Effects of Castrntioti iii Insects 381 



or two Xenos were present. The table shows that the average 

 number in all the infested wasps was about 2.4. These numbers 

 probably represent the few survivers of an originally much greater 

 number which had lived as larvae in the individual larval wasps. 

 Brues ('03) took as many as 31 larvae of X. pallidus of both sexes 

 from a single larva of the Texan P. annularis! 



2. Both sexes of the Xenos may occur in the same Polistes, but 

 when the number exceeds 4, the Xenos are all males. In only one 

 case did I find as many as 3 female Xenos in the same host; in 

 all other cases there were only one or two. In. 45 of the 251 infested 

 Polistes, or in nearly 18 per cent, Xenos of both sexes occurred. 

 Hence while there is undoubtedly a tendency, as Brues has observ- 

 ed ('03), for the sexes to be the same in the same host, this is so 

 far from being a general rule, that the sex of the parasite cannot 

 be supposed to be determined by its host. 



3. When more than one female Xenos is present in the same 

 Polistes, they are of the same size but each is smaller than the 

 females occurring singly in a wasp. 



4. When both sexes inhabit the same Polistes the heads of the 

 females protrude between the more posterior segments, whereas 

 the cephalic ends of the male puparia may protrude between any 

 of the segm.ents behind the first. The heads of the females there- 

 fore usually appear from under the posterior edges of the fourth 

 or fifth abdominal segments. This is obviously an adaptation to 

 the greater length of the female parasite, which has to lie stretched 

 out in the abdomen of its host and could not protrude its head 

 between the more anterior segments without bending its body. 

 Som.etimes both sexes protrude their heads side by side from under 

 the tergite or sternite of the same segment. Sometimes one sex 

 is on the dorsal, the other on the ventral side of the same wasp, 

 but protruding from the same segment. 



5. When the female Xenos protrudes its head between two ter- 

 gites, it lies with its ventral surface uppermost, i.e., its dorso-ven- 

 tral orientation is the reverse of that of its host: w^hen it protrudes 

 its head between two sternites, it lies with its ventral surface down- 

 ward. I.e., with the same dorso-ventral orientation as the wasp. 

 This is obviously an adaptation to copulation with the winged 



