162 INTERNAL ANATOMY OF INSECTS. 
in the Cicadce, in the saw itself a . In the genus Cimbexy 
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, with 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 b . 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sitor is too long to be withdrawn within the abdomen, it 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. c Ibid. 
d Reaum. v. 347. t. xlix./. 10. d,f. c See above, Vol. III. 390. a. 
