L. E. Robinson and J. Davidson 
33 
inconspicuous, and the fact that they are more or less concealed by the 
third pair of legs, makes their recognition in the living animal a little 
difficult. 
In the published descriptions, the spiracles of Argas persicus have 
been stated to have a crescentic or semilunar form. If, however, their 
structure is compared with that of the spiracles of the Ixodidae, it at 
once becomes apparent that the above description applies with accuracy 
to a portion of the spiracle only. As a matter of fact, the actual form 
is ovoid, or more or less circular. Each spiracle appears externally as a 
slightly elevated, convex boss, the surface of which is divided by a deep, 
narrow and curving cleft into two, structurally different portions. The 
anterior portion (see PI. Ill, fig. 6 , a.por.) has a crescentic form, and 
is perforated by innumerable minute pores which are distributed over 
its entire surface. The posterior portion {mac.) is formed of ordinary 
cuticle which is continuous with the general integument. Between 
these two areas the cleft {os.) leads down and opens into a large atrial 
cavity which lies beneath the spiracle, and from which the tracheae take 
their origin. The anterior portion corresponds, therefore, to the area 
porosa of the spiracle of the Ixodidae. The cleft is the ostium of the 
spiracle, and from its relations the posterior imperforate portion can not 
be other than the homologue of the macula. As will be shown in a 
subsequent section of this paper, the internal anatomy of the spiracle 
fully bears out the above comparison. 
The obvious point of difference in structure between the spiracles of 
Ai'gas persicus (this applies, so far as we know, of the spiracles of the 
Argasidae collectively) and those of the Ixodidae is that, in the former, 
the extent of the porose portion of the spiracle is limited to the anterior 
and lateral margins of the macula, while in the more highly specialised 
Ixodidae the porose portion has developed so as to completely surround 
the macula and to isolate it from the general integument. 
Apart from the matter of size, the spiracle in the nymphal stages is 
similar in all respects to that of the adult. 
The Coxal Glands. 
In both adult and nymphal stages a single pair of coxal glands is 
present, the ducts of which open, one on either side of the body, im¬ 
mediately behind the posterior margin of the coxae of the first pair of 
legs (see PI. Ill, fig. 7, o.cx.gl.). The opening is elongate and .slit-like 
and follows the contour of the coxa. It is generally concealed by a pro¬ 
minent fold of the integument, which projects in the interspace between 
Parasitology vi 
3 
