(_Blatta_).
I, Fore-leg and pro-sternum (S) in front of which are the ventral
cervical sclerites (c).
cx, Coxa. tr, Trochanter.
fe, Thigh. tb, Shin.
ta, Tarsal segments.
II, Middle leg and mesosternum.
III, Hind-leg and metasternum.
In IIIA, the episternum (a) and epimeron (b) are slightly separated.]
_Wings._--Two pairs of wings are present in the vast majority of
insects, borne respectively on the mesothorax and metathorax. At the
base of the wing, i.e. its attachment to the trunk, we find a highly
complex series of small sclerites adapted for the varied movements
necessary for flight. Those of the dragon-flies (Odonata) have been
described in detail by R. von Lendenfeld (1881). The long axis of the
wings, when at rest, lies parallel to the body axis. In this position
the outer margin of the wing is the _costa_, the inner the _dorsum_,
and the hind-margin the _termen_. The angle between the costa and
termen is the _apex_. When the wing is spread, its long axis is more
or less at a right angle to the body axis. A wing is an outgrowth from
the dorsal and pleural regions of the thoracic segment that bears it,
and microscopic examination shows it to consist of a double layer of
cuticularized skin, the two layers being in contact except where they
are thickened and folded to form the firm tubular nervures, which
serve as a supporting framework for the wing membrane, enclose
air-tubes, and convey blood. These nervures consist of a series of
trunks radiating from the wing-base and usually branching as they
approach the wing-margins, the branches being often connected by short
transverse nervures, so that the wing-area is marked off into a number
of "cells" or areolets.
[Illustration: After Quail, _Natural Science_, vol. xiii., J. M. Dent
& Co.
FIG. 5.--Wing-Neuration in a Cossid Moth. 2, sub-costal; 3, radial; 4,
median; 5, cubital; 6, 7, 8, anal nervures.]
The details of the nervuration vary greatly in the different orders,
but J. H. Comstock and J. G. Needham have lately (1898-1899) shown
that a common arrangement underlies all, six series of longitudinal or
radiating nervures being present in the typical wing (see fig. 5).
Along the costa runs a costal nervure. This is followed by a
sub-costal which sometimes shows two main branches. Then comes the
radial--usually the most important nervure of the wing--typically with
five branches, and the median with four. These sets arise from a main
trunk towards the front region of the wing-base. From another hinder
trunk arise the two-branched cubital nervure and three separate anal
nervures. In the hind-wing of many insects the number of radial
branches becomes reduced, while the anal area is especially well
developed and undergoes a fan-like folding when the wings are closed.
Great diversity exists in the texture and functions of fore and
hind-wings in different insects; these differences are discussed in
the descriptions of the various orders. The wings often afford
secondary sexual characters, being not infrequently absent or reduced
in the female when well developed in the male (see fig. 6). Rarely the
male is the wingless sex.
In addition to the wings there are smaller dorsal outgrowths of the
thorax in many insects. Paired erectile plates (patagia) are borne on
the prothorax in moths, while in moths, sawflies, wasps, bees and
other insects there are small plates (tegulae)--see Fig. 3, t--on the
mesothorax at the base of the fore-wings.
_Abdomen._--In the abdominal exoskeleton the segmental structure is
very clearly marked, a series of sclerites--dorsal terga and abdominal
sterna--being connected by pale, feebly chitinized cuticle, so that
considerable freedom of movement between the segments is possible. The
first and second abdominal sterna are often suppressed or reduced, on
account of the strong development of the hind-legs. In many insects
ten, and in a few eleven, abdominal segments can be clearly
distinguished in addition to a small terminal anal segment. The female
genital opening usually lies between the seventh and eighth segments,
the male on the ninth. Prominent paired limbs are often borne on the
tenth segment, the elongate tail-feelers (cerci) of bristle-tails and
may-flies, or the forceps of earwigs, for example. In the Embiidae, a
family of Isoptera, it has been shown by G. Enderlein (1901) that
these cerci clearly belong to a partially suppressed eleventh segment,
and R. Heymons (1895-1896) has proved by embryological study that in
all cases they really belong to this eleventh segment, which in the
course of development becomes fused with the tenth. Smaller appendages
(such as the stylets of male cockroaches) may be carried on the ninth
segment. Pairs of processes carried on the eighth and ninth segments
often become specialized to form the ovipositor of the female (see
fig. 14) and the genital armature of the male. A marked modification
of the hinder abdominal segments may be noticed in most insects, the
sclerites of the eighth and ninth being frequently hidden by those of
the seventh. In the higher orders several of the hinder segments may
be altogether suppressed.
[Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve &
Co.
FIG. 6.--Outline of Male ([Male sign]) and Female ([Female sign])
Cockroaches (_Blatta_) from the side, showing Abdominal Segments
(numbered 1-10).]
[Illustration: From Miall and Denny (after Newton), _The Cockroach_,
Lovell Reeve & Co.
FIG. 7.--Brain of Cockroach from side. oe, Gullet; op, optic nerve;
sb, sub-oesophageal ganglion; mn, mx, mx', nerves to jaws; t,
tentorium.]