HEX APOD A 39 



by certain dipterous larvae of the family Syrphidoe, known as rat-tailed 

 maggots. 



Although there are many insects that live in the water and draw their 

 supply of air from above it, many aquatic insects breathe, as do fishes, 

 the air that is mixed with the water. This is accomplished by organs 

 known as tracheal gills. These are hair-like or more or less plate-like 

 expansions of the body-wall, abundantly supplied with tracheae (Fig. 73). 

 These tracheae divide and subdivide, and their terminations or fine 

 branches, tracheoles, are separated from the water that bathes the organ 

 only by its thin walls. In this way the air contained in the tracheae is 

 separated from the air in the water only by a delicate membrane, which 

 admits of the transfer of gases between them. It will be observed that 

 the difference between a tracheal gill and a true gill (as of fishes, Crus- 

 tacea, etc.) is that the true gill is supplied with vessels containing blood, 

 which is purified by being brought in contact with the air in the water, 

 while the tracheal gill is supplied with tracheae containing air to be puri- 

 fied. 



Tracheal gills are usually borne by the abdomen, sometimes by the 

 thorax, and in case of some stoneflies by the head. They pertain al- 

 most exclusively to the immature stages of insects; but stoneflies of the 

 genus Pteronarcys retain them throughout their existence. 



Tracheal gills vary greatly in form; in Corydalus they are hair-like, 

 and occur in tufts near the lateral margins of the abdominal segments; 

 in the caddice-worms they are thread-like, more or less branched, and 

 irregularly distributed over the surface of the abdomen; and in certain 

 damselflies they are in the form of large plate-like caudal appendages. 



( Fi g- 73-) . ... 



The reproductive organs. — The reproductive organs are situated in 



the abdomen, as represented in Figure 67. There is a set on each side 

 of the body; but the two sets usually open by a common tube near the 

 caudal end of the body. In the mayflies and in the earwigs, however, 

 the reproductive organs of each side have a distinct opening. Thus may- 

 flies are often found with two bunches of eggs projecting from the caudal 

 end of the body. 



All insects are developed from eggs; but there are some apparent 

 exceptions. Thus many flies retain their eggs until after they are 

 hatched, and in some flies the young attain a considerable development 

 before they are born. In the plant-lice (Aphtdidce) there is a remarkable 

 alternation of reproduction. This is described more fully in the account 

 of that family. 



The Subclasses and Orders of the Hexapoda 



The class, Hexapoda, is divided into two subclasses, the Apierygota 

 and the Pterygota. The primitive insects were undoubtedly wingless; 

 and there are two orders of living insects, the Thysanura and the Col- 

 lembola, which are still wingless. These orders constitute the subclass 

 Apterygota. All other insects are believed to have descended from 

 winged ancestors and are therefore placed in the subclass, Pterygota. 

 Some of the forms in this subclass, for example the bird-lice, the sucking 

 lice and the fleas have lost their wings during their sedentary parasitic 

 lives but this wingless condition is certainly an acquired one. 



