156 ENTOMOLOGY 



stances the insects have some means of carrying down a supply of air 

 from the surface of the water. Thus Xotonecta bears on its body a 

 silvery film of air entangled in closely set hairs, which exclude the water. 

 Gyrinus descends with a bubble of air at the end of the abdomen. Dy- 

 tiscus and Hydro philus have each a capacious air-space between the 

 elytra and the abdomen, into which space the spiracles open. Nepa 

 and Ranatra have each a long respiratory organ composed of two valves, 

 which lock together to form a tube that communicates with the single 

 pair of spiracles situated near the end of the abdomen. The mosquito 

 larva, hanging from the surface film, breathes through a cylindrical tube 

 (Fig. 230, A, r) projecting from the penultimate abdominal segment; 

 the pupa, however, bears a pair of respiratory tubes on the back of the 

 thorax (Fig. 230, B, r, r), which is now upward, probably in order to 

 facilitate the escape of the fly. The rat-tailed maggot (Eristalis), three 

 quarters of an inch long, has an extensile caudal tube seven times that 

 length, containing two tracheae terminating in spiracles, through which 

 air is brought down from above the mud in which the larva lives. Sim- 

 ilarly, in the dipterous larva, Bittacomor pha clampes (Fig. 173), the 

 posterior segments of the abdomen are attenuated to form a long re- 

 spiratory tube. The larva of Donacia appears to have no special ad- 

 aptations for aquatic respiration except a pair of spines near the end of 

 the body, for piercing air chambers in the roots of the aquatic plants 

 in which it dwells. 



The simplest kind of apneustic respiration occurs in aquatic nymphs 

 such as those of Ephemerida and Agrionidae, whose skin at first is thin 

 enough to allow a direct aeration of the blood. This cutaneous res- 

 piration is possible during the early life of many aquatic species. 



Branchial respiration, however, is the prevalent type among aquatic 

 nymphs and is perhaps the most important of their adaptive character- 

 istics. Thin-walled and extensive outgrowths of the integument, con- 

 taining tracheal branches or, rarely, only blood, enable these forms to 

 obtain air from the water. May fly nymphs (Figs. 19, A; 170), with 

 their ample waving gills, offer familiar examples of branchial respiration. 

 Tracheal gills are very diverse in form and situation, occurring in a few 

 species of May fly nymphs on the thorax or head, though commonly re- 

 stricted to the sides of the abdomen, where they occur in pairs or in 

 paired clusters (Fig. 19, A}. Caudal gills are found in agrionid nymphs 

 (Fig. 171). The aquatic caterpillars of Paraponyx (Fig. 172) are unique 

 among Lepidoptera in having gills, which are filamentous in this instance. 



Caddis worms, enclosed in their cases, maintain a current of water by 



