312 Dr Hindle, The Flight of the House-Fly 
that not only were these investigators working in open country, 
but also their traps were set solely at stations to the west of the 
REET Sitar 
point of liberation and consequently none of the flies that flew 
in other directions would be recovered. 
Owing to lack of opportunities we have been unable to decide 
why, in our experiments, the flies tended to travel either against 
or across the wind. ‘Two explanations seem possible. 
(1) The flies may direct their flight against any current of 
air to which they are subjected. This property is known as 
positive anemotropism and is possessed by some other insects 
and birds. In view, however, of the contrary results obtained by — 
Copeman, Howlett and Merriman (1911) we cannot come to 
definite conclusions on this point and further experiments are 
required to determine if other factors than wind-direction may 
influence the direction of flight. 
(2) The flies may travel against the wind, being attracted 
by any odours it may convey from a source of food. A-poimt in 
favour of this supposition is the nature of the stations at which 
flies were recovered after they had travelled any distance. These 
comprised a butcher’s shop, public-houses and a restaurant, all 
of which places gave off odours that are notoriously attractive 
to flies. 
The maximum distance travelled by any of the flies we 
liberated in Cambridge was 770 yards, which is considerably less 
than that covered by those liberated in the open country at - 
Postwick, in one case as much as 1700 yards. This difference 
may be attributed to the absence of shelter in the case of the 
Postwick flies, whereas in Cambridge food and shelter were 
always plentiful. On the whole, we do not think it likely that 
as a rule flies travel more than a guarter of a mile in thickly 
housed areas. Throughout our experiments the only individual 
that exceeded this distance had travelled 770 yards, of which 
a large part was across open fen land. 
The chief factors influencing the dispersal of the flies are 
temperature, weather, and the time of day when the insects are 
liberated. The effect of temperature is very marked, as when 
it is low the flies become torpid and seek the first available 
shelter. Fine weather is also a necessary condition for long — 
flights, as rain at once drives the flies under cover. The ideal 
conditions for an experiment are two or three days of fine warm 
weather, during which the flies can make their flight, succeeded 
by a wet or showery day, when they are driven indoors and thus 
can be recorded at the various stations. 
With regard to the altitude of the point of liberation, flies set 
free from the roof* tended to disperse slightly better than those 
* A height of about 45 feet. . 
