162 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



exercises a remarkable attraction for the males of Dacus diversus 

 and D. zo7iatus, and suggested that possibly this reagent is allied 

 to a secretion emitted by the females, and that the phenomenon 

 of the male alone being attracted is to be regarded as a repro- 

 duction, by artificial means, of a sexual attraction similar in kind 

 to that which operates in most cases of " assembling." In a later 

 paj^er (1915) Howlett conducted a further series of experiments, 

 confirming his previous results that certain odours are remarkably 

 attractive to male flies of the genus Dacus and that, by the 

 employment of attractive substances, the movements of the flies 

 can to a great extent be controlled in any direction. Three of the 

 common species {D. diversus, ferrugineus and zonatus) normally 

 breed respectively in (1) anthers of Cucurbitacese, (2) fruits of 

 Solanaceae and mango, and (3) peach, guava, mango and other 

 fruits. D. diversus (1) is most strongly attracted by iso-eugenol, 

 zonatus by methyl-eugenol, and ferrugineus (2) by both iso- and 

 methyl-eugenol. The odours of these substances have not yet 

 been identified with those of the plants which constitute the 

 normal breeding places, but male flies have been found attracted 

 to mango, papaya, a Cycad and Colocasia, plants with a very 

 characteristic smell similar to that of eugenol-derivatives. Females 

 have never been seen to frequent these places or to breed in them, 

 but more extended observation on this point is needed. Three 

 explanations suggest themselves, (a) That the smell is a direct 

 sexual guiding smell, emitted by the female, as previously suggested, 

 but, nevertheless, the young crushed females were not found to 

 attract males, {h) The smell is not emitted by the female, but may 

 be termed an " indirect " sexual guide to the plants where the 

 females are accustomed to congregate for breeding purposes. 

 Under these circumstances it is difficult to see why females should 

 not be attracted by the odoriferous chemicals, (c) The odour is a 

 food smell ; if this be so it can only be attractive to males. None 

 of these fundamental questions has so far been solved, but, 

 nevertheless, Howlett's researches have proved a landmark on 

 the applied side of the subject, and have indicated some of its 

 possibilities. Mention needs also to be made of the work of the 

 Severins (1914), who followed up the reactions of another species 



