E. Htndle and G. Merriman 
209 
Reactions to heat. 
The fact that Argas persicus is attracted by radiant heat was first 
noticed in one of our earlier experiments undertaken in order to 
determine whether these ticks reacted to differences in the intensity of 
light. In this experiment a large number of larvae were put into 
a closed glass tube half of which was covered with black paper. 
This was then exposed to an electric light, the tube being arranged 
transversely to the direction of the rays, so that one half was shaded 
and the other brightly illuminated. It was noticed that the ticks 
invariably settled down at the junction of the light and shaded parts. 
At first we wei’e rather at a loss to understand why these animals 
should always select such a position, for when this experiment was 
repeated employing daylight instead of electric light, the ticks settled 
down in the darkest parts of the shaded region and never at the junction 
of the two portions as in the previous case. It was evident from these 
results that in the first case we had overlooked some additional factor 
which was also affecting the parasites. This complicating influence was 
soon found to be the radiant heat fi’om the electric lamp, and the 
reason that the ticks settled down at the junction of the shaded and 
light regions of the tube vvas because this position combined a maximum 
of heat with a minimum of light. If the heat element was eliminated 
it was found that the ticks withdrew into the more shaded regions of 
the tube. 
The effect of radiant heat was also tested employing as the source 
of heat an incubator at a temperature of about 70° C. If a number of 
ticks were liberated within about 6 inches of the incubator they all 
approached it, and as this experiment was performed in the dark, 
phototropism played no part in the reaction. 
A frequent source of error in experiments to decide whether these 
ticks will settle down in either the warm or cold region of a tube 
in which they are confined, is the fact that below a temperature of 
about 15° C., the ticks at once come to rest and remain dormant. As 
a result, if a tube, in which ticks are evenly distributed, be exposed at 
any part to temperatures below 15° C. a certain number will always be 
found in this region owing to their becoming torpid on account of the 
low temperature. 
The effect of moist heat in regard to the feeding of the, tick is 
discussed later. 
Parasitology v 
14 
