﻿THE ENTOMOLOGIST. 



Vol. XXIIL] APRIL, 1890. [No. 828. 



ON AN ADDITIONAL AID TO THE DETERMINATION OP 

 CERTAIN BRITISH GYRINID.E. 



By James Edwards, F.E.S. 



In the case of a genus like Gyrinus, where so much difference 

 of opinion exists amongst authorities as to the value of the 

 specific characters hitherto in use, one experiences an amount of 

 gratification more than ordinary in calling attention to a structural 

 character comparatively easy of observation and practically 

 constant ; I allude to the contour of the median lobe of the 

 cedeagus. 



In dealing with the Homoptera-Cicadina I have long been 

 accustomed to make use of this character in cases where the 

 other characteristics available are difficult to appreciate or other- 

 wise unsatisfactory, and in certain groups I have frequently 

 succeeded by its means in definitely limiting species, which it was 

 otherwise almost impossible to characterize intelligibly. Under 

 these circumstances I, not unnaturally, place its value as a 

 differential specific character very high, mainly on account of the 

 infinitesimally small amount of variation which it presents in the 

 individuals of those assemblages of insects which we are accus- 

 tomed to call a species. Its value, however, is by no means 

 uniform, and each case must be judged entirely on its merits. 

 For instance, there are many species of Typhlocyba whose identity 

 may be decided from an inspection of this part alone ; while, on 

 the other hand, it would be impossible to refer the same organ 

 from several species of Limotettix to the species to which they 

 belonged, on account of their great similarity inter se ; and it is 

 worthy of remark that the latter insects would probably be readily 

 separable by differences in colour and facies, whilst the former 

 resemble one another so closely in those respects that they can 

 only be identified with certainty by a reference to the male 

 genitalia. These examples are taken from the Cicadina ; but so 



ENTOM. — APRIL, 1890. I 



