DISPERSAL UNDER RURAL CONDITIONS 69 



\illage, Postwick in Nort'ulk, wIktc ihu ohserviilioiis wciv made. 

 The work was much hampered by the meteorological conditions, 

 which were for the most part unfavourable, the temperature 

 remaining low while rain showers were frequent and high winds 

 often prevailed. The flies were caugiit in a net and were marked 

 by being placed in a paper bag containing finely powdered coloured 

 chalk of which that of a yellow colour was found to give the best 

 results. Describing the results of the experiments the authors 

 state: " Nevertheless, on each occasion on which several hundred 

 marked flies were liberated, a certain number were subsequently 

 recovered, within forty-eight hours or less, from human habitations 

 in Postwick, at points of the compass which were apparently de- 

 pendent, for the most part, on the direction of the prevailing wind, 

 and at distances ranging from 300 yards to 1700 yards from the 

 refuse deposit. On one particular occasion on which chalk of a 

 bright canary yellow colour was employed for marking the flies, the 

 day being fine and sunny with a gentle north-west breeze, the 

 results were specially interesting. Of these flies several were 

 observed and captured along the stretch of river bank between a 

 point opposite the refuse heap and Postwick Hall, within half an 

 hour of their liberation. Two other marked flies were caught in a 

 large open-fronted house on the lawn of Postwick Hall, a distance 

 of 800 yards in a direct line from the point at which they were set 

 fi'ee — one thii'ty-five minutes and another forty -five minutes after 

 being liberated. Within the next four days, more than forty of 

 these yellow-coloured flies were caught, on hanging fly papers, in 

 the kitchen and outbuildings at Postwick Hall, while isolated 

 specimens were also trapped in various parts of the village, at 

 greater distances and at different points of the compass, from the 

 refuse deposit of the marked flies caught on " tanglefoot " papers 

 and a certain proportion probably were overlooked, oAving to the 

 fact that the colour of the chalk soon gets considerably obscured 

 by the glutinous material becoming spread over the bodies of the 

 flies in their struggles to free themselves." 



In view of the results of the foregoing experiments on the 

 range of flight of house-flies under rural conditions, it seemed to 

 me very desirable that experiments should be carried out under 

 city conditions, where so many factors are present which may affect 



