302 



ANIMAL AGGREGATIONS 



chironomid larva. A part of their results are summarized in Table 

 XXXV. They interpret their findings and the findings of Cobb, 

 Steiner, and Christie (1927) as meaning that in these parasitic 

 nematodes sex depends largely on the number of parasites simultan- 

 eously present, and on the resulting nutritive values of the host. 

 When a single parasite is present, some genotypic males appear to 

 have been inverted into phenotypic females. When two are present, 

 the normal genotypic relation is not sensibly altered. With higher 



TABLE XXXV 



Showing the Number of Nematode Parasites per Chironomid 

 Larva in Relation to Sex Distribution 

 (Data from CauUery and Comas) 

 No account is given here of the intersexes also reported. 



numbers in the nematode population of individual bloodworm hosts, 

 genotypic females apparently were transformed into phenotypic 

 males. The production of females is not quantitative at any level 

 in Caullery and Comas' work, but it may be summarized as showing 

 that the more parasites present in a given host, the fewer females 

 in the population. 



There seems no escape from the conclusion that we are dealing 

 here with cases where the numbers of individuals of the same species 

 in the immediate environment is a sex-determining factor for all 

 those present. Here, even less than in the preceding similar case of 

 Crepidula, has analytical work been done. We do not even know, 

 for example, whether nematodes of different species present in the 



