L. E. Robinson and J. Davidson 
25 
External Anatomy. 
Plates I and II. 
In general structure, Argas persicus is a typical example of the 
family Argasidae. As in the majority of the Acarina, the opisthosoma^ 
of the adult and nymphal stages exhibits no obvious external traces of 
the primitive segmentation, and even in the larva, the arrangement of 
the hairs on the surfaces of the body is such as to give only the merest 
suggestion of metamerism. 
The contour of the body (PI. I, figs. 1 and 2) is oval, narrowing 
anteriorly, the maximum width being attained at about the level of the 
posterior third of the entire length. The dorsal surface of unfed in¬ 
dividuals is flattened, and towards the periphery, where the thickness of 
the body thins out to an acute margin, is usually somewhat reflexed. 
The ventral surface is always more or less convex. 
In fasting specimens, the prevailing colour of the dorsum is a dull 
greyish yellow, darker and opaque in the median field, paler and 
translucent towards the margins. The ventral surface, legs and 
capitulum are always paler in colour than the dorsum. After feeding, 
the general colour alters, gorged specimens presenting a somewhat 
dark, leaden appearance, due to the ingested blood showing through 
the translucent integument. The legs and capitulum, however, retain 
their pale colour and show in stronger contrast. Both surfaces of the 
body then become strongly convex, and in a fully distended individual, 
the margin loses its acuteness. In partially gorged young individuals, 
and also specimens in which the contents of the alimentary canal have 
been absorbed, it is possible to define the form of the stomach with its 
coecal appendages through the dorsal body wall (see PI. I, fig. 1, st, and 
cc.al.). In the latter case, the alimentary canal retains a dark-coloured 
residue of undigested detritus which makes it clearly visible. 
The body is covered with a stout, chitinous cuticle of a leathery 
texture. The greater part of the surface is rough and granular, ex¬ 
cepting the capitulum with its appendages, the legs, and certain parts 
of the body to be specified later. 
Scattered over the entire dorsal surface, but confined to the posterior 
and marginal parts of the ventral surface, are a large number of minute 
dark-brown glistening areas with a more or less circular outline. 
These areas, which we term discs (see Pis. I and II, figs. 1-4, dc.), have 
1 Lankester, E. E. (1904). 
