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 off 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 (Aphidai) 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 

 erasing 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 

 sits free the mature insect, perfect in all its structural details. 



The immature stages of insects that present a complicated development of 



