94 ON THE ORIGIN AND [CHAP. 



menoptera in the Bees and Ants ; among Diptera 

 it is general. Among Trichoptera the larva early 

 acquires three pairs of legs, but as Zaddach has 

 shown, 1 there is a stage, though it is quickly passed 

 through, in which the divisions of the body are indi- 

 cated, but no trace of legs is yet present. Indeed, 

 there appear to be reasons for considering that while 

 among Crustacea the appendages appear before the 

 segments, in Insects the segments precede the ap- 

 pendages, although this stage of development is very 

 transitory, and apparently, in some cases, altogether 

 suppressed. I say u apparently/' because, as I have 

 already mentioned, I am not yet satisfied that it will 

 not eventually be found to be so in all cases. 

 Zaddach, in his careful observations of the em- 

 bryology of Phryganea, only once found a specimen 

 in this stage, which also, according to the researches 

 of Huxley, 2 seems to be little more than indicated 

 in Aphis. It is therefore possible that in other cases, 

 when no such stage has been observed, it not really 

 may be absent, but, from its transitoriness, may have 

 hitherto escaped attention. 



Fritz Miiller has expressed the opinion 3 that this 

 vermiform type is of comparatively recent origin. He 

 says : " The ancient insects approached more nearly to 

 the existing Orthoptera, and perhaps to the wingless 

 Blattidae, than to any other order, and the^ complete 

 metamorphosis of the Beetles, Lepidoptera, &c., is of 

 later origin." " There were/' he adds, " perfect insects 



1 Unters. lib. die Entwick, und den Bau der Gliederthiere, p. 73. 



2 Linnean Transactions, v. xxii. 



3 Facts for Darwin, trans, by Dallas, p. 118. See also Darwin, 

 " Origin of Species," p. 530. 4th ed. 



