CADDIS-WOKMS AND THEIR METAMORPHOSES. 
289 
order Trichoptera : — Wings four, membranous ; the anterior 
generally pilose, with branching nerves ; the posterior larger, 
and folded when at rest ; prothorax very short ; tibise with long 
calcaria at the tip, and often beyond the middle of the limb in 
the four posterior legs ; mouth unfitted for mastication ; man- 
dibles rudimental ; larva hexapod, ordinarily residing in a case 
formed of various materials, in which it retains its station by 
means of two hooked anal processes ; pupa incomplete, inactive 
during the greater period of its existence. 
The larvae of those species which live in movable cases, fix 
them to some substance just before they assume the pupa con- 
dition ; the two open extremities are then closed with a network 
fence, of a form differing according to the species. Food is now 
no longer required ; respiration is effected by means of currents 
of water passing through the gratings. Eeaumur asserts that he 
distinctly saw this gratework to move in alternate motion from 
convex to concave, as the water passed out and in. In some 
cases the pupaB are contained within a double envelope, the 
inner one being a thin membranous covering, the outer one 
being made of small stones. Fig. 5 b represents one of these 
inner cases, drawn from a specimen common in brooks and 
streams ; it is the RhyacopMla vulgaris. The pupae at the close 
of this condition are very like the perfect insects. Like the larvae, 
they are provided with external respiratory filaments ; but we 
may notice a few brownish patches on some of the dorsal seg- 
ments of the abdomen. These, under a microscope, show 
recurved hooks, and no doubt their function is to assist the 
pupae in escaping from their cases. “ The pupae of the larger 
species creep out of the water, crawling up the stems of plants, 
&c., and undergo their final change in the air ; but the smaller 
ones merely come to the surface, where they shed their pupae 
skin in the same manner as gnats, their old envelope serving 
them as a raft.” 
All the larvae of the PJiryganeidw are aquatic ; most of them 
pass this stage of their existence in movable cases of various 
materials and forms ; but others are enclosed in non-movable 
cases. The materials out of which the different cases are 
constructed are bits of stone, sand, wood, grass, leaves, the 
tenanted and untenanted shells of various fluviatile molluscs. 
The cases are, for the most part, cylindrical, and open at each 
end. The fragments of stick, small angular pebbles, &c., 
which the larvae use in the construction of their habitations, 
are held and cemented by means of a sticky secretion, drawn 
out in the form of long delicate silky threads spun from the 
mouth, as in caterpillars. Of these important organs I will 
speak by-and-by. Sometimes one meets with cases made of 
sand, having on either side long slender pieces of stick or rush : 
