414 Mr. F. M. Hewlett on the efed of 



sexual attracting smell of D. zonahis. It is noteworthy 

 that the oil also has an attraction for males of the species 

 D. diversus, and a considerable number of them were 

 caught on the fly-papers in March and April ; the attrac- 

 tion in this case, however, seems to be perhaps a trifle 

 less powerful than with zonatns, though it is difficult to be 

 certain on the point. The number of diversus caught 

 probably did not exceed 25 per cent, of the total of the 

 two species, as towards the end of April diversus became 

 scarce and zonatus very abundant up to the end of June. 

 The quality of the oil affected the result, old oil being 

 more effective than new ; I have been unable to get 

 analyses which would show wherein the difference lies, 

 and what is the precise constituent which is of most 

 importance. Some samples of eucalyptus oil seemed also 

 to possess some slight attraction for zonatus males, but 

 they never came to it in large numbers, nor did they come 

 when there was any oil of citronella exposed in the neigh- 

 bourhood. The distance at which the flies are able to 

 perceive the smell of citronella is doubtful, but seems to 

 be considerable ; half a mile is probably not an extrava- 

 gant estimate if the wind be favourable. By exposing a 

 rag moistened with oil for half an hour or so in places 

 where ordinary collecting fails to reveal the presence of a 

 single fly, it is often possible to catch considerable 

 numbers. 



The smell is in all probability perceived by means of 

 the antennae. To test this a rag wet with citronella was 

 exposed, and of the visiting flies six or eight were caught 

 and their antennae were carefully amputated at the base 

 of the second joint ; they were then liberated, seeming 

 none the worse for the operation, and the rag was watched to 

 see whether they again visited it. None of the flies operated 

 on returned to the rag, though normally flies caiight 

 and liberated anywhere near such a rag will always return 

 to it sooner or later, and generally quite quickly (cf. Plate 

 XL). On one occasion a marked fly was driven away five 

 times, but returned almost immediately after each repulse. 



A curious fact is that the oil has an actually poisonous 

 effect on the fly when the latter is exposed to its vapour 

 in a fairly concentrated form., this effect being independent 

 of the presence or absence of the antennae. 



Four male zonatus were taken and the antennae of two 

 of them were amputated ; they were then confined in glass 



