Insects - Agricultural Entomology - page 8

of the body; the ganglia, however, control many
activities (such as movement of the appendages)
independent of the brain.
The alimentary tract is, essentially, a long,
often much modified, tube stretching from
the mouth to the anus. There are three main
sections: foregut, mid-gut and hindgut, located
mainly within the abdomen. The foregut in-
cludes a crop within which recently ingested food
accumulates. Digestion and absorption of nutri-
ment occurs within the mid-gut, whereas the
hindgut is concerned with the absorption of
water and the storage of waste material prior
to defaecation. The insect gut includes a large
number of long, whitish, blind-ending tubules
(Malpighian tubules), which arise from between
the mid- and the hindgut; these tubules collect
waste material from the body fluids and pass
them into the gut. The haemocoel also contains
an often large organ, known as the fat body,
which forms whitish, yellowish or brownish
groups or layers of cells. The fat body is concen-
trated mainly in the abdomen and serves various
functions, including the synthesis and storage of
fat, glycogen (= carbohydrate) and protein.
The respiratory system includes a series of
small branching tubes (tracheae) and micro-
scopic tubules (tracheoles), which maintain con-
tact with the internal body organs and tissues.
The tracheal system may either be open or
closed. The former opens to the outside through
a series of valve-like pores (spiracles), which
occur along either side of the insect; the spiracles
are sometimes located on characteristic respira-
tory processes. Various types of respiratory
system are recognizable, including:

• apneustic - spiracles absent, i.e. tracheal
system closed (typical of aquatic insects which
breathe through gills);
• holopneustic - spiracles present on the
mesothorax, metathorax and abdominal seg-
ments 1-8 (typical of most adult insects and
many nymphs and larvae);
• metapneustic - spiracles present only on the
anal segment (typical of certain dipterous lar-
vae, including leatherjackets, mosquito larvae
and syrphid larvae);
• propneustic - spiracles present only on the
prothoracic segment (as in mosquito pupae).

Some insects are devoid of both spiracles and a
tracheal system (e.g. Collembola and larvae of
certain endoparasitoids); these forms are termed
atracheate.
In females, the reproductive system is com-
posed of a pair of ovaries, each subdivided into
numerous egg-forming tubules called ovarioles.
Other features include a pair of colleterial glands
(often called cement glands) and a sac-like
spermatheca in which, after mating, sperm is
stored. The ovaries unite to form a central ovi-
duct that opens to the outside through a genital
pore on the ninth abdominal segment. Eggs are
usually deposited through an ovipositor but in
some insects the tip of the female abdomen is
constricted into a tube-like oviscapt and an ovi-
positor is wanting. In some insects (e.g. bees and
wasps), the ovipositor has lost its egg-laying
function and, instead, serves as a sting. Male in-
sects possess two testes, each of which opens via
a long duct (vas deferens) into a seminal vesicle
in which sperm is stored. The seminal vesicles
(vesicula seminalis), along with a pair of acces-
sory glands, open into a single ejaculatory duct
which extends to a gonopore located on the
eighth abdominal segment.

• amphipneustic - spiracles present on the
prothoracic and anal segments only (typical of
many dipterous larvae);

DEVELOPMENT AND GROWTH

Insects usually develop to adulthood through
an egg and several pre-adult feeding stages
(instars), either as nymphs or as larvae. Vivi-

parous insects, however, give rise directly to
live nymphs or larvae and omit an egg
stage.