228 



To account for sex disparity the writer says that there are two 

 alternatives ; either (1) some agency exists which causes a greater 

 rate of mortality of males in one locality, and which is comple- 

 mented by some other agency similarly prejudicial to the opposite 

 sex in another locality; or (2) the males migrate to, or the females 

 from, localities where males predominate, and vice versa. The 

 first alternative is upheld by Kleine, who suggests that a diet of 

 crocodile's blood reacts more disadvantageously on females than 

 on males. This leads to the inference that islands, where males 

 predominate, are unfavourable places for the fly on account of 

 bad food; but since the race remains abundant, it follows that 

 conditions are actually highly favourable, for the fewer the 

 females the higher must be the rate of reproduction in order to 

 maintain these numbers. Other observed facts cast doubt, 

 according to the author, on Kleine' s conclusions. The author 

 considers that the second alternative is the most simple and most 

 logical way of explaining the facts, viz. : that one sex migrates 

 from localities where the other is predominant; he is satisfled 

 moreover, that it is the females which migrate, and supports this 

 view by arguments from analogy witli other orders of insects, 

 by considerations of the physiological condition of the female in 

 early life, and by the fact that islands are on the wliole favour- 

 able breeding places (pupae are most easily collected on them), 

 and it would seem ni(U'e lilcely that females would niigrate from 

 the islands to tbe mainland, than that males would migrate to the 

 islands. 



Considering the meaning of such migrations, the author looks 

 upon it as a factor of natural control ; as no specialised parasites 

 or natural enemies of G. paljmlu have been found, some other 

 factor must operate to keep down the number of individuals to the 

 most favourable level. Migration of females Avould act in this 

 way, and would also serve as a jueans of dispersal of the species. 



When localities actually favourable to increase of the species 

 are compared wilh localities actually unfavourable, it is not 

 improbable, according to tlie author, that points of difference will 

 be discerned, Avhich luay be of value for economic application. 

 The extent to which these conclusions regarding tlie bionomics of 

 G. palpalis in a certain geographical region will apply to the 

 same species in other regions, or to other species, is considered to 

 be as yet wholly problematical. 



Hirst (S.). On Three New Species of Gamasid Mites found on Rats. 

 — Bull. Entoin. Research, f.ondon, iv, pt. '2, Sept. lUl'"!, pp. 

 119-124, 4 figs. 



Altlioiigh a great majority of the Gamasidae are not parasitic 

 in habit, a number of true parasites belonging to this family are 

 known to occur on vertebrates (birds, small mammals, especially 

 bats and rodents) and on arthropods. Gamasid mites belonging 

 to different genera are known to infest rats which frequent human 

 dwellings, and it is possible that these mites are responsible for 



