108 ENTOMOLOGICAL NEWS. [May, 



published showing the frequency and character of these variations. 

 In my 244 specimens of Cynomyia americana 44, or 18 per 

 cent, show some variation. The variation is of three kinds: 1, 

 deficiency in size of a macrochaeta; 2, absence of a macrochaeta 

 normally present; 3, presence of a macrochaeta normally absent. 

 Two individuals show both the second and third kinds of varia- 

 tions, no others had more than one abnormality. Leaving out 

 these two we find that in the remaining 42 the abnormality was 

 unilateral in 23, bilateral in 19. When bilateral the correspond- 

 ing macrochastae on the two sides of the body were always the 

 ones concerned. 



There were eleven specimens in which there seemed to me to 

 be a deficiency in the size of a macrochaeta. The third anterior 

 dorso-central was always the one affected. In one male the ab- 

 normality was on the right side; in four males and one female on 

 the left side; in one male and four females on both sides. Five 

 of the specimens were unusually small: male 7 mm., male 7.5 

 mm., female 8 mm., male 9 mm. and male 9 mm. This macro- 

 chaeta is always the smallest of the dorso-centrals. The amount 

 of variation from the normal size differed much in the different 

 individuals and in some cases seemed to be compensated for by 

 the increased size of a macrochaeta on the cephalic surface of the 

 thorax which is nearly or quite in the line of the dorso-centrals. 

 Similar variation in the size of the third anterior dorso-central 

 occurs in the genera Calliphora and Lucilia. 



Absence of a macrochaeta normally present occurred in twelve 

 individuals (4.9 per centn Of course, it is possible that in some 

 cases the macrochaeta had been originally present and had been 

 broken off, but I was unable, with a magnifying power of twenty 

 diameters, to discover any indication that it had ever been pres- 

 ent. Two specimens which showed the presence of macrochaetae 

 normally absent as well as the absence of macrochaetae normally 

 present will be separately considered. The other cases that fall 

 under the present head were as follows : 



Third anterior dorso-central absent; one male and two females. 

 First left anterior dorso-central absent; one female. 

 Second right posterior achrostical absent; one male. 

 Second left posterior achrostical absent ; two males. 

 Second pair posterior achrostical absent ; three males. 





