8o WHEELER. [Vol. VI 1 1. 



If this be the correct view, it follows that the dorsal body- 

 wall is formed in essentially the same manner in all insects — 

 by a growth and meeting of the germ-band edges. This pro- 

 cess is, therefore, remarkably simple and uniform compared 

 with the processes whereby the envelopes are eliminated. The 

 o-reat variability in the latter case has been dwelt on by Graber 

 ('88) in a paper devoted to dorsal-wall formation in the Insecta. 

 After reviewing all the literature on the subject and contrib- 

 uting many new facts, he proceeds to base a classification of 

 the insects hitherto studied, on the "Keimhiillenzustande." 

 He finds some fault with the current classification on the 

 ground that insects which systematists regard as closely re- 

 lated often present great differences in their respective methods 

 of dorsal-wall formation, whereas remotely related insects often 

 agree very closely in this respect. Thus Lina and Hydro- 

 Philus differ more than Hydrophibis and CEcantJius in the 

 processes whereby the dorsal-wall is formed. In considering 

 Graber' s views, I may pass over the awkward and kakophonous 

 nomenclature which he has introduced, to what I regard as his 

 main error, viz. the superficial analysis of his subject. Graber's 

 term "Keimhiillenzustande," I take it, includes the formation 

 of the envelopes as well as their condition preceding and 

 during their elimination. Now I have attempted to show that 

 there is nothing in the formation of the envelopes nor in the 

 concomitant anatrepsis of the germ-band in the different insect 

 orders to conflict with the current classification. Nor is there 

 anything in the closure of the dorsal-wall in different groups 

 — restricting this term to the confluence of the pleural edges 

 of the germ-band — to support Graber's conclusion. His state- 

 ment must therefore be restricted to the elimination processes. 

 That these are highly variable must be admitted, but they are 

 probably of very little taxonomic value, as Graber would prob- 

 ably have observed, had he attempted to account for the wide 

 differences in allied forms and the agreement of remotely re- 

 lated species. It is my opinion that this high degree of varia- 

 bility in the elimination process is to be traced to the same 

 causes as the variability of the indusium, viz., the rudimental 

 character of the envelopes. Up to the close of the diapause 



