416 Mr. F. M. Hewlett on the effect of 



a movement which seems to be associated with " court- 

 ship " in all species of Dams that occur at Pusa. 



On two occasions a number of males and females have 

 been confined together in order to see whether the citron- 

 ella smell would induce copulation, but without success. 

 Too much importance must not be attached to this result, 

 however, as the conditions were abnormal, and I have 

 never succeeded in getting D. zonatus to copulate in the 

 laboratory. These observations atford at least another 

 argument that the olfactory sense of Diptera, or at any 

 rate of D. zonatus, is not dissimilar in kind from our own : 

 smells which in us give rise to similar sensations {%. e. 

 citronella and $ zonatus) affect the male zonatus in the 

 same way, though its perception of them is far keener than 

 ours. 



Among well-known instances of attraction by smells re- 

 sembling the food of the larva or adult is the case of certain 

 evil-smelling Aroids which are attractive to various flies 

 and beetles accustomed to infest putrescent matter. It 

 has been found that a mixture of certain proportions of 

 acetic acid and ethyl alcohol is most attractive to Droso- 

 phila am2)elo2)hila, whose larvae live in over-ripe fruit. 

 Similarly, I have myself observed Sarcophaga to be very 

 strongly attracted by a flask containing a solution of 

 skatol, a substance normally present in faeces ; many 

 larvae were laid in the flask and were drowned in the 

 liquid. The same fate attended the eggs of Stuvioxys 

 calcitrans which I have obtained in numbers on cotton- 

 wool soaked in valerianic acid, one of the acids present in 

 the fermenting vegetable stuff in which the eggs of this 

 species are naturally deposited ; both valerianic and butyric 

 acids have a similar attraction for an Ortalid fly of the 

 genus Ulidia (?) which is not uncommon at Pusa. 



Our own sense of smell seems to be practically limited 

 to substances havinor a molecular weiarht of about 30 or 

 over; those with molecular weight less than this have 

 no smell or only a very faint one, though they may have 

 an irritant eff"ect on the mucous membrane of the nose. 

 The fact that house-flies will suck freely a dilute solution 

 of formaldehyde (mol. wt. 80) may perhaps indicate that 

 their sense is limited in the same way (Alex. Hill, 

 Nature). I have found that they will sometimes take a 

 solution of hydrocyanic acid (mol. wt. 27), and this might 

 be regarded as evidence supporting this supposition. 



