CHAPTEE XXVII 



INSECTA (continued) 



Order VII. : Hymenopteka {continued) 



Tbnthredinidab (Saw-flies) 



These Hymenoptera are very numerous, and their recogni- 

 tion is economically important, for they are often the cause 

 of serious damage to cultivated plants and trees. Saw-flies 

 differ from all other Hymenoptera in having no constricted 

 "petiole" between "thorax" and "abdomen ";i also the 

 females possess at the end of the abdomen a pair of saw- 

 like structures, hidden when not in use, but capable of being 

 protruded to make an incision in the plant in which the 

 eggs are inserted. 



The larvae feed on plant tissues, and in appear- 



Larva/ ^^'^^ ^^^^ frequently much resemble Lepidopterous 

 caterpillars, and are often mistaken for such. 

 Usually, however, they can be readily distinguished by the 

 fact that, as well as the jointed thoracic appendages, they 

 have six to eight pairs of " prolegs " on the abdomen (Fig. 

 322), whilst true caterpillars have only five pairs at most 

 (Fig. 145). One pair of these prolegs in saw-flies is connected 

 with the fifth body-segment, a segment which is always destitute 

 of appendages in Lepidoptera ; also the prolegs are lacking 

 in hooks such as are found at the free tip of the correspond- 

 ing legs in caterpillars. On the head is one pair only of 

 ocelli, instead of several pairs as in caterpillars. 



Fig. 322 represents some stages in the life- 



Saw flv^ history of a Saw-fly that feeds on the leaves of the 

 pine. The greenish-yellow larva, a, is shown 



' See p. 362 for real distinction between thorax and abdomen. 

 430 



