190 ENTOMOLOGY 
Branchial respiration, however, is the prevalent type among 
aquatic nymphs and is perhaps the most important of their 
adaptive characteristics. Thin-walled and extensive out- 
growths of the integument, containing tracheal branches or, 
rarely, only blood, enable these forms to obtain air from the 
water. May fly nymphs (Figs. 19, 4; 169), with their ample 
waving gills, offer familiar examples of branchial respiration. 
Tracheal gills are very diverse in form and situation, occurring 
ina 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, 4). Cazdal gills are 
found in agrionid nymphs (Fig. 170). 
The aquatic caterpillars of Paraponyx 
(Fig. 171) are unique among Lepidop- 
tera in having gills, which are filamentous 
in this instance. 
Caddis worms, enclosed in their cases, 
maintain a current of water by means 
of undulatory movements of the body,. 
Simulium; A, larva; B, and the larvee and pupze of most black flies 
pupa, showing respira- Tr ae oS ; 
tore Slaiente. (Simuliidz, Fig. 230) secure a continuous 
supply of fresh air simply by fastening 
themselves to rocks in swiftly flowing streams. 
Rectal respiration is highly developed in odonate and ephe- 
merid nymphs. In these, the rectum is lined with thousands 
of tracheal branches, which are bathed by water drawn in from 
behind, and then expelled. 
All these kinds of respiration—cutaneous, branchial and 
occur in young ephemerid nymphs; while mosquito 
rectal 
larvee have in addition spiracular respiration. 
With the arrival of imaginal life, tracheal gills disappear, 
except in Perlidz, and even in these insects the gills are of 
little if any use. 
Marine Insects.—Except along the shore, the sea is almost 
