304 F. M. HOWLETT. 



for diversus and the unattractiveness of fresh solutions of the isomeric substance 

 eugenol. Working on the hypothesis of the existence of generic " smell-groups," 

 and bearing in mind the comparative ease with which iso-eugenol is obtained from 

 eugenol, it seemed to me that there might very probably be some species whose 

 males are attracted by eugenol. Similar considerations had already resulted in the 

 case of caudatus being brought perhaps more or less into line with the others, but 

 there remained of the known Pusa species only the very common cucurbitae, which 

 was not attracted by eugenol or any of the other substances used. The Dacinae 

 of Pusa have for the last three years been observed with some care, and the chance 

 of finding a new eugenol-attracted form seemed slight. Nevertheless, I continued 

 to expose eugenol, and on the 27th of July was rewarded by the capture of a single 

 specimen of the male of an entirely fresh species (fig. 6, PI. xiii.). This individual was 

 seen by me to settle in the bowl ; like caudatus, it exhibited the characteristic 

 beglamoured behaviour and had all the appearance of having found its own 

 particular " bait." It is probably the first victim of deliberate chemical collecting. 

 The investigation here described was undertaken with the idea of ascertaining 

 whether or no the smell-reactions were specifically differentiated. A definite specific 

 differentiation appears to be proved in the case of diversus and eugenol, zonatus and 

 methyl-eugenol, and possibly caudatus and amyl-vanillin and Dacus sp. and eugenol. 

 Between zonatus andferrugineus I have been unable to draw any sharp line. There 

 seem to be differences combined with a general similarity. This may be due to the 

 fact that my analysis of the chemical relations is not carried far enough to differentiate 

 the compounds attractive to these two forms respectively. The ferrugineus compound, 

 if ultimately separated, will certainly be found to be closely similar to methyl- and 

 iso-eugenol. I have made some study of the relations of these two species from other 

 points of view, and it seems to me a fact of some interest that this study reveals a 

 state of affairs apparently in agreement with that which is chemically indicated. 

 In spite of the frequent difference in larval feeding-habits, there exists among adult 

 males a complete series of forms intermediate in colour and pattern between those 

 shown in fig. 1 and fig. 3, PI. xiii., and there are other evidences of close physiological 

 connection. 



We seem to be dealing in the case of these five species of Dacus with a group of 

 insects particularly susceptible to a group of smells (allyl- or propenyl-phenols (?)), 

 each species being specially reactive to one member of the group, the degree to 

 which this reactivity or receptivity is specialised appearing to correspond to the 

 degree of morphological and physiological differentiation of the insect as shown 

 in other directions. The precise degree of specialisation can only be discovered by 

 more controlled and detailed chemical investigation than has hitherto been possible. 

 As the evidence regarding the smell of the females has proved unreliable I have 

 thought it better to publish the results obtained up to the present, in order that 

 other interpretations of these curious reactions of male Dacus and Ceratitis may 

 be given due weight. 



Summary. 

 There are certain smells remarkably attractive to male flies of the genus 

 Dacus and by the employment of these smells the movements of the flies can 

 to a great extent be controlled in any given direction. The reaction is strictly^ 



