156 INTERNAL ANATOMY OF INSECTS. 
somewhat sigmoidal or resembling the letter S, while in 
that of other saw-flies it is cultriform with a concave 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*. In 
Proctotrupes Latr. they appear to be united so as to form 
a tube for the ovipositor, and are produced by a pro- 
longation of the last abdominal segment. The darts 
usually run in two grooves of the sheath, and at their 
apex are retroserrulate>. In some cases the sheath it- 
self is serrated‘. ‘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 triangu- 
lar plate towards the base, which prevents their being 
pushed out too far‘. 
In Szrex 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sitor is too long to be withdrawn within the abdomen, it 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.; 
but as it is infinitely more slender, when in repose it is 
rolled up spirally and concealed within the abdomen. 
It is the puncture of this minute organ that produces the 
curious galls formerly described to you*. But the most 
anomalous ovipositor in this Order appears to be that 
2 Prate XVI. Fie... > Ibid. 
* Reaum. v. 347. ¢. xlix.f.10.d,f. 4 See above, Vor. I. 450—. 
