C. Warburton 295 
species based on characters which, when deliberately sought for, were found 
to be present in all the forms he was able to examine. 
Chaetotaxy is always important in the Acarina, and should be studied 
with especial care. The bristles and hairs are difficult to see in a transparent 
medium, and particularly so in Canada balsam which has nearly the same 
refractive index. It is absolutely necessary to examine them in some medium 
which shall render them more visible. It will be noticed that in the Plate 
the hairs of Munro’s figures of S. scabiei are much longer than those depicted 
by any other investigator, and this is entirely due to his viewing them in a 
gum-arabic medium. It is possible that picric acid or some other reagent 
might stain them faintly, and there is a third method—of viewing the specimen 
in a coloured field of some colloid pigment, when unsuspected length of bristles 
is often betrayed by the length of the light tracts traced by them on the 
coloured background. The particular distribution and the comparative length 
of these bristles would, if constant, be good specific characters. 
The observations of Fiirstenberg and others on the dorsal scales, the 
notothoracic cones and the notogastral spines should be repeated and extended. 
If, for example, Fiirstenberg is accurate in his delineation of the dorsal scales 
of the sarcopt of man and of the sarcopt of the dog there can be no doubt 
that these two forms are essentially distinct. 
The leg armature and chitinous frame-work, and the epimeres should 
once more be subjected to a very close scrutiny. Fiirstenberg and others 
have delineated the skeletal structures of the legs under a high magnification, 
and with considerable detail of structure. Nevertheless it will be noted that 
not a single one of the best accredited figures of the human sarcopt shows 
the presence of a trochantal claw on the anterior legs. Canestrini figures it 
very clearly in the case of two other “species” ( e.g . see Canestrini, vi. PI. 60) 
and Megnin detected it in a horse sarcopt which he judged to be a new species— 
largely on account of its presence. When, however, he re-examined the human 
Sarcoptes for the express purpose of finding it he was at once successful. In 
fact he was driven to the conclusion that it was a constant feature of all the 
forms of Sarcoptes . 
If there are real specific differences between the various forms of Sarcoptes , 
one would expect them to be indicated in their most fixed, and strongly 
chitinised structures, the epimeres. Slight differences are alleged to exist in 
these epimeres, especially those of the second pair of legs, which are sometimes 
nearly straight, at others sharply bent outwards at their free extremities. 
Their extremities, also, may present useful differences, and be knobbed or 
forked as the case may be. All these details will naturally engage the attention 
of anyone undertaking a revision of the subject. 
In the diagnoses of the various forms mention is generally made of certain 
clear areas (clairieres, Blosse), on the dorsum. It appears to me that the 
terms are used in a double sense, sometimes indicating simply a region where 
the striations are absent (as on the notogaster of the female), but more often 
