130 Mr. J. O. Westwood’s Memoirs 
In all the insects of these families which I have examined there 
is a pair of outer crustaceous plates (scabbards or valves) differing 
in size in different genera, all formed alike and arising very near 
to the insertion of the true ovipositor, each valve being composed 
of two joints ;* the first extending from the base to where the 
valve emerges from the abdomen and where the other joint (vary- 
ing very much in length) is united to it by a membrane. These 
plates, which are of equal size, are concave in the inside for the 
reception of the ovipositor. In the Tenthredinide they are smooth 
and rather hairy at the apex, but in Urocerus they are scabrose. 
With regard to the use of these valves, Mr. Marsham, in the volume 
of the * Linnzan Transactions,” has clearly proved that in Jchneu- 
mon manifestator they are only to be considered as the protectors 
of the ovipositor when at rest, and that they are not used in the 
great act of oviposition, but are thrown over the back and unem- 
ployed. 
With regard to the ovipositor itself of the saw-flies ( T’enthre- 
dinide), I cannot do better than give the following interesting 
extract from ‘“ Peck’s Natural History of the Slug Worm,” quoted 
by Kirby and Spence, (vol. iv. p. 154). He compares one of the 
saws and its sheath ‘ with the tenon-saw used by cabinet-makers, 
which, being made of a very thin plate of steel, is fitted with a 
back to prevent its bending. This back is a piece of iron, in which 
a narrow and deep groove is cut to receive the plate, which is fixed; 
the saw of the Zenthredo is also furnished with a back, but the 
groove is in the plate and receives a prominent ridge of the back, 
which is not fixed, but permits the saw to slide forward and back- 
ward as it is thrown out or retracted. The saw of artificers,” he 
adds, ‘‘ is single, but that of the Tenthredo is double, and consists 
of two distinct saws with their backs. The insect in using them 
first throws out one, and, while it is returning, pushes forward the 
other, and this alternate motion is continued till the incision is 
effected, when the two saws, receding from each other, conduct the 
egg between them into its place. In the artificial saw the teeth 
are alternately bent towards the sides or out of the right line, in 
order that the fissure or kerf may be made sufficiently wide for 
the blade to move easily. To answer this purpose in some 
measure in that of the Z'enthredo, the teeth are little twisted, so as 
processes rising from each side of the last segment of the abdomen. They are to 
be seen in the Tenthredinid@ and Ichneumonide, and also in Xiphydria and 
Urocerus, although much more minute in these two genera. 
* In Oryssus each is formed of three joints. (Latr. Gen. Crust. &c., vol. iii. 
p. 247.) 
