram of Mouth-parts of Moth.]
In the singular suctorial mouth of Moths and Butterflies we observe,
first of all, the great development of the maxillæ. Each forms a
half-tube, which can be accurately applied to its fellow, so as to
form an efficient siphon. In many species the two halves can be held
together by a multitude of minute hooks.[61] At the base of each
maxilla is a rudimentary palp (_Mxp_). The mandibles (_Mn_) are also
rudimentary and perfectly useless. The labium, which was so important
to the larva as a spinneret, has disappeared almost completely, but the
labial palps (_Lp_) are large and evidently important.
[61] See Newport’s figure of Vanessa atalanta (Todd’s Cyc., Art.
Insecta), or Burgess on the Anatomy of the Milk-weed Butterfly, in
Anniversary Mem. of Boston Soc. Nat. Hist., pl. ii., figs. 8–10
(1880).
[Illustration: Fig. 25.--Mouth-parts of Gad-fly (_Tabanus_).]
[Illustration: Fig. 25A.--Diagram of Mouth-parts of Gad-fly.]
In Diptera both piercing and sucking parts are usually present. The
Gad-fly (fig. 25) is typical. Here we recognise the labrum (_Lbr_),
mandible (_Mn_), and maxilla (_Mx_′) of the Cockroach transformed
into stylets. The maxillary palp (_Mxp_) is still sensory. A pointed
process, stiffened by chitinous ribs, is developed from the back of
the labrum. This is the epipharynx (_Ep_), a process undeveloped in
the Cockroach, though conspicuous in some Coleoptera. All these parts
are overtopped by the suctorial labium (_Lm_), which has a two-lobed
expansion at the end. In the more specialised Diptera this becomes a
kind of cupping-glass. The Gad-fly is intermediate between the Gnat,
in which all the mouth-parts are converted into piercing organs of
extraordinary length and sharpness, and such flies as the House-fly and
Blow-fly, where the sucking labium forms an organ of the most elaborate
kind, the piercing organs undergoing a marked reduction. Except where
the labium is short, it is doubly or trebly hinged, so that it can be
readily tucked away under the chin.
In Hemiptera the long four-jointed labium (_Lm_) forms a sheath for
the stylets. When not in use the whole apparatus is drawn up beneath
the head and prothorax. The mandibles (_Mn_) are sharp at the tip, and
close like a pair of forceps, enclosing the maxillæ (_Mx_). These are
of unequal length, only one reaching the end of the mandibular case.
Both have saw teeth on the free edge. Palps are entirely wanting.
[Illustration: Fig. 26.--Mouth-parts of Bug. Copied from Landois,
Zeits. f. wiss. Zool., Bd. XVIII., taf. xi., fig. 3.]
[Illustration: Fig. 26A.--Diagram of Mouth-parts of Bug.]
Comparing the four kinds of suctorial mouths, of which the Bee, the
Moth, the Fly, and the Bug furnish examples, we observe that the
sucking-tube is formed in the Moth out of the two maxillæ, in the
other three out of the labium. Of these last the Bee has the edges of
the labium turned _down_, so that the siphon becomes _ventral_; in
the Bug and Fly the edges are turned _up_, and the siphon becomes
_dorsal_. The more specialised flies have the simple arrangement of the
Bug complicated by a system of branching tubes, which are probably a
special modification of the salivary duct. Similar as the mouth-parts
of the four types may be in regard to their mode of working, they
cannot be reduced to any common plan which differs materially from that
presented by the jaws of the Cockroach.
_Composition of Head._
In all Insects fusion of the primitive elements of the head begins
so early and is carried so far, that it is extremely difficult to
discover the precise way in which they are fitted together. The
following facts have been ascertained respecting the development of
the parts in question. At a very early stage of embryonic life the
body of the Insect becomes divided into a series of segments, which
are at fewest fourteen (in some Diptera), while they are not known to
exceed seventeen.[62] Each segment is normally provided with a pair of
appendages. The foremost segment soon enlarges beyond the rest, and
becomes divided by a median groove into two “procephalic lobes.”[63] Of
the appendages the first eight pairs are usually more prominent than
the rest, and of different form; those of the eighth segment, which may
be altogether inconspicuous, never attain any functional importance.
The first four pairs of appendages are budded off from the future head,
while the next three pairs form the walking legs, and are carried upon
the thoracic segments. All the existing appendages of the fore part of
the body are thus accounted for, but the exact mode of formation of the
head has not yet been made out. The chief part of its walls, including
the clypeus, the compound eyes, and the epicranial plates, arise from
the procephalic lobes, and represent the much altered segment of which
the antennæ are the appendages. The labrum is a secondary outgrowth
from this segment, and, in some cases at least, it originates as a pair
of processes which resemble true appendages, though it is unlikely
that such is their real character. No means at present exist for
identifying the terga and sterna of the head, nor have the gena, the
occipital frame, and the cervical sclerites (described below) been
assigned to their segments.[64] It is worthy of notice that in the
stalk-eyed Crustacea, the head, or what corresponds to the head of
Insecta, consists of either five or six somites, taking into account
a diversity of opinion with respect to the eyestalks, while only four
pairs of appendages can be certainly traced in the head of the Insect.
The mandibles and maxillæ exist to the same number in both groups, and
are homologous organs, so far as is known; the numerical difference
relates therefore to the antennæ, of which the Crustacean possesses two
pairs, the Insect only one. Whether the pair deficient in the Insect is
altogether undeveloped, or represented by the pair of prominences which
give rise to the labrum,[65] is a question of much theoretical interest
and of not a little difficulty.
[62] Balfour, Embryology, Vol. I., p. 337.
[63] Huxley, Med. Times and Gazette, 1856–7; Linn. Trans.,
Vol. XXII., p. 221, and pl. 38 (1858).
[64] “I think it is probable that these cervical sclerites represent
the hindermost of the cephalic somites, while the band with which the
maxillæ are united, and the genæ, are all that is left of the sides
and roof of the first maxillary and the mandibular somites.”--Huxley,
Anat. Invert. Animals, p. 403.
[65] Balfour, Embryology, Vol. I., note to p. 337.
The following table shows the appendages of the head and thorax in
the two classes. The homologies indicated are, however, by no means
established.[66]
CRAYFISH. | COCKROACH.
|
| Antennæ.
---- | ----
Eyestalks. |
Antennules. |
Antennæ. |
Mandibles. | Mandibles.
Maxillæ (1). | Maxillæ (1).
Maxillæ (2). | Maxillæ (2).
---- | ----
Maxillipeds (1). | Thoracic Legs (1).
Maxillipeds (2). | Thoracic Legs (2).
Maxillipeds (3). | Thoracic Legs (3).
[66] J. S. Kingsley in Q. J. Micr. Sci. (1885), has reviewed the
homology of Insect, Arachnid, and Crustacean appendages, and comes
to conclusions very different from those hitherto accepted. He
classifies the appendages as pre-oral (Insect-antennæ) and post-oral,
and makes the following comparisons:--
HEXAPODA. | ACERATA. | CRUSTACEA.
| |
(=Insecta | (=Arachnida |
+ Myriopoda?) | + Limulus.) |
(1) Antennæ. | Absent. | Absent.
(2) Mandibles. | Cheliceræ. | Antennules.
(3) Maxillæ. | Pedipalpi. | Antennæ.
(4) Labium. | 1st pair legs. | Mandibles.
(5) 1st pair legs. | 2nd pair legs. | 1st Maxillæ.
(6) 2nd pair legs. | 3rd pair legs. | 2nd Maxillæ.
(7) 3rd pair legs. | 4th pair legs. | 1st Maxillipeds.
Pelseneer (Q. J. Micr. Sci., 1885), concludes that both pairs of
antennæ are post-oral in _Apus_, and probably in all other Crustacea.
_Neck._
The neck is a narrow cylindrical tube, with a flexible wall
strengthened by eight plates, the cervical sclerites, two of which
are dorsal, two ventral, and four lateral. The dorsal sclerites lie
immediately behind the head (fig. 14); they are triangular, and closely
approximated to the middle line. The inferior plates (fig. 27) resemble
segments of chitinous hoops set transversely, one behind the other,
rather behind the dorsal sclerites, and close behind the submentum.
There are two lateral sclerites on each side of the neck (fig. 27), a
lower squarish one, which is set diagonally, nearly meeting its fellow
across the ventral surface, and an oblong piece, closely adherent to
the other, which extends forwards and upwards towards the dorsal side.
_Thorax._
The elements of the thoracic exoskeleton are simpler in the Cockroach
than in Insects of powerful flight, where adaptive changes greatly
obscure the primitive arrangement. There are three segments, each
defended by a dorsal plate (_tergum_) and a ventral plate (_sternum_).
The sterna are often divided into lateral halves. Of the three terga
the first (_pronotum_) is the largest; it has a wide free edge on each
side, projects forwards over the neck, and when the head is retracted,
covers this also, its semi-circular fore-edge then forming the apparent
head-end of the animal. The two succeeding terga are of nearly equal
size, and each is much shorter than the pronotum, contrary to the rule
in winged Insects.[67]
[67] Many Orthoptera, which seize their prey with the fore legs, have
a very long pronotum.
[Illustration: Fig. 27.--Ventral Plates of Neck and Thorax of Male
Cockroach. I, prosternum; II, mesosternum; III, metasternum. × 6.]
All the terga are dense and opaque in the female; in the male the
middle one (_mesonotum_) and the hindmost (_metanotum_) are thin and
semi-transparent, being ordinarily overlaid by the wing-covers. While
the thoracic terga diminish backwards, the sterna increase in extent
and firmness, proportionally to the size of the attached legs. The
prosternum is small and coffin-shaped; the mesosternum partly divided
into lateral halves in the male, and completely so in the female.
The metasternum is completely divided in both sexes, while a median
piece, carrying the post-furca, intervenes between its lateral halves
in the male. Behind the sterna, especially in the case of the second
and third, the flexible under-surface of the thorax is inclined,
so as to form a nearly vertical step. In the two hinder of these
steps a chitinous prop is fixed; each is Y-shaped, with long, curved
arms for muscular attachment, and a central notch, which supports
the nerve-cord. The hindmost of these, known as the _post-furca_,
lies immediately behind the metasternum, and its short basal piece
is attached between the lateral halves of that plate. Behind the
mesosternum is a somewhat slighter prop, the _medi-furca_. A third
piece of similar nature (the _ante-furca_), which is well developed in
some Insects--_e.g._, in Ants--is apparently wanting in the Cockroach,
though there is a transverse oval plate behind the prosternum, which
may be a rudimentary furca.
Fig. 27 shows two conical processes which lie in the middle line of
the ventral surface of the thorax, one in front of the metasternum,
the other in front of the mesosternum. These are the thoracic pits,
tubular apodemata, serving for the insertion of muscles. The occurrence
of stink-glands in the thorax of Hemiptera,[68] and of so-called
poison-glands in the thorax of _Solpuga_, led us to look for glands in
connection with these processes, but we have found none.
[68] Also in _Phasmidæ_ (see Scudder, Psyche, Vol. I., p. 137).
_Thoracic Appendages. Legs; Wings._
Three pairs of legs are attached to the thoracic segments; they
regularly increase in size from the first to the third, but hardly
differ except in size; the peculiar modifications which affect the fore
pair in predatory and burrowing Orthoptera (_Mantis_, _Gryllotalpa_),
and the third pair in leaping Orthoptera (Grasshoppers, &c.), being
absent in the cursorial Blattina. Each leg is divided into the five
segments usual in Insects (see fig. 28). The coxa is broad and
flattened. The trochanter is a small piece obliquely and almost
immovably attached to the proximal end of the femur, on its inner side.
The femur is nearly straight and narrowed at both ends; along its inner
border, in the position occupied by the stridulating apparatus of the
hind leg of the Grasshoppers, is a shallow longitudinal groove, fringed
by stiff bristles. The tibia is shorter than the femur in the fore leg,
of nearly the same length in the middle leg, and longer in the hind
leg; it is armed with numerous stiff spines directed towards the free
end of the limb. There are usually reckoned five joints in the tarsus,
which regularly diminish in length, except that the last joint is as
long as the second. All the joints bear numerous fine but stiff hairs
upon the walking surface. The extremity of the fifth joint is segmented
off, and carries a pair of equal and strongly curved claws.[69]
[69] Professor Huxley (Anat. Invert. Animals, p. 404) points out that
the so-called _pulvillus_ ought to be counted as a sixth joint. The
same is true of the foot of Diptera and Hymenoptera, where there are
six tarsal joints, the last carrying the claws. (Tuffen West on the
Foot of the Fly. Linn. Trans., Vol. XXIII.)
[Illustration: Fig. 28.--The three Thoracic Legs of a Female Cockroach.
I, _s_, sternum; _cx_, coxa; _tr_, trochanter; _fe_, femur; _tb_,
tibia; _ta_, tarsus. In IIIA the coxa is abducted, and the joints _a_
(episternum) and _b_ slightly separated. × 4.]
At the base of each leg are several chitinous plates (fig. 28), upon
which no small labour has been bestowed by different anatomists. They
are arranged so as to form two joints intermediate between the coxa and
the sternum, and these two joints admit of a hinge-like movement upon
each other, while their other ends are firmly attached to the coxa and
sternum respectively. (Compare III and IIIA, fig. 28.) These parts in
the Cockroach may be taken for two basal leg-joints which have become
adherent to the thorax. In other cases, however, they plainly belong
to the thorax, and not to the leg. In the Mole-cricket, for instance,
similar plates occur; but here they are firmly united, and form the
lateral wall of the thorax. In the Locust they become vertical, and
lie one in front of the other. Most authors have looked upon them as
regular elements of a typical somite. They regard such a segment as
including two pleural elements--viz., a dorsal plate (_epimeron_), and a
ventral plate (_episternum_). We have already (p. 34) given reasons for
doubting the constancy of the pieces so named. It is not inconvenient,
however, to denote by the term _episternum_ the joint which abuts upon
the sternum; for the joint which is applied to the coxa no convenient
term exists, and its occurrence in Insects is so partial, that the
want need not be supplied at present.[70] Both joints are incompletely
subdivided. In the first thoracic segment of the Cockroach they are
less firmly connected than in the other two.
[70] The nomenclature adopted by Packard (Third Report of U.S.
Entomological Commission) seems to us open to theoretical objections.
Cockroaches of both sexes are provided with wings, which, however,
are only functional in the male. The wing-covers (or anterior pair
of wings) of the male are carried by the second thoracic segment. As
in most _Orthoptera genuina_, they are denser than the hind wings,
and protect them when at rest. They reach to the fifth segment of
the abdomen, and one wing-cover overlaps the other. Branching veins
or nervures form a characteristic pattern upon the surface (figs. 4,
29), and it is mainly by means of this pattern that many of the fossil
species are identified and distinguished. The true or posterior wings
are attached to the metathorax. They are membranous and flexible, but
the fore-edge is stiffened, like that of the wing-covers, by additional
chitinous deposit. When extended, each wing forms an irregular quadrant
of a circle; when at rest, the radiating furrows of the hinder part
close up fan-wise, and the inner half is folded beneath the outer.[71]
The wing reaches back as far as the hinder end of the fourth abdominal
segment. The wing-covers of the female are small, and though movable,
seem never to be voluntarily extended; each covers about one-third of
the width of the mesonotum, and extends backwards to the middle of the
metanotum. A reticulated pattern on the outer fourth of the metanotum
plainly represents the hind wing; it is clearly rather a degeneration
or survival than an anticipation of an organ tending towards useful
completeness.
[71] On wing-plaiting and wing-folding in Blattariæ see Saussure,
Etudes sur l’aile des Orthoptères. Ann. Sci. Nat., Sér. 5^e (Zool.),
Tom. X.
[Illustration: Fig. 29.--Wings and Wing-covers of Male Cockroach. × 4.]
The rudimentary wing of the female Cockroach illustrates the homology
of the wings of Insects with the free edges of thoracic terga, and
this correspondence is enforced by the study of the development of the
more complete wings and wing-covers of the male. The hinder edges of
the terga become produced at the later moults preceding the completely
winged stage, and may even assume something of the shape and pattern
of true wings; it is not, however, true, though more than once stated,
that winged nymphs are common. Adults with imperfectly developed wings
have been mistaken for such.
_Origin of Insect Wings._
The structure of the wing testifies to its origin as a fold of the
chitinous integument. It is a double lamina, which often encloses
a visible space at its base. The nervures, with their vessels and
tracheal tubes, lie between the two layers, which, except at the base,
are in close contact. Oken termed the wings of an Insect “aerial
gills,” and this rather fanciful designation is in some degree
justified by their resemblance to the tracheal gills of such aquatic
larvæ as those of Ephemeridæ, Perlidæ, Phryganidæ, &c. In the larva
of _Chloeon (Ephemera) dipterum_ (fig. 30), for example, the second
thoracic segment carries a pair of large expansions, which ultimately
are replaced by organs of aerial flight. The abdominal segments carry
similarly placed respiratory leaflets, the tracheal gills, which by
their vigorous flapping movements bring a rush of water against their
membranous and tracheated surfaces.
Gegenbaur[72] has argued from the resemblance of these appendages to
wings, that the wing and the tracheal leaflet are homologous parts, and
this view has been accepted as probable by so competent an observer as
Sir John Lubbock.[73]
[72] Grundzüge der Vergl. Anat. (Arthropoden, Athmungsorgane.)
[73] Origin and Metamorphoses of Insects, p. 73.
The leaflets placed most advantageously for propulsion seem to have
become exclusively adapted to that end, while the abdominal gills
have retained their respiratory character. At the time of change
from aquatic to terrestrial life, which takes place in many common
Insects when the adult condition is assumed, and which, according to
Gegenbaur, was a normal event among primitive Insects, the tracheal
gill is supposed to disappear, and in its place, at the next moult, an
opening, the stigma, is formed by the rupture of an air-tube. Gegenbaur
supposes that the primitive Insects were aquatic, and their tracheal
system closed. The tracheal gill he takes to be the common structure
which has yielded organs so unlike as the wing and the stigma.
[Illustration: Fig. 30. Chloeon (Chloeopsis) dipterum. Larva in eighth
stage, with wings and respiratory leaflets. × 14. Copied from Vayssière
(loc. cit.).]
The zoological rank of the Insects (Ephemeridæ, Perlidæ, and
Libellulidæ), in which tracheal gills are most usual, is not
unfavourable to such an explanation. Lubbock has given reasons for
regarding _Campodea_ and the Collembola (of the order Thysanura)
as surviving and not very much altered representatives of the
most primitive Insects, and he has shown that no great amount of
modification would be required to convert the terrestrial _Campodea_
into the aquatic _Chloeon_-nymph.[74] We must not forget, however,
that tracheal gills are by no means restricted to these families of
low grade. Trichoptera, a few Diptera, two Lepidoptera (_Nymphula_ and
_Acentropus_), and two Coleoptera (_Gyrinus_ and _Elmis_),[75] have
tracheal gills, and a closed tracheal system in the larval condition.
We cannot suppose that these larvæ of higher orders represent an
unbroken succession of aquatic forms, but if we refuse to adopt this
alternative, we must admit that the closed tracheal system with
tracheal gills may be an adaptive modification of the open system with
stigmata.
[74] Palmén cites one striking proof of the low position of
Ephemeridæ among Insects. Their reproductive outlets are paired and
separate, as in Worms and Crustacea.
[75] These examples are cited by Palmén.
It is well known[76] that in certain Ephemeridæ (_e.g._, _Tricorythus_
and _Cœnis_) a pair of anterior tracheal gills may become transformed
into large plates, which partly protect the gills behind (fig. 31).
A similar modification of the second and third thoracic gills in
_Prosopistoma_ and _Bætisca_ brings all the functional respiratory
organs under cover, and these enlarged plates resemble stiff and simple
wings very closely.
[76] Eaton, Trans. Ent. Soc., 1868, p. 281; Vayssière, Ann. Sci.
Nat., Zool., 1882, p. 91.
[Illustration: Fig. 31.--Tricorythus. Adult larva, with three
functional leaflets. The next leaflet in front is converted into a
protective plate. × 7.
A, protective plate of Tricorythus larva, seen from the outside. × 26.
B, the same from within, showing the attached respiratory appendage.
C, protective plate of Cœnis larva, without respiratory appendage.
All the figures are copied from Vayssière.]
Palmén[77] has subjected Gegenbaur’s hypothesis to a very searching
examination. He observes that:--
[77] Zur Morphologie des Tracheensystems (1877).
1. In _Campodea_, and presumably in other primitive Insects, the
tracheal system is not closed and adapted for aquatic respiration,
but open. Tracheal gills are not by any means confined to the lowest
Insects. (See above, p. 65.)
2. Tracheal gills are not always homodynamous or morphologically
equivalent. In Ephemeridæ, some are dorsal in position, some ventral
(first abdominal pair in _Oligoneuria_ and _Rhithrogena_); they may be
cephalic, springing from the base of the maxilla, as in _Oligoneuria_
and _Jolia_; _Jolia_ has a branchial tuft at the insertion of each of
the fore legs.[78] In Perlidæ the tracheal gills may have a tergal,
pleural, sternal, or anal insertion. In some Libellulidæ also, anal
leaflets occur.[79]
[78] We take these instances from Eaton, Monograph of Ephemeridæ,
Linn. Trans., 1883, p. 15.
[79] Charles Brongniart has lately described a fossil Insect from the
Coal Measures of Commentry, which he names _Corydaloides Scudderi_,
and refers to the Pseudo-Neuroptera. In this Insect every ring of the
abdomen carries laminæ, upon which the ramified tracheæ can still be
made out by the naked eye. Stigmata co-existed with these tracheal
gills. (Bull. Soc. Sci. Nat. de Rouen, 1885.)
Some Crustacea are furnished with respiratory leaflets, curiously
like those of Tracheates, with which, however, they have no genetic
connection. In Isopod Crustacea the exopodites of the anterior
abdominal segments often form opercula, which protect the succeeding
limbs. In the terrestrial Isopods, _Porcellio_ and _Armadillo_, these
opercula contain ramified air-tubes, which open externally, and
much resemble tracheæ. The anterior abdominal appendages of _Tylus_
are provided with air-chambers, each lodging brush-like bundles of
air-tubes, which open to the outer air. Lamellæ, projecting inwards
from the sides of the abdominal segments, incompletely cover in
the hinder part of the ventral surface of the abdomen, and protect
the modified appendages. (Milne Edwards, Hist. Nat. des Crustacés,
Vol. III.)
3. Tracheal gills never perfectly agree in position and number with the
stigmata throughout the body. Sometimes they occur on different rings,
sometimes on different parts of the same ring. Gegenbaur’s statements
on this point are incorrect.
4. Tracheal gills may co-exist with stigmata. In Perlidæ the tracheal
gills persist in the imago, and may be found, dry and functionless,
beneath the stigmata. In Trichoptera they gradually abort at successive
moults, and in some cases remain after the stigmata have opened.
5. Stigmata do not form by the breaking off of tracheal appendages, but
by the enlargement of rudimentary tracheal branches, which open into
the main longitudinal trunks. In larvæ with aquatic respiration these
branches exist, though they are not functional.
Palmén’s objections must be satisfactorily disposed of before
Gegenbaur’s explanation, interesting as it is, can be fully accepted.
Palmén has proved, what is on other grounds clear enough, that stigmata
are more ancient than tracheal gills, aerial tracheate respiration than
aquatic. But there is nothing as yet to contradict the view that the
first Insect-wings were adapted for propulsion in water, and that they
were respiratory organs before they became motor. It is Gegenbaur’s
explanation of the origin of stigmata, and not his explanation of the
origin of wings, which is refuted by Palmén.
_Abdomen._
In the abdomen of the female Cockroach eight terga (1–7; 10) are
externally visible. Two more (8, 9) are readily displayed by extending
the abdomen; they are ordinarily concealed beneath the seventh tergum.
The tenth tergum is notched in the middle of its posterior margin. A
pair of triangular “podical plates,” which lie on either side of the
anus, and towards the dorsal surface, have been provisionally regarded
by Prof. Huxley as the terga of an eleventh segment. Seven abdominal
sterna (1–7) are externally visible. The first is quite rudimentary,
and consists of a transversely oval plate; the second is irregular
and imperfectly chitinised in front; the seventh is large, and its
hinder part, which is boat-shaped, is divided into lateral halves, for
facilitating the discharge of the large egg-capsule.
In the male Cockroach ten abdominal terga are visible without
dissection (fig. 33, p. 70), though the eighth and ninth are greatly
overlapped by the seventh. The tenth tergum is hardly notched. Nine
abdominal sterna are readily made out, the first being rudimentary, as
in the female. The eighth is narrower than the seventh, the ninth still
narrower, and largely concealed by the eighth; its covered anterior
part is thin and transparent, the exposed part denser. This forms the
extreme end of the body, except that the small sub-anal styles project
beyond it. The podical plates resemble those of the female.
Pleural elements are developed in the form of narrow stigmatic plates,
with the free edge directed backwards. These lie between the terga and
sterna, and defend the spiracle.[80]
[80] Gerstaecker has found in the two first abdominal segments of
_Corydia carunculigera_ (_Blattariæ_) pleural appendages, which
are hollow and capable of protrusion. They have no relation to the
stigmata, which are present in the same segments, and their function
is quite unknown. See Arch. f. Naturg., 1861, p. 107.
The modifications of the hindmost abdominal segments will be more fully
considered in connection with the reproductive organs.
[Illustration: Fig. 32.--Under side of Abdomen of Male and Female
Cockroach. × 4.]
The high number of abdominal segments found in the Cockroach (ten or
eleven) is characteristic of the lower orders of Insecta. It is never
exceeded; though in the more specialised orders, such as Lepidoptera
and Diptera, it may be reduced to nine, eight, or even seven. The
sessile abdomen of the Cockroach is primitive with respect to the
pedunculate abdomen found in such insects as Hymenoptera, where the
constricted and flexible waist stands in obvious relation to the
operations of stinging and boring, or to peculiar modes of oviposition.
The first abdominal segment, which is especially liable to dislocation
and alteration in Insects, occupies its theoretical position in the
Cockroach, though both tergum and sternum are reduced in size. The
sternum is often altogether wanting, while the tergum may unite with
the metathorax.
The externally visible appendages of the abdomen are the cerci and the
styles of the male Cockroach. The cerci are found in both sexes; they
are composed of sixteen rings each, and project beneath the edge of the
tenth tergum. They are capable of erection by special muscles, and are
supplied by large nerves.[81] The sub-anal styles are peculiar in their
insertion, being carried upon the sternum of their segment (the ninth).
[81] Jointed cerci are commonly found in Orthoptera (including
Pseudo-Neuroptera); in the Earwig they become modified and form the
forceps. The “caudal filaments” of _Apus_ are curiously like cerci.
The cerci are concealed in the American _Cryptocercus_, Scudd. (Fam.
_Panesthidæ_).
[Illustration: Fig. 33.--Profile of Male and Female Cockroach. × 4.]
The abdominal segments are never furnished with functional legs in
adult Insects, but representatives of the lost appendages are often met
with in larvæ. According to Bütschli,[82] all the abdominal segments
are provided with appendages in the embryo of the Bee, though they
disappear completely before hatching. Some Hymenopterous larvæ have as
many as eight pairs of abdominal appendages, Lepidopterous larvæ at
most five (3–6; 10).[83]
[82] Entw. der Biene. Zeits. f. wiss. Zool. Bd. XX. Or, see Balfour’s
Embryology, Vol. I., p. 338.
[83] From more recent observations it is probable that abdominal
appendages are usually present in the embryos of Orthoptera,
Coleoptera, Lepidoptera, and possibly Hymenoptera. The subject is
rapidly advancing, and more will be known very shortly.