The outer cellular layer (ectoderm or "hypodermis") of insects as of
other Arthropods, secretes a chitinous cuticle which has to be
periodically shed and renewed during the growth of the animal. The
regions of this cuticle have a markedly segmental arrangement, and the
definite hardened pieces (sclerites) of the exoskeleton are in close
contact with one another along linear sutures, or are united by
regions of the cuticle which are less chitinous and more membranous,
so as to permit freedom of movement.
_Head._--The head-capsule of an insect (figs. 1, 2) is composed of a
number of sclerites firmly sutured together, so that the primitive
segmentation is masked. Above is the crown (_vertex_ or _epicranium_),
on which or on the "front" may be seated three simple eyes (ocelli).
Below this comes the front, and then the face or clypeus, to which a
very distinct upper lip (_labrum_) is usually jointed. Behind the
labrum arises a process--the _epipharynx_--which in some blood-sucking
insects becomes a formidable piercing-organ. On either side a variable
amount of convex area is occupied by the compound eye; in many insects
of acute sense and accurate flight these eyes are very large and
sub-globular, almost meeting on the middle line of the head. Below
each eye is a cheek area (_gena_), often divided into an anterior and
a posterior part, while a distinct chin-sclerite (_gula_) is often
developed behind the mouth.
[Illustration: From Miall and Denny, _The Cockroach_, Lovell Reeve &
Co.
FIG. 1.--Head and Jaws of Cockroach (_Blatta_). Magnified 10 times. A,
Front; B, side; C, back; v, vertex; f, frons; cl, clypeus; lbr,
labrum; oc, compound eye; ge, gena; mn, mandible; ca, st, pa, ga, la,
cardo, stipes, palp, galea, lacinia of first maxilla; sm, m, pa', pg,
sub-mentum, mentum, palp, galea of 2nd maxilla.]
_Feelers._--Most conspicuous among the appendages of the head are the
feelers or antennae, which correspond to the anterior feelers
(antennules) of Crustacea. In their simpler condition they are long
and many-jointed, the segments bearing numerous olfactory and tactile
nerve-endings. Elaboration in the form of the feelers, often a
secondary sexual character in male insects, may result from a distal
broadening of the segments, so that the appendage becomes serrate, or
from the development of processes bearing sensory organs, so that the
structure is pinnate or feather-like. On the other hand, the number of
segments may be reduced, certain of them often becoming highly
modified in form.
[Illustration: After Marlatt, _Entom. Bull._ 14, n. s. (U.S. Dept.
Agric.).
FIG. 2.--Head of Cicad, front view. Ia, frons; b, clypeus (the pointed
labrum beneath it); II, mandible; III, first maxilla; (a, base; b,
sheath; c, piercer), III', inner view of sheath; IV, second maxillae
forming rostrum (b, mentum; c, ligula).]
_Jaws._--The mandibles of the Hexapoda are usually strong jaws with
one or more teeth at the apex (fig. 1, A, B, mn), articulating at
their bases with the head-capsule by sub-globular condyles, and
provided with abductor and adductor muscles by means of which they can
be separated or drawn together so as to bite solid food, or seize
objects which have to be carried about. They never bear segmented
limbs (palps) and only exceptionally (as in the chafers) is the
skeleton composed of more than one sclerite. The mandibles often
furnish a good example of "secondary sexual characters," being more
strongly developed in the male than in the female of the same species.
In most insects that feed by suction the mandibles are modified. In
bugs (Heteroptera) and many flies, for example, they are changed into
needle-like piercers (fig. 2, II), while in moths and caddis-flies
they are reduced to mere vestiges or altogether suppressed.
As previously mentioned, a pair of minute jaws--the _maxillulae_--are
present in the lowest order of insects, between the mandibles and the
first maxillae. They usually consist of an inner and an outer lobe
arising from a basal piece, which bears also in some genera a small
palp (see APTERA).
In their typical state of development, the _first maxillae_ offer a
striking contrast to the mandibles, being composed of a two-segmented
basal piece (_cardo_ and _stipes_, fig. 1, C, ca, st) bearing a
distinct inner and outer lobe (_lacinia_ and _galea_, fig. 1, C, la,
ga) and externally a jointed limb or palp (fig. 1, C, pa). Such
maxillae are found in most biting insects. In insects whose mouths are
adapted for sucking and piercing, remarkable modifications may occur.
In many blood-sucking flies, for example, the galea is absent, while
the lacinia becomes a strong knife-like piercer and the palp is well
developed. In bugs and aphids the lacinia is a slender needle-like
piercer (fig. 2, III), while the palp is wanting. In butterflies and
moths the lacinia is absent while the galea becomes a flexible
process, grooved on its inner face, so as to make with its fellow a
hollow sucking-trunk, and the palp is usually very small.
The _second pair of maxillae_ are more or less completely fused
together to form what is known as the _labium_ or "lower lip." In
generalized biting insects, such as cockroaches and locusts
(Orthoptera), the parts of a typical maxilla can be easily recognized
in the labium. The fused cardines form a broad basal plate
(_sub-mentum_) and the stipites a smaller plate (_mentum_)--see fig.
1, C, sm, m--jointed on to the sub-mentum, while the galeae, laciniae
and palps remain distinct. In specialized biting insects, such as
beetles (Coleoptera), the labium tends to become a hard transverse
plate bearing the pair of palps, a median structure--known as the
_ligula_--formed of the conjoined laciniae, and a pair of small
rounded processes--the reduced galeae--often called the "paraglossae,"
a term better avoided since it has been applied also to the maxillulae
of Aptera, entirely different structures. The long sucking "tongue" of
bees is probably a modification of the ligula. In bugs and aphids
(Hemiptera), the fused second maxillae form a jointed grooved beak or
rostrum (fig. 2, IV) in which the slender piercers (mandibles and
first maxillae) work to and fro.
This second pair of maxillae (or labium) form then the hinder or lower
boundary of the mouth. In front or above the mouth is bounded by the
labrum, while the mandibles and first maxillae lie on either side of
it. A median process, known as the _hypopharynx_ or tongue, arises
from the floor of the mouth in front of the labium, and becomes most
variously developed or specialized in different insects. The salivary
duct opens on its hinder surface. It does not appear to represent a
pair of appendages, but the maxillulae of the Aptera become closely
associated with it. According to the view of R. Heymons, the
hypopharynx represents the sterna of all the jaw-bearing somites, but
other students consider that it belongs to the mandibular and first
maxillary segments, or entirely to the segment of the first maxillae.
_Neck._--The head is usually connected with the thorax by a distinct
membranous neck, strengthened in the more generalized orders with
small chitinous plates (_cervical sclerites_). These have been
interpreted as indicating one or more primitive segments between the
head and thorax. Probably, however, as suggested by T. H. Huxley
(_Anat. Invert. Animals_, 1877), they really belong to the labial
segment which has not become completely fused with the head-capsule.
It has been shown by C. Janet (1889), from careful studies of the
musculature, that the greater part of the head-capsule is built up of
the four anterior head-segments, the hindmost of which has the
mandibles for its appendages, and this conclusion is in the main
supported by the recent work on the head skeleton of J. H. Comstock
and C. Kochi (1902) and W. A. Riley (1904).
_Thorax._--The three segments which make up the thorax or fore-trunk
are known as the _prothorax_, _mesothorax_ and _metathorax_ (see fig.
3). The dorsal area of the prothorax is occupied by a single sclerite,
the _pronotum_ (fig. 3, d), which is large and conspicuous in those
insects, such as cockroaches, bugs (Heteroptera) and beetles, which
have the prothorax free--i.e. readily movable on the segment
(mesothorax) immediately behind--smaller and of less importance where
the prothorax is fixed to the mesothorax, as in bees and flies. The
dorsal area of the mesothorax, and also of the metathorax, may be made
up of a series of sclerites arranged one behind the
other--_prescutum_, _scutum_, _scutellum_ and _post-scutellum_ (fig.
3, e, f, g, h), the scutellum of the mesothorax being often especially
conspicuous. Ventrally, each segment of the thorax has a _sternum_
with which a median _pre-sternum_ and paired _episterna_ and _epimera_
are often associated (see figs. 3, 4). The recent suggestion of K. W.
Verhoeff (1904) that the hexapodan thorax in reality contains six
primitive segments is entirely without embryological support.
_Legs._--Each segment of the thorax carries a pair of legs. In most
insects the leg is built up of nine segments: (1) a broad triangular,
sub-globular, conical or cylindrical haunch (_coxa_); (2) a small
_trochanter_; (3) an elongate stout thigh (_femur_); (4) a more
slender shin (_tibia_); and (5-9) a foot consisting of five _tarsal
segments_. The fifth (distal) tarsal segment carries a median adhesive
pad--the _pulvillus_--on either side of which is a claw. The pulvillus
is probably to be regarded as a true terminal (tenth) segment of the
leg, while the claws are highly modified bristles. Numerous bristles
are usually present on the thighs, shins and feet of insects, some of
them so delicate as to be termed "hairs," others so stout and hard
that they are named "spines" or "spurs." In the relative development
and shape of the various segments of the leg there is almost endless
variety, dependent on the order to which the insect belongs, and the
special function--walking, running, climbing, digging or swimming--for
which the limb is adapted. The walking of insects has been carefully
studied by V. Graber (1877) and J. Demoor (1890), who find that the
legs are usually moved in two sets of three, the first and third legs
of one side moving with the second leg of the other. One tripod thus
affords a firm base of support while the legs of the other tripod are
brought forward to their new positions.
[Illustration: After Marlat, _Ent. Bull._ 3, n.s. (U.S. Dept. Agr.).
FIG. 3.--Thorax of Saw-Fly (_Pachynematus_).
I, Dorsal view.
II, Ventral view.
III, Lateral view.
IV, Lateral view with segments separated. _Prothorax_:
a, Episternum.
b, Sternum.
c, Coxa of fore-leg.
d, Pronotum. _Mesothorax_:
e, Prescutum.
f, Scutum.
g, Scutellum.
h, Post-scutellum.
i, Mesophragma.
j, _Epimeron_.
k, _Episternum_.
l, Coxa of middle leg. _Metathorax_:
m, Scutum.
o, Epimeron.
p, Coxa of hind leg.
n, _First Abdominal Segment_.
t, Tegula at base of fore-wing.]
[Illustration: After Miall and Denny, _The Cockroach_, Lovell Reeve &
Co.