136 Annals Entomological Society of America [Vol. XI, 



to eleven. (Plates V, VI and VII). Here we have the only 

 instance of the association of one of the genitalia with terga 

 rather than with sterna, so far as the sclerites are concerned, 

 but the membrane forming the tenth and eleventh sterna is 

 also in close contact with the base of this appendage. Wheeler, 

 in his work on the embryo of Xiphidium, shows this appendage 

 attached to the eleventh somite. It seems likely that variation 

 in the form of these appendages accompanies a change in 

 function. Evidence of this may be noted in the varied cerci of 

 the Orthoptera, Neuroptera, Mecoptera, Trichoptera, and 

 Diptera. 



The main conclusion emphasized is the descent of the male 

 and female genitalia from the anlages of abdominal, pedal 

 extremities of a myriapod-like ancestor, as seen in the embryos 

 of insects. The stylets upon which so much emphasis has been 

 laid by investigators, mainly represented by Heymons, cer- 

 tainly have little place in the adult insect. That they are 

 present in the Thysanura seems to have little weight after one 

 has worked upon representatives not only of that order but of 

 many other groups, because even where such stylets are present 

 there are also present far more important structures, the above 

 mentioned genitalia. Even in Heymons's figures it is clear that 

 the two descended from entirely separate anlages, and the 

 interpretation that the genitalia descended from the anlages of 

 stylets seems entirely unnecessary, and in fact, in one case at 

 least, entirely wrong. 



That there are traces in the adult insect of ten, eleven, or 

 possibly twelve somites, would seem but natural considering the 

 probability of a far greater ancestral number, but only somites 

 eight, nine, ten, and eleven bear a direct relation to the genitalia. 



