THE BREEDING SEASON 13 



insects as with other animals, depends, among other things, 

 upon the nature of the diet, upon the chemical conditions of 

 the surrounding medium, upon the moisture of the air, or 

 upon other circumstances which are often unknown. Thus, 

 failure to breed in a new environment is experienced by many 

 Lepidoptera. For example, Death 's-Head hawk moths, which 

 are commonly blown over to this country from the Continent, 

 but do not breed here, deposit their eggs on young potato ^plants, 

 and these develop into moths which emerge in the autumn. 

 The eggs, however, are quite infertile, so that, as a result, the 

 Death's-Head has never established a footing in Britain, though 

 stray specimens are often captured. 1 In the case of other insects, 

 such as the mosquito (Anopheles), there is direct evidence that 

 food is an important factor in egg-formation. Thus it was 

 found that mosquitoes fed on bananas refused to breed, but 

 when fed on human blood they invariably laid eggs after two 

 or three days. 2 It is interesting to note also that in the mos- 

 quitoes and other Culicidae, the males are generally unable 

 to suck blood, this habit being apparently correlated with the 

 function of oviposition. Dr. Gordon Hewitt informs me that 

 among the Empidae, which are carnivorous, the females, during 

 the nuptial flights, are always fed by the males on small insects, 

 and that they seem incapable of discharging their sexual func- 

 tions unless they are fed in this way. 3 



In some insects oviposition takes place long after the death 

 of the males. Thus, Lefroy and Hewlett state that in the 

 mango weevil (Cryptorhynchus gravis) the males die in August 

 while the females live until the following March to lay eggs. 4 



MOLLUSCA 



Among the marine Mollusca, in curious contrast to so many 

 forms of life, winter is the usual time for the deposition of the 



1 Country Side, October 27, 1906. 



2 " Report of Malaria Expedition to Nigeria," Liverpool, Trop. Med. 

 Memoir, IV. See also Ross (Nature, vol. Ixxx., 1909), who says that females 

 of Culex and Stegomya apparently only desire to suck blood after fertili- 

 sation. 



3 Hewlett, "Coupling of Empis" Ent. Mag., vol. xliii., 1907. 

 * Lefroy and Hewlett, Indian Insect Life, Calcutta, 1909. 



