P. A. Buxton 
115 
matters and deal only with the Sarcoptes of the horse, terming it S. scabiei 
var. equi Gerlach, without prejudice to the question of its rank as variety or 
species. I shall be glad to receive at the Quick Laboratory, Cambridge, 
specimens of Sarcoptes or other mange mites from any animal; the specimens 
should be sent alive if possible. When sufficient material has accumulated I 
hope to carry out a comparative study of the Sarcoptes of various animals. 
I shall not deal with the life-history of S. scabiei var. equi. A good biological 
study of the closely allied, if separable, S. scabiei var. hominis has recently 
been published by Munro (1919), and, since it bears on my anatomical studies 
of the various stages of S. scabiei var. equi described in this paper, an outline 
of the life-cycle will be useful. From the egg hatches the larva, and from 
the larva emerges the nymph; I do not think that there is any external 
difference to be found between male and female. It is believed that the 
nymph, on moulting, produces either an adult male or an immature female. 
It is currently held that the immature female is the stage in which copulation 
occurs, but I have produced evidence (p. 139) for believing that in Sarcoptes 
the act is performed only by the adult female. We believe then that in the 
male there are three stages after the egg, and in the female four, that is to say 
the larva, the nymph, the immature female and the adult female. 
THE ADULT FEMALE. 
The following measurements are taken from ten females measured. 
Length from tip of chelicera to end of body, 357-432 microns (average 388 
microns); breadth 250-295 microns (average 271). The ratio of length to 
breadth varies between 100/79 and 100/66, the average of the ten being 100/70. 
It will be seen that there are considerable individual differences not only in 
absolute size, but also in the relation between length and breadth. These 
differences are in all probability due to the action of the great longitudinal 
and dorsi-ventral muscles, which must be capable of producing great altera¬ 
tions in the shape of the mite. 
Method. All the specimens were measured alive, and then cleared in gum 
arabic in order that I might satisfy myself by the examination of the toco- 
stome that I was dealing with adult females. It is essential that this method 
should be followed in measuring Sarcoptes because it is readily compressed by 
a cover-slip, and altered in size and shape by fixation. According to the earlier 
authors there are considerable differences in size and shape between the 
various species of Sarcoptes , but we do not know how many individuals they 
measured nor under what conditions, so that it is doubtful whether the measure¬ 
ments of one author are comparable with those of another. The dimensions 
of S. scabiei var. equi , as given by Canestrini and Kramer, are length 450-500 
microns, breadth 310-370 microns. These measurements are considerably in 
excess of my own, and were possibly taken from mounted specimens. Gerlach 
gives length 440, breadth 300 microns for the female, figures much closer 
to my own. 
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