ADAPTATIONS OF AQUATIC INSECTS 189 



terized by the presence 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 

 holopneustic insects have become fitted for an aquatic life. In 

 these instances the insects have some means of carrying down 

 a supply of air from the surface of the water. Thus Noto- 

 nccla 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. Dytiscus and Hy- 

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

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

 Rcinatra 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. 229, A, r) pro- 

 jecting from the penultimate abdominal segment; the pupa, 

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

 thorax (Fig. 229, 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. 172), the 

 posterior segments of the abdomen are attenuated 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 Agrionidse, whose 

 skin at first is thin enough to allow a direct aeration of the 

 blood. This cutaneous respiration is possible during the early 

 life of many aquatic species. 



