1918] Marchand—A Thermotropism in Anopheles punctipennis 135 
filter paper in the same way as before, in order to provide a foot- 
hold for the insects. The mosquitoes were attracted under these 
conditions in the same way as if food had been present, each one 
attacking the surface of the filter paper which covered the glass 
plate, and all bending their proboscis in repeated efforts to pierce 
the surface. The number of mosquitoes used was about five or 
six at a time. The arrangement was that given in the diagram 
(Fig. 2). If several mosquitoes are used in biting experiments, 
there will usually be some individuals which will show no inclina- 
tion to bite, but the percentage of individuals not attracted by the 
heated glass plate, was about the same, and not greater than in the 
biting experiments. The males showed the same tropism as the 
females but much less strongly. As soon as the glass plate had 
cooled off, the mosquitoes became indifferent. However, the 
experiment could be repeated with the same mosquitoes as also 
with different sets and always with the same results. 
These observations, taken together with those made by Howlett, 
indicate that this thermotropic reaction is a very important factor 
to be considered in the analysis of the bloodsucking instinct. 
I have, since, tried to confirm these facts by observations on 
other species of mosquitoes, but so far have used only the hiber- 
nating females of Aédes sylvestris. These, however, did not show 
even a trace of the thermotropic reaction observed in A. puncti- 
pennis. On the other hand they also consistently refused to bite, 
though various food other than blood was readily accepted. Fe- 
males of Anopheles are known to bite occasionally in winter, and 
therefore, usually hibernate in stables where blood can be obtained 
(Grassi and others; see Howard, Dyar and Knab),! while Aédes 
which hibernates in cellars, seems not to bite at all during the winter 
even if brought into a heated room. The absence of thermotropism 
would, therefore, in this case, be only an adaptation to the con- 
ditions of hibernation, during which no blood food is taken, and it 
is perfectly possible, that Aédes sylvestris will be found thermo- 
tropic during the “biting season” unless, in this genus, other 
tropisms are involved. 
1 Loe. cit. p. 206-209. 
