ADAPTATIONS OF AQUATIC INSECTS 189g 
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- 
necta bears on its body a silvery film of air entangled in closely 
set hairs, which exclude the water. Gyrinius 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 
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. 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, 7), 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 trachez 
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 Agrionidz, whose 
skin at first is thin enough to allow a direct aération of the 
blood. This cutaneous respiration is possible during the early 
life of many aquatic species. 
