LIBRARY

14
COLLEGE ENTOMOLOGY
the cuticle and is the dried, more or less indestructible portion of insects preserved
in collections.
Structure and comJ)osition oj the cuticle.
Epicuticle -- the very thin outer layer, not more than 1 J,L thick, somewhaL
amber in color; impermeable.
Exocuticle - rather thick middle layer; colorless.
Endocuticle - thickest inner layer; colorless or partly amber; furnishes
rigidity to the walls.
Composition. - These various layers are composed of from 20 to 50 per
cent chitin, a nitrogen polysaccharide, together with proteins and other
complex nonchitinous substances.
Renewal. - The discontinuous growth of insects is due to the fact t.hat the
exoskeleton is neither sufficiently renewed and extended nor clastic
enough to accommodate for long the growing bodies of the immature
insect, although it is rarely altered during mature stages. The entire
outer integumen1.. is cast off from time to time in a process called molt1:ng
or ecdysis. The cast cuticle or skin is the exuvice. The new exoskeleton is
secreted by the epidermal (hypodermal) cells, a single layer of large cells
just beneath the cuticle. In this renewal process a considerable amount
of the chitin of the cuticle is absorbed before the skin is cast, retained
until after molting, and utilized in the formation of the new integument.
Endoskeleton. - There are innumerable illgrowths from the cuticle for
muscle attachments which are called apodemes. Groups of these are
variously known as the tentorium or apodemes of the head; phragrnata,
apociemes of the dorsal plates or tergites; lateral apodemes, apodcmes of
the side plates or pleurites; and jUTcm, apodemes of the ventral plates or
sternites. These structures constitute the endoskeleton and are of value
in the classification of certain groups of insects.
Segmentation. - The divisions or segmentation of the insect body in all
the growing and adult stages are a distinctive characteristic of these
small animals. In fact, the word insect is derived from insectum, referring
to the incisions of the body. In the larvre and pupre the segmentation,
though often distinct, is not so evident as in the adult with its three important
body regions: head, thorax, and abdomen.
Head. - The somewhat oval or globular head is generally known as the
head capsule because of its hard rigidity and toughness. It is thought to have
been derived from no less than six primary segments and is composed of a
number of sclerites separated by sutures. It supports organs of sight and sense
- the eyes and antenna: - and of feeding - the mouth parts.
The sclerites derived from primitive segments are (1) ocular, (2) antennal,
(3) intercalary, (4) mandibular, (5) maxillary, (6) labial.
Epicranial suture. - This important suture is a prominent line in the form of
an inverted "Y" arising from the neck region as a single stem or median suture
and pranching on the vertex towards the bases of the antennre. It divides