CROWDING AND SEX DETERMINATION 299 



They do not make clear whether the transformations affected by 

 the presence of the large individual can take place in the absence 

 of physical contact. They are not conclusive concerning the possi- 

 ble presence of some chemical product or the effect of such a product 

 transmitted through the sea- water; and the biophysical possibilities 

 remain to date unexplored. All we know is that in some manner the 

 presence or absence of larger individuals affects the state of develop- 

 ment of the primary and secondary sex organs of associated smaller 

 individuals, which is exciting enough information to deserve greater 

 experimental attention than has been so far given to it. 



SEX IN NEMATODE PARASITES 



While working on grasshoppers to determine the fatal dose of the 

 eggs of one of their parasites, the nematode hairworm, Mermis 

 subnigrescens, Cobb, Steiner, and Christie (1927) discovered that 

 the sex of the resulting adult parasite was male or female according 

 to the size of the dose. The parasite is an important factor in the 

 control of grasshoppers, and the experiments were primarily con- 

 cerned with this aspect. 



The parasite M. subnigrescens is found quite commonly in a num- 

 ber of varieties of common grasshoppers: for example, the red- 

 legged grasshopper Melanoplus fetnurrubrum and its relatives. The 

 grasshoppers become parasitized by swallowing eggs of the parasite 

 that have been deposited on their food plants; these eggs contain 

 well-developed hairworms. 



Cobb, Steiner, and Christie (1927) state that their researches have 

 shown that ''when immature females of the hairworm are kept in 

 solitary confinement before they can have copulated with males 

 they produce viable eggs." In thousands of observations made by 

 these workers the average number of hairworms per infested grass- 

 hopper in nature has been from i to 3, and these are always females. 



Usually overinfestation of a grasshopper by hairworms resulted 

 in the premature death of the grasshopper as well as of the para- 

 sites, though on rare occasions highly infested grasshoppers have 

 been found in nature containing more than a hundred hairworms. 

 In all such extreme cases the parasites were small and young. These 



