512 



HYMENOPTERA 



delicate, membranous, depressed spaces, in front of each of which 

 there stands up a flap of membrane. The function of the cenchri 

 is quite unknown. The median pkte is fastened to the hind 

 margin of the metanotum, and looks quite like one of the dorsal 

 plates of the following abdominal segments, from which, however, 

 it is separated by a more or 'less conspicuous membrane. In 

 the majority of the Tenthredinidae the median plate is divided 

 along the middle, but in the Cimbicides this is not the case. 

 The mesosternum is very large, and the metasternum small, 

 so that the middle and hinder pairs of coxae are placed close 

 together. The abdomen consists of nine segments, there being 

 eight dorsal plates in addition to the median plate, and seven 

 ventral plates besides the terminal armature. There is a pair of 

 short cerci, each of a single segment. The trochanters are 

 divided ; each tibia bears two spurs at the extremity, and the 

 tarsi are 5 -jointed. 



The most characteristic and interesting of the structures with 

 which the Insects of this family are provided is the apparatus 

 from which the name of sawfly is derived. As long as two 



centuries ago these instru- 

 ments excited the admira- 

 tion of Yallisnieri and of 

 Keaumur, who described 

 them at length ; and it is 

 truly astonishing that any 

 part of a living being 

 should be changed into 

 tools so mechanically per- 

 fect as these saws are 

 (Fig. 344). They serve 

 the pui*pose of assisting 

 the female in depositing 

 tlie eggs in a suitable 

 situation, the place selected 

 being frequently the ten- 

 der stems of shrubs or 



Fig. 344. Saws of Gimhex sylvarum. A, The 

 pair spread out and placed in a horizontal posi- 

 tion ; a, the lower margin of the saw proper ; other plants, Or the in 

 h, the upper margin of the support : B, 

 teeth of the saw more highly magnified. 



terior 



course possessed only by the female. 



of leaves. These 

 rgans are therefore of 

 They are placed on the 



