162' INTERNAL ANATOMY OF INSECTS. 



in the Cicada, in the saw itself a . In the genus Cimbex, 

 belonging to the same tribe, the saw differs in shape, 

 being somewhat sigmoidal or resembling the letter S, 

 while in that of other saw-flies it is cultriform with a con- 

 cave edge : other minor differences distinguish them, 

 which need not be particularized. 



A similar structure, Vith regard to the organ in ques- 

 tion, obtains in the rest of the Hymenoptera, even those 

 that use it as a weapon of offence ; but the backs of the 

 saws in them, composed of a single piece, become a 

 sheath for the darts. The valves, however, vary. In most 

 of those with an exerted sting, as Pimpla, they are linear, 

 exerted, and as long as the aculeus itself 5 . In Procto- 

 trupes they appear to be united so as to form a tube for 

 the ovipositor, and are produced by a prolongation of 

 the last abdominal segment. The darts usually run in 

 two grooves of the sheath, and at their apex are retro- 

 serrulate c . In some cases the sheath itself is serrated d . 

 The shanks of the darts are connected with the valves ; 

 so that when these open they are pushed out : sometimes 

 on their outer side they have a triangular plate towards 

 the base, which prevents their being pushed out too far e . 



In Sirex and many ichneumons, in which the ovipo- 

 sitor is too long to be withdrawn within the abdomen, it 

 remains always exerted ; but in general it is retracted 

 within that part when unemployed. In the gall-fly (Cy~ 

 nips) this instrument is really as long as in Pimpla, &c. ; 



* Valisn. Esperienz. $c. Musca de Rosai. Reaum. v. 100 . 

 De Geer ii. 916. The last writer thought he saw in the back of 

 the saw itself a longitudinal cavity (918), which applied to the groove 

 would form an open canal. 



b PLATE XVI. FIG. 1. e Ibid. 



d Reaum. v. 347. t. xlix./. 10. </,/. See above, VOL. III. 390. a. 



