XV 
The Determination and Persistence of Direction in Flight.— 
It is clear that the steady flight in one direction which has 
been so often recorded involves something more than the 
liberation of the instinct. There must be some stimulus which 
determines and keeps the direction of flight. And the most 
usual stimulus is probably the wind-current. 
Commander J. J. Walker has very kindly tabulated on 
pp. Xx-xxv the data recorded by the late Mr. J. W. Tutt in 
“ Ent. Record’ (1898-1902), omitting those referring to 
Pyrameis cardui and Danaida plexippus and adding a few 
more recent observations. Among the references which give 
sufficient data it will be found that flight was against 
the wind eighteen times, with it nine times and across 
it four times. But these latter include Mr. C. B. Williams’ 
many records from British Guiana (p. xviii). These facts 
support the conclusion that there may often be a definite 
relationship to the wind-current. The reaction to the stimulus 
probably differs in different species and in different parts 
of the world, being determined by natural selection based on 
local conditions. 
Flight against the Wind.—This reaction may be doubly 
advantageous. A gentle current is obviously far more effec- 
tive as a stimulus when flight is against rather than with it; 
furthermore, as Mr. A. W. Bacot suggested to me, steady 
flight against a wind flowing towards a hot dry area is pretty 
sure to carry an insect into a cooler, moister locality. This 
is the reaction we should expect to find especially common in 
countries where the food-plant is lable to be temporarily 
exhausted by drought over a large area, or by drought 
combined with larval attack. 
Flight with the Wind.—It is probable that this reaction has 
been developed in localities where flight with a prevalent 
wind is more likely to carry the species to plenty than flight 
against it, e.g. for a butterfly which would be carried into a 
dried-up area by flying against a wind blowing across it. 
Backwards and Forwards Flight.—It is obvious that a species 
which reacts to the wind-current in either of the above ways, 
but especially the former because of the greater sensitiveness 
already explained, will reverse its direction with each reversal 
