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 Tor 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 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 rill 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 pentamerous. 



Ovipositor 5 6-valved, the vagina darting 

 forth two retroserrulate spiculae. 



* VOL. III. p. 67. See above, p, 160. 



b Whoever consults De Geer ii. 941 . t. xxxiii./. 14, 15. t. xxxvi. 

 /. 27. and t. xxxix./. 7, 8, will be convinced that the metamorphosis 

 of Tenthredo L. is incomplete rather than obtected. 



e The Hymenoptera, 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. 



d See above, p. 350. 



