NATURAL SELECTION 277 



5 (46), 7 (15), 1 (7) genera and species respectively. From the 

 point of view of use only the tuft of bristles of the cardo of the 

 maxillae, the comb of bristles on the fore tarsi, the modifica- 

 tion of the sting-sheaths of the female, the structure of the 

 male genitalia and colour need be considered. None of the 

 other characters, as far as is known, bears any relation to the life 

 of these insects. 



Colour. — It is possible that in some species the colours give 

 warning of the powerful sting : in a few desert species the pale 

 yellowish colour may be protective : as a general rule, a few 

 principal types of colour-pattern are common to the majority 

 of species. The adaptive value of the colour-pattern of any 

 European species is at present very doubtful. 



Bristle-tuft on the maxilla. — This is found in the females (and, 

 in a less developed state, in the males) of the species of Deutera- 

 genia and Pseudagenia : according to Adlerz (1903, pp. 37-8), 

 these bristles are used to collect spiders' web, with which the 

 entrance to the nest is in part closed. 



Female sting-sheath. — This is considerably modified in the 

 parasitic species of the Ceropalinae. Adlerz (1902, 1903) has 

 shown that, unlike other Psammocharids, the female, by means 

 of it, conveys her eggs into the lung-book of a spider already a 

 prey of another species. 



Male genitalia and apical abdominal sternites. — The modifica- 

 tions of these probably provide the best specific characters in 

 the family, but it is not possible at present to relate the differ- 

 ences in the male structure to the corresponding differences in 

 the female. We shall return to this subject later (p. 296). 



Comb on the fore tarsi, especially of the female. — This structure 

 has a very interesting distribution amongst the species. It is 

 absent in the Ceropalinae (7 species) and Pepsinae (48) ; in 

 the Macromerinae it is absent in three genera (10) and present 

 in one (1) ; in the Psammocharinae it is absent in 10 species 

 of Psammochares, present in the four other genera (5) and 

 3 1 species of Psammochares ; in the Homonotinae it is absent 

 in five genera (13) and present in two genera (2). Thus the 

 comb is present in 39 out of 127 species, and occurs in three out 

 of five subfamilies. It is sometimes a generic character (Ctena- 

 genia, Macromerinae), sometimes only specific (Psammochares) ; 

 in the latter case it is impossible to draw a sharp line between 

 species with a very small comb and those without one at all. 



