22 PROCEEDINGS ENTOMOLOGICAL SOCIETY 



Calliphora viridescens Desvoidy, a common blow fly referred by 

 some authors to the family Muscidse and by others to the Sarco- 

 phagidae is a species known colorationally from its North American 

 congeners by the black cheeks and beard, but structurally solely 

 by the presence of a third pair of posterior intraalar bristles. 



An opportunity of studying the relative constancy of this char- 

 acter was afforded during the year 1908 at Harrisburg, Pa., by 

 rearing it in considerable numbers from the putrid body of a snake. 

 No less than 540 full sized individuals, comprising three species, 

 having been reared from the body of the reptile which in life, 

 could not, have exceeded 24 inches in length. 



Of the number mentioned above 247 proved to be C. viridescens. 

 The entire series was examined individually and the results tabu- 

 lated, they are as follows: 



Number of specimens examined 247 



Number of males examined ' 131 



Number of females examined 116 



Number of males abnormally bristled 20 



Number of females abnormally bristled 32 



Total number of abnormal specimens 52 or over 20 per cent 



Number of males with supernumerary bristles 20 



Number of females with supernumerary bristles 30 



Number of females with less than 3 intraalars 2 



In nearly all cases where supernumerary intraalar bristles 

 occurred they were smaller than normal. In other words there 

 was an apparent tendency of the most cephalad towards obso- 

 lescence. In several cases 110 less than five macrochaetae replaced 

 the usual three, the more frequent number, however, was four 

 and the aberration was bisymmetrical or otherwise. Two indi- 

 viduals possessed but a single pair of posterior intraalars on each 

 side and could therefor not be distinguished structurally from 

 Calliphora erythrocephala. 



In no group of the Diptera have chsetotactic characters proven 

 of greater service than in that exceedingly large and homogeneous 

 assemblage designated in the works of the more conservative 

 authors as the family Tachinidae. 



Some nineteen years have elapsed since Dr. S. W. Williston 2 

 pointed out the remarkable structural variation to which the spe- 

 cies Belvosia bifasciata Fabr. is subject. He remarked the vari- 

 ation in the ciliation of the fascialia, comparative length of the 

 antennal joints, angularity of the fourth vein and in the male, of 

 the length of the anterior claws. So it is perhaps not surprising 

 that a corresponding variation is found in the number and ar- 

 rangement of the macrochaetae particularly of the thoracic region, 

 but also of the abdomen in this species. 



2 Insect Life, vol. v, p. 238. 



