2 9 ;o THE JOINTED ANIMALS 



pieces. The dorsal areas of the three segments are termed the pronotum, meso- 

 notum, and metanotum; the lateral regions the pleurae; and the inferior regions 

 the sterna. To the pleurae are articulated the three pairs of legs, each of which 

 consists primarily of five segments, named respectively, from the base to the apex, 

 coxa, trochanter, femur, tibia, and tarsus; the last, which constitutes the foot, being 

 generally tipped with two claws, and subdivided into several often as many as five 

 smaller segments. To the sides of the upper surface of both the meso- and meta- 

 thorax 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 de- 

 generate 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 con- 

 nected 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 (Aphidtz) 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 eggshell and the attainment of matu- 

 rity, the young undergoes a succession of molts, or castings of the skin. In some 

 case's 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 grass- 

 hoppers, 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 (L,epidoptera), 

 an extraordinary change of form takes place during growth, the young upon hatch- 

 ing being so totally unlike the adult that no one unacquainted with the facts of in- 



