8 
INSECTS. 
and metathorax are usually attached a pair of wings, which are very characteristic 
organs of all the higher insects, although absent in the lowest forms, and in many 
species degenerate through parasitic habits. The wings differ much in structure, 
thickness, clothing, etc., in different orders of insects, but in all cases they seem 
to consist of an upper and a lower membranous layer, traversed by narrow bands 
of thicker material, the nervures. 
The abdomen in insects is marked oft from the thorax by the absence of true 
appendages. It may consist of as many as ten distinct segments, but never of 
more, and generally of fewer. Each segment is protected above by a dorsal plate, 
or tergum, and below by a ventral plate, or sternum, the two being connected 
laterally by membrane. The last segment is often provided with a pair of 
appendicular structures, which may be long, many-jointed, and antenniform, or 
short and one-jointed, like the pincers of an earwig. And, in addition to these, 
certain other structures, such as the stings of bees and wasps, and the ovipositors 
of locusts and ichneumon flies, are frequently connected with the hinder segments 
of the abdomen. The only other external structures that need be mentioned here 
are the stigmata, or apertures, of the respiratory organs. These pierce the lateral 
surfaces of the thoracic and abdominal segments, and vary much in number, size, 
and form, being generally far more plainly seen in the larvae than in the adults. 
There may be as many as eleven pairs, but usually the number falls short of this. 
In exceptional cases, as in the plant-lice (. Aphidce ) belonging to the order 
Hemiptera, and in certain parasitic flies of the group Pupipara, the young are born 
in an advanced stage of development, the eggs developing within the body of the 
parent without being first deposited. But in the vast majority of species the 
young make their first appearance in the world in the egg-stage. 
Between the time of its escape from the egg-shell and the attainment of 
maturity, the young undergoes a succession of moults, or castings of the skin. In 
some cases the change of structure that an insect presents during the course of its 
growth is, comparatively speaking, trifling, the young being hatched in a condition 
in which in outward form it substantially resembles the parent in everything but 
size, and, in the case of species that bear wings in the adult, in the entire absence 
of these organs. A familiar instance of this method of growth is found in the 
cockroaches and grasshoppers, in which the young emerge from the egg as 
miniature and wingless copies of their parents. 
In other cases, however, as in the flies (Diptera) and butterflies (Lepidoptera), 
an extraordinary change of form takes place during growth, the young upon 
hatching being so totally unlike the adult that no one unacquainted with the facts 
of insect development would suppose the two to belong to the same category of 
animals. In these two orders, as well as in some others, the new-born young 
has the appearance of a fleshy grub; and the grub-like condition is retained 
unchanged, except in size, until the time for the last moult approaches. It then 
undergoes a startling change of condition, and, losing its organs of sense and 
ceasing to feed, passes into a state of quiescence, during which the final changes 
in its organisation are more or less rapidly passed through, and the final moult 
sets free the mature insect, perfect in all its structural details. 
The immature stages of insects that present a complicated development of 
