Dr Hindle, The Flight of the House-Fly. 311 
or yellow. The mouth of the bag was then closed and the whole 
gently shaken for one or two minutes so that the flies were 
thoroughly dusted with the chalk. The balloon was then removed 
and, after being taken to the point selected for the liberation, the 
trap was opened and the flies allowed to escape in any direction 
they chose. The flies were recovered either by means of fly-papers 
or balloon traps, several of which were exposed at the various 
observation stations. The traps and papers were examined for 
several successive days after the liberation of a number of coloured 
flies and as the observation stations extended as far as 900 yards 
from the point of liberation, comprising both thick and sparsely 
populated localities, an accurate idea of the distribution of the 
insects was thus obtained. Full meteorological data were kindly 
supplied by Messrs W. E. Pain, Chemists, Sidney St., Cambridge. 
Their observations were made in the centre of the town and in 
consequence indicate the exact conditions under which the flies 
travelled. 
Discussion of results. 
Unfortunately, nearly all our experiments in Cambridge were 
seriously handicapped by the great difficulty of obtaining flies 
in sufficient numbers and also by the adverse meteorological 
conditions. Throughout August the weather was so bad that 
from the nineteenth to the thirty-first not a single fly could be 
liberated. During the early part of September nearly all the 
flies became infected with Hmpusa muscae and this, in conjunction 
with the cold weather, brought the investigation to a sudden end. 
In the earlier experiments we should have preferred to have 
liberated at least double the number of flies, but owing to the 
difficulty of procuring them this was impossible. Our results, 
therefore, are not as complete as we could have wished. 
Nevertheless, owing chiefly to the large number of stations 
employed for the recovery of the flies and their being situated 
in every direction, we have been able to obtain certain definite 
results. 
The most striking feature is the marked effect of the direction 
of the wind on the courses taken by the flies. After a careful 
examination of all our results, we can state definitely that flies 
tend to travel either directly against or across the wind. ‘The 
only exceptions to this rule were those recovered within a radius 
of about 150 yards from the point of liberation and probably 
these flies were individuals that had merely selected the first 
shelter they could find. These results differ considerably from 
those of Copeman, Howlett and Merriman (1911) who found that 
the flies tended to go with the wind, but it should be remembered 
