ADAPTATIONS OF AQUATIC INSECTS 169 



of spiracles, but have also developed an adaptive, or closed (apneustic) 

 type, for utilizing air that is mixed with water. 



Through minor modifications of structure and habit, many holo- 

 pneustic insects have become fitted for an aquatic life. In these in- 

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

 from the surface of the water. Thus the backswimmer, Notonecta, 

 bears on its body a silvery film of air entangled in closely set hairs, 

 which exclude the water. The whirligig beetle, Gyrinus, descends 

 with a bubble of air at the end of the abdomen. Dytiscus and Hydro- 

 philus have each a capacious air-space between the elytra and the 

 abdomen, into which space the spiracles open. The water scorpions, 

 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. 232, A, r) projecting from the penultimate 

 abdominal segment; the pupa, however, bears a pair of respiratory 

 tubes on the back of the thorax (Fig. 232, 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. Similarly, in the dipterous larva, Bittacomorpha 

 clavipes (Fig. 175), the posterior segments of the abdomen are attenu- 

 ated to form a long respiratory tube. The larva of Donacia appears 

 to have no special adaptations 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 is, however, 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 (Blood gills) enable 

 these forms to obtain air from the water. May fly nymphs (Figs. 20, 

 A ; 170), with their ample waving gills, offer familiar examples of branch- 

 ial respiration. Tracheal gills are very diverse in form and situation, 

 occurring in a few species of May fly nymphs on the thorax or head, 



