HYMENOPTERA. I5 



greater number of their legs ; these varying from twenty to twenty-two, whereas 

 those of the Lepidoptera have but sixteen at most. They also differ by the shining 

 and almost naked skin, and the curious habit possessed by many of curling in the 

 posterior segments, raising them at the same time and depressing them with a 

 rhythmic movement. This action, which may be for the purpose of frightening 

 away foes, coupled with the melancholy -looking eyes, gives them a grotesque 

 appearance, not observable in the caterpillars of the Lepidoptera, save in a few 

 instances. When full grown, the majority of the larvaa leave the food-plant and 

 spin in or on the surface of the ground, or under dry leaves and moss, a barrel- 

 shaped cocoon in which they pass the winter, turning to a chrysalis only a short 



1, Sirexjuvencus, female larva, pupa (all of nat. size) ; 2, CORN SAW-FLY and larvae in the rye stalks ; 

 3, Pachymerus calcitrator, a wasp parasitic on the above ; 4, Larva and pupa of Cephus (enlarged). 



time before the perfect insect emerges. At least a thousand species are known, 

 though this is probably but a small moiety of those that exist. 



STEM SAW-FLIES, Family CEPHID^E. 



The larvae of these slender, delicate, armoured insects pass their lives in the 

 stems of plants or young shoots of trees ; and the adults are characterised by the saw 

 of the female being partially concealed by two integumental flaps. As an example 

 of the typical genus, we may take the corn saw-fly (Cephus pyymceus), of which 

 the perfect insect flies actively in the sunshine, flitting from blossom to blossom 

 among buttercups in May, and thence onwards through the summer. The larvae 

 cause serious damage on the Continent to rye crops, and more rarely in wheat fields, 

 where they crawl up and down within the stems, feeding on the delicate tissues. 

 When full fed, they construct a transparent cocoon in which to pass the winter, 



