THE ZOOLOGICAL POSITION OF THE COCKROACH.
Sub-kingdom ARTHROPODA.
Class I. Crustacea.
" II. Arachnida.
" III. Myriopoda.
" IV. Insecta.
Order 1. Thysanura.
" 2. _Orthoptera._
" 3. Neuroptera.
" 4. Hemiptera.
" 5. Coleoptera.
" 6. Diptera.
" 7. Lepidoptera.
" 8. Hymenoptera.
The place of the Cockroach in the Animal Kingdom is illustrated by the
above table. It belongs to the sub-kingdom Arthropoda, to the class
Insecta, and to the order Orthoptera.
_Characters of Arthropoda._
Arthropoda are in general readily distinguished from other animals by
their jointed body and limbs. In many Annelids the body is ringed, and
each segment bears a pair of appendages, but these appendages are soft,
and never articulated. The integument of an Arthropod is stiffened
by a deposit of the tough, elastic substance known as Chitin, which
resembles horn in appearance, though very different in its chemical
composition. In marine Arthropoda, as well as in many Myriopoda and
Insects, additional firmness may be gained by the incorporation of
carbonate and phosphate of lime with the chitin. However rigid the
integument may be, it is rendered compatible with energetic movements
by its unequal thickening. Along defined, usually transverse lines
it remains thin, the chitinous layer, though perfectly continuous,
becoming extremely flexible, and allowing a certain amount of
deflection or retraction (fig. 1). The joints of the trunk and limbs
may thus resemble stiff tubes. Muscles are attached to their inner
surface, and are therefore enclosed by the system of levers upon which
they act (fig. 2B). In Vertebrate animals, on the contrary, which
possess a true internal skeleton, the muscles clothe the levers (bones)
to which they are attached (fig. 2A). The whole outer surface of an
Arthropod, including the eyes, auditory membrane (if there is one),
and surface-hairs, is chitinised. Chitin may also stiffen the larger
tendons, internal ridges and partitions, and the lining membrane of
extensive internal cavities, such as the alimentary canal, and the
air-tubes of Insects.
[Illustration: Fig. 1.--Diagram of Arthropod limb extended, retracted,
and flexed. Graber has given a similar figure (Insekten, fig. 8).]
[Illustration: Fig. 2.--Vertebrate and Arthropod joints. A, Vertebrate
joint, the skeleton clothed with muscles. B, Arthropod joint, the
skeleton enclosing the muscles.]
In most Arthropoda the body is provided with many appendages. In
Crustacea there are often twenty pairs, but some Myriopoda have not far
from two hundred pairs. Some of these may be converted to very peculiar
functions; in particular, several pairs adjacent to the mouth are
usually appropriated to mastication. One or more pairs of appendages
are often transformed into antennæ.
The relative position of the chief organs of the body, viz.:--heart,
nerve-cord, and alimentary canal, is constant in Arthropoda. The heart
is dorsal, the nerve-cord ventral, the alimentary canal intermediate.
(See fig. 3.) The œsophagus passes between the connectives of the
nerve-cord. Not a few other animals, such as Annelids and Mollusca,
exhibit the same arrangement.
Arthropoda are not known to be ciliated in any part of the body, or in
any stage of growth. Another histological peculiarity, not quite so
universal, is the striation of the muscular fibres throughout the body.
In many Invertebrates there are no striated muscles at all, while in
Vertebrates only voluntary muscles, as a rule, are striated.
The circulatory organs of Arthropoda vary greatly in plan and degree of
complication, but there is never a completely closed circulation.
The development of Arthropoda may be accompanied by striking
metamorphosis, _e.g._, in many marine Crustacea, but, as in other
animals, the terrestrial and fluviatile forms usually develop directly.
Even in Insects, which appear to contradict this rule flatly, the
exception is more apparent than real. The Insect emerges from the egg
as a fully formed larva, and so far its development is direct. It is
the full-grown larva, however, which corresponds most nearly to the
adult Myriopod, while the pupa and imago are stages peculiar to the
Insect. It is not by any process of embryonic development, but by a
secondary metamorphosis of the adult that the Insect acquires the power
of flight necessary for the deposit of eggs in a new site.
[Illustration: Fig. 3.--Longitudinal section of Female Cockroach, to
show the position of the principal organs. _Oe_, œsophagus; _S.gl_,
salivary gland; _S.r_, salivary reservoir; _Cr_, crop; _G_, gizzard;
_St_, chylific stomach; _R_, rectum; _Ht_, heart; _N.C_, nerve-cord. ×
7.]
_Characters of Insects._
Insects are distinguished from other Arthropoda by the arrangement
of the segments of the body into three plainly marked regions--head,
thorax, and abdomen; by the three pairs of ambulatory legs carried
upon the thorax; by the single pair of antennæ; and by the tracheal
respiration. Myriopods and Arachnida have no distinct thorax. Most
Crustacea have two pairs of antennæ, while in Arachnida antennæ are
wanting altogether. Crustacea, if they possess special respiratory
organs at all, have branchiæ (gills) in place of tracheæ (air-tubes).
In Arachnida, Myriopoda, and Crustacea there are usually more than
three pairs of ambulatory legs in the adult.
The appendages of an Insect’s head (antennæ, mandibles, maxillæ) are
appropriated to special senses, or to the operations of feeding,
and have lost that obvious correspondence with walking legs which
they still retain in some lower Arthropoda (_Peripatus_, _Limulus_,
_Arachnida_). The thorax consists of three[8] segments, each of which
carries a pair of ambulatory legs. No abdominal legs are found in any
adult insect. The middle thoracic segment may carry a pair of wings or
wing-covers, and the third segment a pair of wings.
[8] In some Insects there are traces of a fourth thoracic segment.
The lower or less-specialised Insects, such as the Cockroach, have
nearly as many nerve-ganglia as segments, and the longitudinal
connectives of the nerve-cord are double. In the adult of certain
higher Insects[9] (_e.g._, many Coleoptera, and some Diptera) the
nerve-ganglia are concentrated, reduced in number, and restricted to
the head and thorax; while all the connectives, except those of the
œsophageal ring, may be outwardly single.
[9] So also in some larvæ (_Calandra_, _Œstrus_, &c.).
The heart, or dorsal vessel, is subdivided by constrictions into a
series of chambers, from which an aorta passes forwards to the head.
Air is usually taken into the body by stigmata or breathing-pores,[10]
which lie along the sides of the thorax and abdomen. It circulates
through repeatedly-branching tracheal tubes, whose lining is
strengthened by a spiral coil. Air-sacs (dilated portions of the
air-tubes) occur in Insects of powerful flight.
[10] In some aquatic Insects the exchange of gases is effected by
“pseudobranchiæ,” and the tracheal system is closed.
The generative organs are placed near the hinder end of the body.[11]
Most Insects are oviparous.[12] The sexes are always distinct; but
imperfect females (“neuters”) occur in some kinds of social Insects.
Agamogenesis (reproduction by unfertilised eggs) is not uncommon.
[11] Dragon-flies have the male copulatory apparatus, but not the
genital aperture, in the fore part of the abdomen.
[12] Aphis and Cecidomyia are at times viviparous, and a viviparous
Moth has been observed by Fritz Müller (Trans. Entom. Soc. Lond.,
1883).
_Orders of Insects._
The orders of Insects are usually defined with reference to the
degree of metamorphosis and the structure of the parts of the mouth.
Five of the orders (3, 5–8) in the table on page 9 undergo complete
metamorphosis, and during the time of most rapid change the insect
is motionless. In the remaining orders (1, 2, 4) there is either no
metamorphosis (_Thysanura_), or it is incomplete--_i.e._, the insect
is active in all stages of growth. Among these three orders we readily
distinguish the minute and wingless Thysanura. Two orders remain,
in which the adult is commonly provided with wings; of these, the
Orthoptera have biting jaws, the Hemiptera, jaws adapted for piercing
and sucking.
The name of Black Beetle, often given to the Cockroach, is therefore
technically wrong. True Beetles have a resting or chrysalis stage, and
may further be recognised in the adult state by the dense wing-covers,
meeting along a straight line down the middle of the back, and by the
transversely folded wings. Cockroaches have no resting stage, the
wing-covers overlap, and the wings fold up fan-wise.
_Further Definition of Cockroaches._
In the large order of Orthoptera, which includes Earwigs, Praying
Insects, Walking Sticks, Grasshoppers, Locusts, Crickets, White Ants,
Day-flies, and Dragon-flies, the family of Cockroaches is defined as
follows:--
Family _Blattina_. Body usually depressed, oval. Pronotum shield-like.
Legs adapted for running only. Wing-covers usually leathery, opaque,
overlapping (if well developed) when at rest, anal area defined by
a furrow (fig. 4). Head declivent, or sloped backwards, retractile
beneath the pronotum. Eyes large, ocelli rudimentary, usually two,
antennæ long and slender.
[Illustration: Fig. 4.--Generalised sketch of Cockroach wing-cover.]
About eight hundred species of Cockroaches have been defined, and to
facilitate their arrangement, three groups have been proposed, under
which the different genera are ranked.[13]
Group 1. Both sexes wingless (_Polyzosteria_).
Group 2. Males winged, females wingless (_Perisphæria_, _Heterogamia_).
Group 3. Both sexes with more or less developed wings (about 7 genera).
[13] For descriptions of the species Fischer’s Orthoptera Europæa
(1853) or Brunner von Wattenwyl’s Nouveau Système des Blattaires
(1865) may be consulted. The classification adopted by the last-named
author is here summarised.
BLATTARIÆ.
A.--Femora spinous (_Spinosæ_).
Fam. 1.--_Ectobidæ._ Seventh abdominal sternum undivided in female.
Sub-anal styles absent in male. Wings with triangular apical area.
_Ectobia_, including _E. lapponica_ (_Blatta_) and other genera.
Fam. 2.--_Phyllodromidæ._ Seventh abdominal sternum undivided
in female. Sub-anal styles usual in male (0 or rudimentary
in _Phyllodromia_). Wings without triangular apical area.
_Phyllodromia_, including _P. germanica_ (_Blatta_) and other
genera.
Fam. 3.--_Epilampridæ._
Fam. 4.--_Periplanetidæ._ Seventh abdominal sternum divided in
female. Sub-anal styles conspicuous in male. _Polyzosteria_,
_Periplaneta_, &c.
B.--Femora not spinous (_Muticæ_).
Families.--_Chorisoneuridæ_, _Panchloridæ_, _Perisphæridæ_,
_Corydidæ_, _Heterogamidæ_, _Blaberidæ_, _Panesthidæ_.
Many useful references will be found in Scudder’s Catalogue of N.
American Orthoptera, Smiths. Misc. Coll., viii. (1868).
In Group 3 occur the only two genera which we shall find it necessary
to describe--viz., _Blatta_, which includes the European Cockroaches,
and _Periplaneta_, to which belong the Cockroaches of tropical Asia and
America.
Genus _Blatta_. A pulvillus between the claws of the feet. The
seventh sternum of the abdomen entire in both sexes; sub-anal styles
rudimentary in the male.
Genus _Periplaneta_. Readily distinguished from Blatta by the divided
seventh abdominal sternum of the female, and the sub-anal styles of
the male.
Two species of Periplaneta have been introduced into Europe. These are--
1. _P. orientalis_ (Common Cockroach, Black Beetle). Wing-covers and
wings not reaching the end of the abdomen in the male; rudimentary in
the female.
2. _P. americana_ (American Cockroach). Wing-covers and wings longer
than the body in both sexes.