297 ) 



XII. Notes on the value of the genitalia of insects as guides 

 in Phylogcny. By W. Wesche, F.R.M.S. Com- 

 muoicateJ by J. E. Collin, F.E.S. 



[Read June 3id, 1908.] 



The stiuly of organs such as genitalia or month parts 

 throuc^h a series of Orders has yielded much matter of 

 interest, and the value of the mouth in classification has 

 been fully recognised, A further investigation through 

 the families of an Order is still more interesting, but it is 

 a study beset with difficulties and pitfalls on every side. 

 The material is so vast that the anatomist must pick his 

 species, and suitable material is often not sufficiently 

 abundant, especially in specialised forms. Conclusions 

 derived from one form are obviously dangerous enough, 

 but conclusions derived from the study of a number may 

 be absolutely wrong when applied to a particular species. 

 I have in my mind a typical case, where a very careful 

 worker who had specialised on a particular family, denied 

 the presence of two-jointed palpi in that family. As a 

 matter of fact the vast majority of species are one-jointed, 

 but at least three, of which one is a very common species 

 in England, are two-jointed. This is the kind of danger 

 which besets the worker in insect anatomy, and should be 

 allowed for in estimating the value of his work. So if I 

 now attempt some generalisations, I do so conscious of the 

 difficulties of my endeavour. 



Since the year 1900 I have made studies of the mouth 

 parts and the genitalia of a number of species, and I 

 have paid special attention to those contained in the Order 

 Diptera. 



A more general study of various families, selecting in 

 each the most aberrant as well as the most widely dis- 

 tributed, or the most common species, shows that certain 

 families are more conservative in type than others. But 

 this conservatism may only apply to the mouth, while the 

 genitalia are more variable, as is the case in tlie Syrphidai ; 

 while in the huge group of the calyptrate Muscidio, as 

 long as the mouth remains unspecialised (a part ordinarily 



TRANS. ENT. SOC. LOND. 1908. — PART 11. (sEPT.) 



