SPECIPIO DIFFERENCES. 71 



forward as far as the catch, which passes between the 

 two branches and under the basal segment, thus re- 

 taining the whole organ in its place. 



Specific differences. 



The colouring varies greatly in different specimens, 

 though if a large number of individuals are compared 

 together a common plan can generally be detected. 

 All species founded on single specimens must, how- 

 ever, be regarded as unsatisfactory, and, indeed, in all 

 cases it is desirable to rely as much as possible on 

 differences of form. The shape of the saltatorial ap- 

 pendage, and of the feet, afford excellent specific cha- 

 racters in many genera. 



Among the parti- coloured species, the magnitude of 

 the differently coloured areas often differs greatly in 

 different specimens. Of this OrcheseUa cincta and 

 Isotoma trifasciata afford characteristic examples. 



Having examined a very large number of speci- 

 mens of most of the more common species, I have 

 found myself compelled to reduce considerably the 

 number of supposed species. Future researches, while 

 they will undoubtedly reveal new forms, will, I believe, 

 carry the process of condensation still further. Indeed, 

 I doubt whether there is any case of two species which 

 differ in colour only, inhabiting the same localities, and 

 using the same food. Such species, founded on 

 chromatic differences only, will therefore, I beheve, 

 eventually prove to be mere varieties. 



On the other hand, there are some cases in which 

 species essentially distinct are at first sight curiously 

 similar. In some instances these may almost be re- 

 garded as approximating to mimetic species. For 

 instance, Isotoma viatica (PI. XXXVI) and Lipura 

 corticina (PI. XLY), though belonging to different 

 families, are curiously similar as regards their dorsal 

 aspect. 



