HYMENOPTERA. 373 



flies, are found to grow, and increase to twice their former size 

 after they are laid, probably by absorbing the sap of the plant 

 through their thin shells. 



Most of the larvae or young of the saw-flies strikingly resem- 

 ble caterpillars, being usually of a cylindrical form, of a greenish 

 color, and having several pairs of legs. Hence they are some- 

 times called false caterpillars. With the exception of such as 

 belong to the genera Lyda and Cephus, in which the legs are 

 only six, and the proplegs are entirely wanting, these false cater- 

 pillars have a greater number of legs than true caterpillars, being 

 provided with from eighteen to twenty-two ; but their proplegs 

 have not the numerous little hooks that arm those of caterpillars. 

 They have the means of spinning silk from their lower lips, but 

 not often in any great quantity. They are mostly naked and 

 without hairs ; a few have forked prickles on their backs ; some 

 are covered with a white flaky substance, that easily rubs ofT; 

 and others have a dark colored slimy skin, which has caused them 

 to be called slugs or slug- worms. They shed their skins about 

 four times, and, after the last moulting, often materially change in 

 appearance. Not only do these insects resemble caterpillars in 

 their forms, but they have nearly the same habits. They are 

 generally found on the leaves of plants, which they devour. 

 Many kinds are altogether solitary ; a few live together in swarms, 

 under silken webs, which they spin for a common place of shelter ; 

 others are found also in swarms, but without any webs over them, 

 and, when disturbed, they throw up their heads and tails, in a 

 very odd way ; some roll up leaves, and live in the hollow thus 

 formed, like the Tortrices ; others make portable cases of bits 

 of leaves, which they carry about on their backs, like the Tinese ; 

 certain kinds live within the stems of plants, and devour the pith ; 

 and wheat, in Europe, is said to suffer considerable injury from 

 internal feeders (Cephus pygmceus) of this kind. When fully 

 grown, most of them go into the ground, and enclose themselves 

 in thin silken cocoons, of an oblong oval shape, coated with 

 grains of earth. Some make much thicker cocoons, in texture 

 resembling parchment, and fasten them to the plants on which 

 they live, or conceal them in crevices, or under leaves and stones 

 on the ground. They generally remain for a long time unchanged 



