ASSOCIATION OF CHROMOSOMES IN DIPTERA 221 



pupae could be debermined by the flies which subsequently 

 hatched from the food. In no case is there any question as 

 to the genus of the flies concerned and only in a few cases is 

 the species doubtful. Such cases are mentioned in the text. 

 Of the families Asihdae, Bombyliidae, Syrphidae, Sapromyzi- 

 dae, Ortalidae and Trypetidae, only adult flies were used. 



For the identification of the Sarcophagidae, the writer is 

 indebted to Mr. R. R. Parker, for that of Culex pipiens to Mr. 

 Fred. Knab, for that of the Drosophilidae to Dr. A. H. Sturte- 

 vant,^ and for all other identifications to Mr. C. W. Johnson 

 who has very kindly examined a large series of specimens. 



REALITY OF CHROMOSOME PAIRING IN THE DIPTERA 



Since Stevens' observations on chromosome pairing in the 

 Diptera were more or less incidental to other features, and since 

 her conclusions have been directly opposed by those of Taylor 

 and of Lomen on Culex — material upon which part of Stevens' 

 work was based — it seems desirable first of all to ascertain 

 definitely whether or not the so-called pairing phenomena in 

 flies do in reality represent the association of independent chromo- 

 somes. In the opinion of Taylor ('14) and of Lomen ('14) the 

 duality of the chromatic elements in Culex (and hence by in- 

 ference in the other Diptera), is due, not to a pairing of two 

 chromosomes but to the precocious splitting of one. Hence 

 they conclude that the haploid number is present in both germinal 

 and somatic cells, and that the somatic divisions are essentially 

 the same as the maturation divisions. According to their 

 idea each chromosome divides in anaphase, giving rise to two 

 daughter chromosomes which remain separated during the rest- 

 ing stage and prophase (thus simulating a pair), and go to 

 opposite poles in the succeeding division. 



Before considering the contentions of Taylor and of Lomen 

 further, I will present some of the evidence that has led me to 

 conclude that the double chromatic elements in flies are really 



^ Several species of Drosophila included here are undescribed, and are given 

 Sturtevant's manuscript names. 



