ASSOCIATION OF CHROMOSOMES IN DIPTERA 243 



Dasyllis thoracica (figs. 155-158). D. thoracica furnishes evi- 

 dence very similar to that presented by Asilus sericeus. No 

 two of its five pairs of chromosomes (fig. 156) appear to be the 

 same size. The smallest and next smallest pairs are very dis- 

 tinct, as is also the largest. Possible confusion arises then, 

 only in connection with the two intermediate pairs, but since 

 one of these appears to be the X-Y pair its dimorphism, if the 

 apparent dimorphism is real, serves to differentiate it from 

 the other intermediate pair. I have been unable to obtain 

 sufficient spermatogonia! figm*es to determine definitely the 

 sex-chromosome relations, but evidence from the first sperma- 

 tocyte divisions makes it probable th,at the relations shown in 

 figure 156 are correct. In the first spermatocytes (fig. 157), 

 one of the intermediate pairs (corresponding to XY in figure 156) 

 appears to have a univalent attachment (X in the figures) at 

 one end, which strongly suggests the unpaired end of an X- 

 chromosome. Analysis of the first spermatocyte group (fig. 

 157) then, reveals one small spherical chromosome (1), one small, 

 elongate chromosome (2), one larger, symmetrical chromosome 

 (3), one similar, but asymmetrical chromosome (4), and one 

 largest chromosome (5), each distinct from all of the others. 

 In the diploid group each of these is represented by a pair of 

 chromosomes. A diploid group showing the intimately paired 

 association in prophase, similar to that in the Muscidae, etc. 

 is given in figure 155. A second spermatocyte, haploid group 

 is shown in figure 158. The seeming duality of the largest 

 chromosome here is simply due to the metaphase split, and is 

 not related to the apparent sex-chromosome dimorphism of 

 the first division- 



Deromyia winthemi (figs. 159-164). The six pairs of chromo- 

 somes in this species (figs. 159-161) are graduated into four 

 sizes, of which the largest and smallest are represented by one 

 pair each, and the two intermediates by two pairs each. In 

 some figures (161, 164) even these intermediates appear to be 

 individually differentiated, but the distinctions are not great. 

 The sex-chromosomes (X and Y) are very dissimilar, and, as 

 shown by the figures, are more often dissociated than are homol- 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 21, NO. 2 



