GENERAL CHARACTERS OF THE 

 COLLEMBOLA 



HEAD. Among the Collembola we have considerable dif- 

 ferences in the form of the head, from the elongate forms, where 

 it is borne almost horizontally, to the shorter-bodied Sminthur- 

 idae, in which it approximates the vertical position. The mouth - 

 parts are typically mandibulate, each mandible bearing usually 

 four or five teeth on its inner edge. That the "Entognatha" 

 could constitute a separate group merely by virtue of having the 

 mouth-parts withdrawn (?) into the head, has been proven fal- 

 lacious by embryology. Dr. J. W. Folsom, in his work on the 

 development of the mouth-parts in Anurida maritima Guer., has 

 shown that, developmentally, the mouth-parts are external as 

 in the Orthoptera and other pterygote insects ; but that, by a 

 downward folding of the genae, which unite ventrally with the 

 labium, the mouth-parts are pocketed and thus appear with- 

 drawn. 



The eyes are of a form, which, I think, has its counterpart 

 in no other order of the Hexapoda. Compound eyes are un- 

 known, and simple ocelli in the positions where they are borne 

 by other insects are likewise unknown. The two dorso-lateral 

 eye patches of pigment material, nearly always dark, are in the 

 normal position for compound eyes ; yet they bear simple ocelli, 

 and only a few of these. The normal number throughout most 

 of the genera appears to be sixteen, each eye spot containing 

 eight, which are usually arranged approximately in the form of 

 a letter S, with the lower (posterior) part straighter than the 

 upper part. The eye patch is commonly more or less convex, 

 especially so in the Sminthuridae, where it is quite biscuit-form, 

 with most of the eyes set round the edges. It is evident that the 

 group is degenerating in regard to the eyes at least, and it seems 

 not unreasonable to suppose that one of the stages to be passed 



