ASSOCIATION OF CHROMOSOMES IN DIPTERA 241 



that the latter contains three large chromosomes (X and the 

 two largest sized autosomes), while the former (139) has only- 

 two large chromosomes but has an extra small member which 

 must be Y. No sufficiently clear first maturation divisions 

 have been found to show the X-Y relations of that stage, unless 

 the apparently single element projecting from the largest chromo- 

 some in figure 138 is the unmatched end of X. If so, one of 

 the smaller pairs is concealed. The figure is drawn just as it 

 appears, but I am not sure of its significance. 



Spogostylum simson (figs. 141, 142). No males of this species 

 were secured, but very clear figures were observed in ovarian 

 follicle cells. Two of these are given to indicate the similarity 

 between the pairing here and in the other species. Figure 141 

 is a metaphase plate showing the diploid group and the associa- 

 tion in pairs. Figure 142 illustrates a similar cell in prophase 

 with corresponding chromosomes forming closely united double 

 threads in the characteristic manner. As the figures indicate, 

 this group differs markedly from both species of Anthrax in 

 the size and form relations of its members. Apparently there 

 is no dominating type of chromosome group in the Bombyliidae 

 such as is seen in the majority of other families. 



Asilidae 



Twelve species of this family have been studied as indicated 

 in the synopsis (p. 217), but only a few of them need be 

 considered. Those chosen are selected particularly to illus- 

 trate the various numbers and sizes of the autosomes, and the 

 varying degrees of inequality of the sex-chromosomes. Pair- 

 ing is constant in all of them. 



Asihis sericeus (figs. 143-145). This species has perhaps the 

 most simple group found in the family, containing as it does only 

 five pairs of chromosomes, and lacking any conspicuous inequality 

 between the sex-chromosomes. Yet it is one of the most inter- 

 esting groups I have found, for each pair appears to differ from 

 all the rest in respect to size. The two large pairs are admittedly 



