408 INSECTS INJURIOUS TO VEGETATION. 



The female saw-flies use these ingeniously contrived tools 

 to saw little slits in the stems and leaves of plants, wherein 

 they afterwards drop their eggs. Some, it appears, lay their 

 eggs in fruits ; for ]\L\ Westwood discovered their young within 

 apples that had fallen from the trees before they had grown to 

 the size of walnuts. The wounds made in plants by some 

 kinds of saw-flies swell, and produce galls or knobs, that serve 

 for habitations and for food to their young. The eggs, them- 

 selves, of all these flies, are found to grow, and increase to 

 twice their former size after they are laid, probably by absorb- 

 ing the sap of the plant through their thin shells. 



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

 semble caterpillars, being usually of a cylindrical form, of a 

 greenish color, and having several pairs of legs. Hence they 

 are sometimes 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 caterpillars 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 spin- 

 ning silk from their lower lips, but not often in any great 

 quantity. They are mostly naked and without hairs; a few 

 have forked pricldes on their backs ; some are covered with a 

 white flaky substance, that easily rubs ofl"; 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 appear- 

 ance. Not only do these insects resemble caterpillars in their 

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

 erally 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 



