L. E. Robinson and J. Davidson 
401 
The Sensory Organs. 
Although Argas persicus quickly responds to such external stimuli 
as light and heat, the only recognisable sensory organs which it appears 
to possess are the tactile hairs, which are present on various parts of 
the body and appendages, and Haller’s organ, the function of which has 
been finally and definitely settled by the recent experimental investi¬ 
gations of Hindle and Merriman 1 . 
The tactile hairs are identical in their general form with the 
corresponding structures in the Ixodid ticks, and call for no special 
* 
comment. The most important, conspicuous by reason of their relatively 
large size and concentration, are those with which the extremity of the 
terminal article of the palp is furnished. Long tactile hairs are also 
found, sparsely scattered on the legs, more especially on the tarsal 
articles. Hindle and Merriman have called special attention to the 
well-marked tendency to bring as much of its body surface into contact 
with surrounding objects as possible, which this tick shows in all stages 
of its development 2 . Whether this “ positive thigmotropism ” is the 
manifestation of a special instinct, or whether it is only a secondary 
result of the tick’s inherent desire for concealment and avoidance of 
light, appears to be open to question. It is, however, certain that the 
entire surface of the tick’s body is extremely sensitive to the stimulus 
of contact. 
Haller s organ, in its structure and situation, is identical with the 
corresponding sensory organ in other ticks. The only material difference 
in the structure of the organ, from that of such a form as Haemaphysalis 
punctata, is that the vesicle containing the sensory hairs is relatively 
deeper, and the pore which communicates with the exterior is larger; 
and also, the sensory hairs are apparently confined to the lower portion 
of the vesicle, while the crowded hairs with which the vesicle is filled 
apparently serve a protective function only. 
As already pointed out, although no eyes are present, Argas persicus 
displays a marked dislike to exposure to light, and it would appear, in 
the absence of any recognisable photo-sensitive organ, that the light 
sense is distributed over the entire body surface. The investigations of 
Hindle and Merriman support this view, and they make special allusion 
to the fact that in the larval and early nymphal stages, when the cuticle 
1 Hindle, E. and Merriman, G. (1912). 
2 Hindle, E. and Merriman, G. (1912), p. 208. 
