\n\i'i \ i [OK -I \ i i< i 169 



mi It--, luit have al-o developed an adapt i\ c, or closed (a/'n< 

 type, t'nr utili/inu r air tliat is mixed with water. 



Through minor niodiln at ion- of >tru<ture and habit, many holo- 

 pnrustiV insert- have become lillcd lor an aquatic life. In tin- 

 Stances the insects have some means of can-yini: down a -upply of air 

 from the -urfacc of the water. Thus the ba< kswimmer. \oltmecta, 

 hear- on its body a silvery film of air entangled in closely -et hair-, 

 whieh exclude the water. The whirligig beetle, (tyrinus. de-ceiid- 

 with a bubble of air at the- end of the abdomen. Dyti^cns and Hydro- 

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

 abdomen, into which space the spiracles open. The water scorpion-. 

 \ef>a and J\<tnatra, 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 

 mai^ot (Eristalis)y 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, Bittacomor pha 

 chrcipcs (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 Agrionidas, 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- 

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

 taining trachea! branches or, rarely, only blood (Blood gills) enable 

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

 -1 : rjo). with their ample waving irjn s , offer familiar examples of branch- 

 ial respiration, Trachcal gills are very diverse in form and situation, 

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



