4 Protection of Meat Commodities against Bloicflies 



they had been swarming round it when it was placed in the sun. In my 

 own experiments, the baits were duplicated, one series being placed in 

 the sunniest part of a large garden, another series being placed in the 

 shadiest portion of an adjoining insectary, which, having side walls of 

 large mesh chicken wire, was quite open to flies. Both batches were 

 equally readily infested within a few minutes of exposing, but in few 

 cases did oviposition occur upon the baits exposed to the sunshine, and 

 even then the eggs were few and scattered, whereas the baits in the 

 shade were readily blown. 



This apparent discrepancy in observations may be explained by an 

 undoubted difference in response to hght between the Calliphora type 

 of blowfly and the Lucilia type. The flies used by Miss Lodge were 

 chiefly Lucilia. Among the flies that visited my baits, Lucilia was in 

 the minority. In fact numerous samples taken from various parts of the 

 city all showed a marked predominance of Calliphora vomitoria, this fly 

 constituting on the average 75 per cent, of the sample, the remaining 

 percentage being made up almost equally of Calliphora erythrocephala 

 and Lucilia caesar. In addition to being in the minority, Lucilia had 

 also to compete for foothold on the baits not only with Calliphora but 

 with hordes of Acalyptrate Muscidae as well. 



The difference in response to hght has been pointed out by Herms(2) : 

 " The more frequent presence of Calliphora vomitoria in houses and like 

 situations is due chiefly to a relatively low degree of responsiveness to 

 light, so that odours from darker places may attract it more readily. 

 On the other hand, Lucilia caesar is very strongly phototactic and con- 

 sequently would seek the open, and if by chance it should find its way 

 into darker places, its responsiveness to large luminous areas would 

 soon lead it to escape. It may be assumed that the two species are 

 equally chemotactic, which assumption is justified at least by observa- 

 tion. Since C. vomitoria is less strongly phototactic, individuals least so 

 might easily be attracted into fairly dark places and would not soon be 

 compelled to leave, because of their phototropism." 



This difference in habit was strikingly observed during the hot 

 August sunshine of 1916 in the trenches outside Delville Wood, on the 

 Somme, where blowflies were abnormally abundant. In the shade 

 afforded by the deeper portion of the trench, round the traverses, any 

 moist patch on the chalk wall would be hidden by a dense indigo- 

 coloured cluster of Calliphora, large as a soup plate, unredeemed by the 

 metallic green of Lucilia, whereas, where the trench was shallow or 

 blown in, the green shimmer of Lucilia was everywhere. 



