26 HYMENOPTERA CHAP. 



in colonies. Great difficulties attend their study on account of 

 several points in their economy, such as, that the sexes are 

 different, and frequently not found together ; also that there may 

 be two generations of a species in one year, these being more or 

 less different from one another. Another considerable difficulty 

 arises from the fact that these bees are subject to the attacks of 

 the parasite Stylops, by which their form is more or less altered. 

 These Insects feed in the body of the bee in such a way as to 

 affect its nutrition without destroying its life ; hence they offer a 

 means of making experiments that may throw valuable light on 

 obscure physiological questions. Among the effects they produce 

 in the condition of the imago bee we may mention the enfeeble- 

 ment of the sexual distinction, so that a stylopised male bee 

 becomes less different than it usually is from the female, and a 

 stylopised female may be ill developed and less different than 

 usual from the male. The colours and hair are sometimes altered, 

 and distortion of portions of the abdominal region of the bee are 

 very common. Further particulars as to these parasites will be 

 found at the end of our account of Coleoptera (p. 298). We may 

 here remark that these Stylo2is are not the only parasitic Insects 

 that live in the bodies of Andrenidae without killing their hosts, 

 or even interrupting their metamorphoses. Mr. K. C. L. Perkins 



recently captured a specimen of Halictus 

 rubicundus,from which he, judging from the 

 appearance of the example, anticipated that 

 a Stylops would emerge ; but instead of this 

 a Dipterous Insect of the family Chloropidae 

 appeared. Dufour in 1837 called attention 

 to a remarkable relation existing between 

 Andrena aterrima&nd a parasitic Dipterous 

 larva. The larva takes up a position in 

 the interior of the bee's body so as to be 

 partly included in one of the great trachea! 

 FIG. 13. Parasitic Dipterous vesicles at the base of the abdomen ; and 

 larva in connection with th bee th maintains the parasite in its 



trachea! system of An- _ r 



drena aterrima. (After position, and at the same time supplies it 



with air by causing two tracheae to grow 



into its body. Dufour states that he demonstrated the continuity 

 of the tracheae of the two organisms, but it is by no means clear 

 that the continuity was initially due to the bee's organisation. 



