HETEROCHROMOSOMES IN MOSQUITOES. 1 17 



during the growth stages (Stevens, '08). In Metapodius ter- 

 minalis (Wilson, '09) we find either an unpaired heterochro- 

 mosome or an unequal pair of idiochromosomes, and a varying 

 number of supernumeraries within the same species. In no 

 case before have undoubted heterochromosomes, following the 

 usual rule as to sex distribution, been found in one or more genera 

 or species and entirely absent in nearly related species of the 

 same family. In Anopheles punctipennis the heterochromosomes 

 can be traced from the youngest spermatogonia observed, through 

 the two generations of spermatocytes to the spermatids and even 

 to a stage in which the spermatids are transforming into sper- 

 matozoa. The unequal pair in the male is represented by an equal 

 pair in the female, and as in other similar cases, it is evident 

 that an egg fertilized by a spermatozoon containing the smaller 

 heterochromosome must produce a male, while an egg fertilized 

 by a sperm containing the larger heterochromosome will develop 

 into a female. 



Now in Culex and Theobaldia all of the chromosomes behave 

 alike so far as condensation is concerned, and there is neither an 

 unpaired heterochromosome nor an unequal pair to be detected 

 in any stage. In other words there is no visible differentiation 

 of one pair of chromosomes which can be associated with the 

 determination of sex. There seems to be absolutely no reason 

 for supposing that the mechanism by which sex is determined 

 is not identical in these three species of mosquitoes; and the inevi- 

 table conclusion would seem to be either that the heterochro- 

 mosomes of the male are not a necessary factor in sex determi- 

 nation, or that the sex-determining chromosomes are not neces- 

 sarily differentiated as heterochromosomes. 



I have kept no record of the sex of the pupae dissected, but 

 should say that the numbers of males and females were about 

 equal for all of the species studied. In a large collection of 

 Theobaldia larvae which, judging by their size, might have all 

 come from one laying, most of the males pupated a day or two 

 sooner than the females. All of the pupae were taken out and 

 dissected each day. The first day nearly all were males and 

 the last day nearly all females. An average for the four or five 

 days, I feel sure, must have given nearly equal numbers. 



