302 F. M. HOWLETT. 



I am impressed with the fact that in individuals of opposite sexes of the same 

 species we have often to deal with two almost totally distinct personalities, if we 

 may use the term. Nevertheless, and granting this in the case particularlv of blood- 

 sucking insects such as Culicidae, in which there is a marked difference in 

 feeding-habits, it somehow seems on the face of it unlikely that in fruit-flies we 

 should find differences sufficiently marked as to justify either of the above 

 explanations. The facts observed might be, on the other hand, simply and easilv 

 explained by a hypothesis that the smells have some definite sexual significance. 

 It is true that young crushed females did not attract males, but although the 

 evidence with regard to the smell of the female is invalidated, I think it may be 

 fairly confidently asserted that a very faintly discernible aromatic smell does 

 actually exist. Recent experience with very weak odours has, however, shown me 

 that it is well to exercise caution in forming conclusions as to their exact nature, 

 the character of the smell often changing in a quite unexpected degree with an 

 increase in strength, and I would not press this point. 



The attraction of Cemtitis capitata by kerosene is now known to be similarlv 

 confined to the males and is apparently a quite analogous case. It seems at present 

 to supply another argument for some directly sexual explanation of the phenomena. 



In my former paper on this subject I wrote as follows :■ — " .... we might look 

 on each species as tuned to respond to three or four notes on the scale of smell, and 

 we should expect to find the most delicate adjustment and most accurate ' tuning ' 

 in the direction of the sexual smell, since errors of perception would here be most 

 disadvantageous to the species. There would be a correlation between the degree 

 of specialisation of the larva in the matter of diet and the defijiiteness of the smell 

 which would prompt the female to lay eggs. In many cases the food-smell of the 

 adult fly would be least narrowly adjusted. At all times other senses such as those 

 of sight and touch might play a more or less important part as auxiliaries or controls. 



" If we accept for the moment some such view as this, then among those species 

 in which the male finds the female by smell we must regard each one as an 

 assemblage of individuals in which one sex is tuned to respond to a certain definite 

 kind of molecular vibration corresponding to some compound or mixture of 

 compounds emitted by the other sex, and these compounds would thus constitute 

 definite specific characters. AVe might even perhaps go further and define some of 

 the larger groups by those ' generic ' smells which characterise certain kinds of 

 chemical substances, .... and which depend on the presence of certain atoms or 

 of atomic groups of some particular configuration." 



In the course of the investigation of which particulars have just been given, 

 I made two observations which in the light of the latter part of the passage quoted 

 are of interest. 



The species of Dacus found at Pusa are diversus, ferrugineus, zonatus, ciicurbifae 

 and caudatus. The form hitherto known as ferrugineus var. 7nangifera€, Cotes, is, 

 as Prof. Bezzi suspects, not a " true " variety, but merely a poorly-developed 

 individual ; by giving insufficient or improper food it is quite easy to rear 

 " mangiferaey I may also confirm here Prof. Bezzi's suspicion that the Australian 

 A tryoni may be identical with our D. zonatus. An examination of good specimens 



