Intercrossing — Genus 



149 



the form of their male claspers (fig. 86 c, d^ e,f) ; these 

 differences however are not perfectly constant, and it 

 is evident that the two forms are still in process of 

 separation from each other. Similar differences have 

 been noticed in the races of various butterfly-species, 

 while two nearly related wasps, Vespa vulgaris and 

 V. gennattica, have been observed trying to effect 

 sexual union, but vainly, on account of the difference 

 in their genital armature. There can be no doubt 

 that intercross- 

 ing among in- 

 sects is largely 

 prevented by 

 this means, to 

 which the 

 name of " me- 

 chanical selec- 

 tion " has been 

 applied (92). 



Genus. — 1 1 

 is now neces- 

 sary to deal 

 with the more 

 comprehensive ^ ^^ «^ 



A' ' ' ' t- 



ai Visions 1 n t O p,^ ?,e.—A. Cicadaseptemdcdm, male ; c. d. its genital 



which species claspers; B. C. septetndccitn, var Cassinii, male; 



are grouped. 

 Returning to 

 the two butterflies mentioned in the early pages of 

 this chapter, as examples of distinct species, we 

 remember that the brown one is known as Polyom- 

 matus astrarche, the blue as Polyommatus icarus, the 

 names astrarche and Icarus being specific, serving, 

 that is, to distinguish the species to which they 

 are applied. But it will be noticed that the first 

 of the two names applied to each of these insects, 



e. f. its genital claspers. (A. B. natural size, c. d. 

 e. /. enlarged). From Marlatt (after Riley and 

 Hagen), Bull. 14 (n.s.) Div. Ent. U.S. Dep'.. Agr. 



