384 SYSTEM OF INSECTS. 
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 Trichoptcra extrude 
their eggs at once in a mass a . 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 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. 
Def. Metamorphosis incomplete b . 
Trophi in most not used for mastication c . 
Wings four : neuration generally areolate d . 
Prothorax obsolete, giving place to an ample 
collar. 
Tarsi pen tarn erous. 
Ovipositor 5 — 6-valved, the vagina darting 
forth two retroserrulate spiculae. 
8 Vol. III. p. 67. See above, p. 160. 
b Whoever consults De Gecr ii. 941 — . t. xxxiii./. 14, 15. t. xxxvi. 
/. 27. and t. xxxix./. 7, 8, will be convinced that the metamorphosis 
of Tcnthredo L. is incomplete rather than obtectcd. 
e The Hymenoptertt, though they have all the usual oral organs, 
cannot be denominated masticators generally ; these organs, especi- 
ally the mandibles, being chiefly used in their economy. 
'' See above, p. 3wO. 
