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 Aedes 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 iVnopheles 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 Aedes 

 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 Aedes sylvestris will be found thermo- 

 tropic during the "biting season" unless, in this genus, other 

 tropisms are involved. 



1 Loc. cit. p. 206-209. 



