Mr Brindley, Notes on certain parasites, food, etc. 167 



Notes on certain parasites, food, and capture hy birds of the 

 Common Earwig (Foi-ficula aiiricularia). By H. H. Brindley, M.A., 



St John's College. 



[Read 18 February 1918.] 



(rt) Effects of pa7'asitism. 



In a paper entitled " The effects of Parasitic and other kinds 

 of castration in Insects " (Jour. Exper. Zool. viii. Philadelphia, 

 1910) Wheeler expresses the opinion (p. 419) that Giard has given 

 good reasons for supposing that the dimorphism exhibited by the 

 forcipes of male earwigs from the Farn Islands, Northumberland 

 (Bateson and Brindley, " On some cases of variation in secondary 

 sexual characters statistically examined," Proc. Zool. Soc. Lond. 

 1892, p. 585), is due to "differences in the number of gregarines 

 they harbour in their alimentary tract." The reference to Giard 

 is C.R. Acad. Sci. cxviii. 1894, p. 872, where he writes " J'ai tout 

 lieu de croire qu'une interpretation du meme genre (referring to 

 the changes evoked in Carcinus by the action of parasites) pent 

 s'appliquer pour la distribution des longueurs des pinces des 

 Foi'ficules males. II est possible, en effet, d'apres la longueur de 

 la pince, de prevoir qu'une Forficule male possede des Gregarines 

 et qu'elle en possede une plus ou moins grande quantite." 



In criticism of the above statements Capt. F. A. Potts and 

 myself published a letter in Science, Philadelphia, Dec. 9, 1910, 

 p. 836, in which we gave reasons for disagreeing with Wheeler's 

 conclusion : viz., (i) that in the absence of any further account by 

 Giard the above passage could not be taken as direct evidence 

 that he had examined the intestine of Forficula for gregarines and 

 found a correspondence between their presence and the condition 

 of the male forcipes ; (ii) that out of several thousand earwigs 

 collected by us on the Farn Islands in 1907 over 50 males of 

 different forceps lengths were carefully dissected with the results 

 that the gregarine Clepsydrina ovata was found to occur commonly 

 in the alimentary canal, that it occurred indifferently and was 

 absent indifferently in " low " and " high " males, and that 

 no correlation could be traced between the number of parasites 

 and the length of its forcipes. Moreover, no difference in the 

 development of the testes or other internal sexual organs could 

 be detected in low and high males respectively. 



Since the above was written I have (August 1917) examined 

 the alimentary canal of 51 earwigs out of a large batch obtained 

 at Porthcressa, St Mary's, Isles of Scilly, where the males exhibit 



