Def. Metamorphosis varying. Larva a hexapod.
Wings four in most, and reticulated with numerous areolets.
Prothorax distinct.
Scapulæ and Parapleuræ parallel and oblique.
Tail of the female without a terebrant, or pungent multivalve ovipositor[1191].
6. Hymenoptera[1192] (Piezata F.). Mr. MacLeay considers Sirex L. as being osculant between the Order we are now entering upon and the Trichoptera, and Tenthredo, L. as belonging to the latter. He appears to ground this opinion chiefly upon a consideration of their larvæ and a slight difference in their ovipositor. As the Order, as settled by Linné, has always been deemed one of the most natural ones, and all the great Entomologists of the present æra have agreed with him in thinking it so; it seems to me that to prove them mistaken in this opinion, the question should have been discussed at more length, and that it requires arguments of more weight than any Mr. MacLeay has at present produced to set it aside. He appears in general to lay great stress upon an agreement in larvæ and the kind of metamorphosis; and I am ready to acknowledge that it forms a strong presumption in favour of any hypothesis of affinity between certain tribes. But when it is had recourse to as fundamental and infallible, I think it is pushed far beyond what it will bear, or is warrantable. I may be wrong; but in my apprehension, a striking agreement in their general structure in the perfect state, which is the acme of their nature, affords a much more satisfactory reason for keeping two tribes together, than any difference observable in their larvæ or metamorphosis, for separating them. Let any one compare the structure of these two tribes with the Trichoptera on one side, and the Hymenoptera on the other, and it will require but a glance to convince him of their greater affinity to the latter; and the simple inspection only of Jurine's plates of the wings of Hymenoptera is calculated to produce the same effect. With regard to their larvæ, the resemblance between the case-worms and the pseudo-caterpillars of the saw-flies seems to me very distant, and the numerous prolegs of the latter have scarcely a legitimate representative in the former. The larvæ of the genus Lyda lose the prolegs intirely, and in one species, which much resembles the vermiform larvæ of Hymenoptera, the real legs are so extremely short as to be scarcely discernible[1193]; so that it requires no great stretch of faith to believe that saw-flies or Sirices may exist in whose larvæ the legs disappear[1194]. But it is this very tribe, whose larvæ thus approach to those of the other Hymenoptera, in which Mr. MacLeay finds the greatest external resemblance to the Trichoptera[1195]. In fact the difference between the saw-flies and Siricidæ, and the remainder of the Hymenoptera, amounts to little more than what takes place in the Diptera Order between the Tipulidæ, Asilidæ, Muscidæ, &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[1196]: but in fact, in these last there are two spiculæ, answering to the two saws of Tenthredo, so that the vagina 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 Trichoptera extrude their eggs at once in a mass[1197]. I do not mean, however, that it should be inferred from what I have here said, that there is no tendency in the saw-flies towards a Trichopterous 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.
Def. Metamorphosis incomplete[1198].
Trophi in most not used for mastication[1199].