ANATOMY AND PHYSIOLOGY 135 
larve of aquatic habit, as Gyrinus and 
Cnemidotus, possess tracheal gills, as do 
also caterpillars of the genus Parapony+ 
(Fig. 171), which feed on the leaves of 
several kinds of water plants. 
Though manifold in form, tracheal 
gills are generally more or less foliaceous 
or filamentous, presenting always an ex- 
tensive respiratory surface; their integu- 
ment is thin and the tracheze spread 
closely beneath it. These adaptations 
are often supplemented by waving move- 
ments of the gills, as in May fly nymphs, 
and by frequent movements of the insect 
from one place to another. 
Especially noteworthy are the rectal 
tracheal gills of odonate nymphs. In 
these insects the lining of the rectum 
forms numerous papillz or lamellz, which 
Caterpillar of Para- 
ponyx obscuralis, to show 
tracheal gills. Length, 
15 mm.—After Harr. 
TGs 172) 
Larva of Bittaco- 
morpha clavipes, show- 
ing respiratory tube. 
Natural size.— After 
Hart. 
contain a profusion of delicate tracheal 
branches; these are bathed by water 
drawn into the rectum and then expelled, 
at rather irregular intervals. A similar 
rectal respiration occurs also in ephemerid 
nymphs and mosquito larve. 
A few forms, chiefly Perlide, are 
exceptional in retaining tracheal gills 
in the adult stage; in some imagines 
they are merely vestiges of the nymphal 
gills, but in others, such as Pteronarcys 
(Fig. 18), which habitually dips into 
the water and rests in moist situations, 
the gills probably supplement the spira- 
cles. Further details on the respiration 
of aquatic insects are given in Chapter 
aye 
