Vol. XXXl] ENTOMOLOGICAL NK\VS 163 



As a result of a number of similar experiments he concludes 

 that (a) the bite of a mosquito is a reaction to the stimulus 

 provided by a hot surface, (b) that the mosquito is attracted 

 to the hot surface mainly by the warm air rising from it, 

 and (c) that the strength of the reaction is, within certain 

 limits, proportional to the differential temperature of the 

 surface, i. e., the difference between its temperature and the 

 general air temperature at the time, and that this difference 

 must be positive. 



Hewlett's experiments were not known to the writer when 

 in the fall of 1915 some observations were made on mosquitoes 

 of the species Anopheles ptinctipennis Say, which confirm 

 the results obtained by Hewlett in Stegomyia, and render it 

 probable that the basic tropism underlying the bloodsucking 

 instinct is a thermotropism (Marchand, 1918). If female 

 adults of A. punctipennis were placed in a lamp-chimney 

 which on one end was closed by a covering made of cheese- 

 cloth, and at the other end was brought in contact with a 

 heated glass-plate, with a sheet of filter paper between serv- 

 ing as a foothold, the mosquitoes were seen immediately to 

 go to that end of the lamp-chimney where the glass plate was 

 and to alight on the filter paper. Here they began to attack 

 the surface with their probosces, trying to bite through the 

 glass plate. In fact they could be seen bending their pro- 

 bosces as a result of their strong efforts to pierce the surface. 

 They consequently reacted to the heated glass-plate in the 

 same way as if it were the skin of a vertebrate host. This 

 experiment was repeated with different sets of mosquitoes, 

 usually five or six at a time, and always with the same result. 

 The number of specimens refusing to attack the glass-plate 

 v seemed not greater than that usually observed of specimens 

 refusing to accept blood food when offered. Males showed 

 the same reaction but much less strongly; in this respect 

 my observation differs from Hewlett's. Male Anopheles 

 are also slightly attracted by human skin but are not able 

 to pierce it. On the other hand, hibernating females of 

 Culex did not show even a trace of this thermotropic reaction. 

 These mosquitoes, however, also persistently refused to take 



