Vol. xxvii] 



ENTOMOLOGICAL NEWS. 



327 



which is an appendange of the ventral side of abdominal seg- 



ment two consists usually of three segments and two, usually 



unchitinized folds. Segment I is the basal or proximal, slen- 



der, heavily chitinized shaft. Segment 2 is the moderately 



chitinized median segment, while segment 3 is the apical or 



distal segment which folds 



forward on the median 



segment and is usually un- 



chitinized. The median 



segment ordinarily carries 



across its ventral surface a 



fleshy ridge or fold, the 



inner fold, and across its 



posterior end the terminal 



fold. This latter is prob- 



ably erectile in many Spe- 



Internal fold. 

 Terminal fold 



Diagram showing the parts usually found 



in the z >'^P teroi 



cies. Either or both fleshy 

 folds may be lacking and in the Legion Lestes segment 3 is 

 much reduced. The differences between penes are usually in 

 the shape of the distal or third segment. 



I have introduced this present series of papers with this 

 one on Acanthagrion to have it appear in conjunction with a 

 paper on some species of the same genus by Mr. Williamson. 



The study of the penes in this group at once cleared up the 

 hitherto obscure relations of the various forms which had 

 variously been termed varieties and subspecies. Those that 

 have been studied are, as far as the penis is concerned, good 

 species. Mr. Williamson and I were both much surprised 

 when we found the great structural differences existing in the 

 penes of the so-called "subspecies" (ablittuin, ascendens, ;;//'- 

 warwm 4 ), of yracile. The penes of the thirteen species of 

 Acanthagrion I have been able to examine vary more among 

 themselves in form than those of any similar group of closely 

 related species of Zygoptera. If I had seen the penes only, I 



4 The minantin referred to by Mr. Kennedy is the species described 

 by me (anted, p. 314) as kennedii. It was determined independently by 

 Dr. Calvert and myself as niinarum, but is probably not the ininnrnin 

 of de Selys. E. B. WILLIAMSON. 



