28 
Argas persicm 
The external sensory organs are apparently limited to the tactile 
hairs and Hallers organ (see PI. II, hg. 4, Hal.org., and PI. VI, fig. 26). 
The tactile hairs, which are distributed over the surface of the body and 
appendages, are for the most part isolated, but in certain places, notably 
the terminal exti’emities of the palps, they are grouped together, in a 
dense cluster (see PI. IV). 
Haller’s organ is a small vesicle containing sensory hairs situated on 
the tarsi of the first pair of legs. Its cavity communicates with the 
exterior by a minute pore. The olfactory function of Haller’s organ is 
now definitely establishedh Argas g)ersicus is extremely sensitive to 
light but definite organs of vision can not be identified. 
In general appearance, the adults of both sexes resemble each other 
very closely. The male is usually smaller than the female, but this rule 
is by no means invariable. The sexes are readily distinguished by the 
shape and size of the genital aperture (see PI. Ill, figs. 3 and 4, gen. 
ap.). In the male, its transverse diameter is considerably narrower than 
the width of the basis capituli and the lips of the opening have an 
elliptical or half-moon shaped contour. The genital aperture of the 
female is a sharply defined transverse slit, the length of which is as 
great as, or exceeds, the width of the basis capituli. 
The nymphs closely resemble the adults in general appearance. 
Their smaller size and the absence of a genital aperture are the more 
obvious points of distinction. The discs are not so numerous, especially 
in the neonymph. 
The larvae are small hexapodous creatures, the 4th pair of legs not 
being developed ; the spiracles are absent and the discs of the later 
stages are represented by a single circular area in the centre of the 
dorsum. The capitulum of the larva is situated so far forward as to 
nearly approach the almost terminal situation of this structure as seen 
in the Ixodidae. Until engorgement takes place, the greater part of 
the capitulum is visible from the dorsal aspect ; after engorgement, 
the anterior margin of the body generally extends so far forward as to 
entirely conceal it. The capitulum of the nymph also occupies a 
position relatively more anterior than that of the adult, though never 
so far advanced as that of the larva. In the unfed neonymph”, it is 
generally possible, from the dorsal aspect, to see the tip of the hypostome 
protruding beyond the anterior margin of the body. 
1 Hindle, E. and Merriman, G. (1912). 
- =lst stage nymph. 
