12 THE PHYSIOLOGY OF REPRODUCTION 



as a result, the Death's-Head has never established a permanent 

 footing in Britain, though stray specimens are often captured.^ 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.^ It is interesting to note also that in the mosquitoes 

 and other Culicidse, the males are generally linable to suck blood, 

 this habit being apparently correlated with the function of 

 oviposition. Among the Empidse, which are carnivorous, the females, 

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

 insects, and they seem incapable of discharging their sexual functions 

 unless they are fed in this way.* The Hon. Charles Rothschild, how- 

 ever, has suggested a more probable explanation of this phenomenon, 

 namely, that the females would eat the males were they not supplied 

 by a specific pabulum to divert their attention.* Hewlett^ has 

 shown that with the fruit-fly of Pusa (Dacus) sexual attraction is 

 brought about by an odour emitted by the female and that this can 

 be imitated artificially by oil of citronella. 



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.® 



MOLLUSCA 



Among the marine' Mollusca, in curious contrast to so many 

 forms of life, winter is the usual time for the deposition of the eggs.'' 

 On our own coasts Nudibranchs come to shore to lay their eggs from 

 January to April. Patella spawns from October until the end of the 

 year. Purpura lapillus is said to be most active during the same 

 season, but it breeds to some extent throughout the year. Buccinum 

 undatum breeds from October until May, whereas Littorina breeds 

 all the year round.^ 



1 Country Side, October 27, 1906. 



2 " Eeport of Malaria Expedition to Nigei'ia," Liverpool, Trop. Med. Memoir, 

 IV. See also Ross {Nature, vol. Ixxx., 1909), who says that females of Gvilex 

 and Stegomya apparently only desire to suck blood after fertilisation. 



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

 * Letter to the author. 



5 Howlett, " The Effect of Oil of Citronella on two species of Dacus,'' Trans. 

 Ent. Soc. Lond; 1912. 



" Lefroy and Howlett, Indian Insect Life, Calcutta, 1909. 



' Lo Bianco, " Notizie biologische riguardanti speci^mente il periodo di 

 maturita sessuale degli animali del golfo di Napoli," Mitth. Zool. Stat. Neapol., 

 vols. viii. and xiii. Much valuable inforniation concerning the breeding habits 

 of Mollusca and other animals, inhabiting the Bay, is given in these papers. 

 See also vols, xviii., xix., and xx. ; and Isset, Biologia Marina,, Milan, 1918, 



8 Cook, " Mollusca," Camh. Nat. Hist., vol. iii., London, 1895. 



