48 
TRANSFORMA TIONS OF INSECTS. 
do not always have spiracles, but they often breathe like fishes, 
by branchiae or gills. 
The imago or adult insect, which is produced by metamor- 
phosis from the water larva and nymph, becomes an air breather 
— and spiracles are developed in its sides exactly in the places 
where the gills were attached during its fish-like life. In the 
larvae of the May flies the branchiae are formed of expansions of 
the skin, which are very delicate, thin, and variously folded and 
fringed, and they are attached in pairs to the first seven seg- 
THE AQUATIC LARVA AND NYMPH OF THE CADDIS FLY ( Phryganea flavicOMis). 
ments of the abdomen. The tracheae are included in the folds 
and are continued into the body of the larva, and they transmit 
the purified air to it; but the gills disappear during metamorphosis. 
Some packets of filaments of thin tissue derived from the 
skin occupy the same position in the larvae and nymphs of caddis 
flies, and have the same function and fate as the branchiae of the 
May fly larvae. These branchiae are not found in all aquatic larvae, 
for many which do not become metamorphosed eventually into 
winged insects still continue to breathe air, although they live 
in the water. Such larvae come to the surface to breathe every 
now and then, and they do so by the means of their swimming 
