LIBRARY

THE ANATOMY OF INSECTS 29
MUSCULAR SYSTEM
A considerable portion of the insect body is composed of muscular tissues
which may almost completely occupy the head and thoracic cavities, serving
as powerhouses for motivating especially the organs of ingestion and locomotion.
The muscles are whitish, grayish, pale yellowish, or rarely pinkish in
color and are distinctive in being cross-striated and therefore easily distinguished
from other body tissues. They are grouped in flat or strap-like fibers or
bundles which acL as small or large units. In the thoracic regions of winged insects
the muscles may be massed in compact lobes. The number of distinct
muscles is very great. Lyonet, in studying the European goat moth, distinguished
4,041 separate muscles which he figured with great care. The skeletal
muscles are mostly elongated, parallel fibers aUached to or united with t.he cuticle
by minute tonofibrilla; or cuticular fibrils. Certain groups of fibers may converge
into tendons at points of insertion at the apodemes. Visceral muscles may
be longitudinal, circular, or arranged in a network of branching fibers.
Appendages are usually operated by two sets of muscles, the abductor muscles
that open out or extend an appendage away [rom its basc, and the adductor
muscles which pull or draw the appendagc towards the base. The wings are
moved by the anterior and posterior pleural muscles operating on the sclerites to
which the bases of the wings are attached. These large muscles are only slightly
elongated in pulling the sclerites and vibrate with great rapidity to enable swift
and almost tireless lIight.
FAT BODY OR ADIPOSE TISSUE
Under the layer of muscular tissues and more or less surrounding the other
systems lies the fat body or adipose tissue, a defmite system derived from the
mesoderm, which is composed of large loose or tightly compressed cells which
are whitish, yellowish, orange, or greenish in color. It is composed not only of
fats but also of proteins and glycogen and constitutes the stored-up food reserves.
It may be present in the larval, pupal, and adult stages. This food material
is liberated into the blood and is available at any time in the growth of
the larVal and histogenesis of the pupre and during the life of the adult. In the
larvre it is often very extensive, especially in caterpillars, maggots, grubs, and
the larvre of hymenopterous forms of all kinds. In the immature forms of insects
that are without Malpighian tubules the fat body may act. as organs of
secretion and become loaded with urates. This condition may also occur in
certain fly pupre and in the larvre and pupre of insects with complete transformations,
in which case the urates finally discharge into the alimentary canal
upon transformation into the adult condition. Some of the fat body may be
transferred to the adults or may be stored after maturity has been attained. It
may then provide food reserves for hibernation or for egg production in the
females.