297 



CHEMICAL REACTIONS OF FRUIT-FLIES. 

 By F. M. HowLETT. 



(Plates XIII-XVI.) 



In 1912 I published some observations* on the strong attraction of oil of citronella 

 for the males of Dacus (now Bactrocera) zonatus, Samid., and D. diver sus, Coq., and 

 put forward some theoretical considerations regarding the influence of smells on such 

 insect activities as pairing, egg-laying, and food-finding, on the assumption that 

 their olfactory sense may have a specialised receptivity for a few particular 

 " directive " smells or groups of smells, while remaining relatively unaffected by 

 smells outside these regions of susceptibility. If, as I then believed, citronella 

 represented a sexual smell attractive to males of two different species, it was 

 probable either that there were two substances present, each of which attracted 

 one species, or that the two species were not really distinct. 



The publication of Prof. Bezzi's excellent monograph on the Indian Trypelids 

 showed me that whereas I had been dealing with what I took to be only two species 

 of Dacus, individuals of what I had regarded as merely a melanic variety of 

 D. zonatus were really the form identified by him as Z). ferrugineus. These 

 individuals had also been seen attracted by citronella in the course of the earlier 

 experiments (March), but disappeared later. It should be noted that the characters 

 distinguishing zonatus horn, ferrugineus are purely those of colour ; I have up to now 

 not seen any definite morphological distinction, and the question of the relationship 

 between these two forms is one of some interest, to which I refer later. 



In these observations I discriminate as far as is practicable between zonatus and 

 ferrugineus, taking as my criterion of the latter a more or less dark thorax and a 

 wing approximating in pattern to that shown in fig. 3, PI. xiii. Typical forms of 

 zonatus Sindi ferrugineus are shown in figs. 1 and 3, PI. xiii., but between these two 

 many gradations in colour and wing-pattern can be found. My investigation this 

 year thus started with the knowledge that males of three species, diver sus, Coq., 

 zonatus, Saund., 'diid ferrugineus, F., were all attracted by citronella. If these three 

 species are distinct, there must (I imagined) be three distinct compounds present 

 in citronella, each attracting one species only. 



In the course of the year 1912 I had tried a very large number of experiments with 

 about twenty varieties of essential oils in order to discover whether there were 

 other substances attractive to the flies. These oils were tried alone, mixed with 

 one another in various proportions, and mixed with citronella, certain vegetable 

 oils, and other substances. From these experiments it was soon evident that the 

 flies perceived the smell of small quantities of citronella mixed with almost any 

 other substance, even though the smell was so faint as to be almost imperceptible 

 to me even at close range. In the absence of citronella no defijiite attraction was 

 noted save in the following cases :- — 



(1) One sample of oil of eucalyptus, origin and composition unknown ; 



(2) a mixture of clove oil, coconut oil and kerosene ; 



(3) a mixture of clove oil and eucalyptol. 



* Trans. Ent. Soc, Oct. 1912. 



