Effects of Castration ui Insects 383 



sitization was very great. In one nest there were 86 wasps, 44 or 

 51 percent of which contained X. nigrescens. There were from one 

 to seven in each wasp (an average of 2.6 per host), and of the total 

 number of Xenos (94), 91 were males and only 3 females. In the 

 other nest there were 42 wasps, and 36 or more than 85 per cent 

 were stylopized. The total number of the parasites — ^in this case 

 X, pallidus — was 125 (81 males and 44 females); the highest num- 

 ber in a single wasp being 10, the average per host 3.6. 



Fuller consideration must be given to the effects of the stylopids 

 on their hosts. This may properly begin with a resume of the 

 excellent work of Perez (1886) who examined stylopized speci- 

 mens of 47 species of Andrena. The effects produced by Stylops 

 in these bees is so considerable as to render their specific deter- 

 mination difficult. This is not surprising perhaps, when we con- 

 sider the vast number of closely related species in the genus. All 

 the known specimens of certain "species" (F. Smith's Andrena 

 insolita, separata and victima) have been found to be stylopized, 

 which gives force to Perez's opinion that these are not true species 

 but merely parasitized individuals of forms that are already known 

 under other specific names. Perez describes minutely the fol- 

 lowing modifications as characteristic of stylopized Andrenae: 

 (l) The abdomen is shortened and swollen and therefore more 

 globular, the shortening being due to an attenuation of the termi- 

 nal segments. (2) The head is usually smaller than that of nor- 

 mal specimens. (3) The villosity of the abdomen is more 

 abundant, longer and more silky, especially on the terminal seg- 

 ments, and its color is often greatly altered, becoming lighter 

 and more reddish or fulvous. The villosity of the thorax may 

 undergo similar but less pronounced changes. (4) The puncta- 

 tion of the body becomes finer, denser and more superficial in 

 correlation with the pilosity, which arises from the punctures. 

 These changes are common to both sexes and therefore affect 

 specific characters. They give the specimens a peculiar pseudo- 

 specific facies. Perez therefore rightly warns against basing 

 new species of Andrena on stylopized individuals. 



The following changes affect the secondary sexual characters: 

 (i) The normal males of the genus Andrena, as in many other 



