206 
Sensot'if Perceptions in Ticks 
The path taken by each tick under the influence of both lights 
sinQultaneously and of each light separately, closely approximated that 
of the tracing of the track represented in Diagram 1. 
In this diagram D and E show respectively the direction of the rays 
of light coming from each source. The tick was placed at the point A 
and up to the point B was exposed to the action of both lights. It will 
be noticed that the path taken by the tick is the resultant of the actions 
of the two beams of light coming from D and E respectively. When it 
reached the point B the light coming from E was shut off by means 
of a screen and at once the tick altered its direction and walked 
directly away from the only remaining light coming from D. It 
was allowed to walk in this direction until it reached G, at which 
point the position of the screen was altered so as to again expose 
the tick to the action of the rays coming from E whilst intercepting 
those from D. Here again the tick altered its direction until it walked 
directly away from E. Similar results were obtained with both 
nymphal and adult stages. 
These experiments show that the light acts equally on both sides of 
the tick and as, with the exception of Haller’s organ {vide infra), 
no sensory organs appear to be' present in A. persicus it may reasonably 
be assumed that the light acts on all parts of the surface. This view 
is supported by the fact that the sensitiveness to the action of light is 
most marked on the larval and early nymphal stages, in which the 
integument is very thin, and least evident in the adults, these being 
protected by a thicker integument. 
II. Perception of differences in the intensity of light. 
Although in the foregoing experiments it has been shown that 
these ticks exhibit the property of negative phototropism, probably 
the most important factor which influences them in their reactions to 
light is a perception of differences in the intensity of the illumination. 
This property may easily be demonstrated by placing a large 
number of ticks in a vessel, part of which is shaded from the light, 
when the ticks congregate in the shaded regions, with the exception of 
the unfed larvae which do not react very deflnitely. 
This perception of differences in intensity is much more strongly 
developed than the negative phototropism as may be shown by the 
following experiment (Diagram 2). 
