SYSTEM OF INSECTS. 375 
tween the Tipulide, Asilide, Muscide, &c., in which 
also the metamorphosis differs. 
Another argument upon which Mr. MacLeay seems 
to lay some stress, is taken from the number of parts 
into which the ovipositor of the saw-flies is resolvable, 
which he finds to consist of four pieces ; while in what he 
considers as the genuine Hymenoptera, it; is formed only 
of three*: but in fact, in these last there are two spicule, 
answering to the two saws of Tenthredo, so that the va- 
gina in which these move may be considered as a double 
sheath: only, as these were to be pushed out at the same 
time, and the others alternately, it was necessary that in 
the latter each sheath should be separate, to admit of this 
motion; but as to its composition, the weapon in both is 
essentially the same. At any rate this structure could 
furnish a reason only for the formation of a separate 
group in the same Order, but none for the transfer of 
such group to another, which had no such instrument 
at all; since, as we have seen, the 77ichoptera extrude 
their eggs at once ina mass®. I do not mean, however, 
that it should be inferred from what I have here said, 
that there is no ¢endency in the saw-flies towards a Tri- 
chopterous type, for in them nature seems pointing that 
way, but the distance is too great, and the number of 
types of form necessary to fill up the interval too many, 
to warrant in my opinion their removal from the one 
Order to the other. 
Der. Metamorphosis incomplete‘. 
* Hor. Entomolog. 429. » Vor. III. p. 67. See above, 
p- 155. © Whoever consults De Geer ii. 
941—. ¢. xxiii. f. 14, 15. é. xxxvi. f. 27. and ¢. xxxix. f- 7, 8, will 
be convinced that the metamorphosis of T'enthredo L. is incomplete 
rather than obfected. 
