48 TRANSFORMATIONS OF INSECTS. 
do not always have spiracles, but they often breathe like fishes, 
by branchiz 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 
larvee of the May flies the branchiz 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- 
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THE AQUATIC LARVA AND NYMPH OF THE CADDIS FLY (PAryganea flavicornis), 
ments of the abdomen. The trachee 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 larve and nymphs of caddis 
flies, and have the same function and fate as the branchiz of the 
May fly larve. These branchie are not found in all aquatic larve, 
for many which do not become metamorphosed eventually into 
winged insects still continue to breathe air, although they live 
in the water. Such larve come to the surface to breathe every 
now and then, and they do so by the means of their swimming 
