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definitely misleading, because the area is not rugose. Just anterior to the 
plastron there are two ring-shaped markings (st). These lie beneath the integu¬ 
ment and are crossed by one of its folds. Their nature is entirely obscure. They 
consist of chitin and under high magnifications it is seen that their outline is 
slightly irregular. Megnin described them as rudiments of stigmata; Munro 
figured them but did not mention them in his text; other authors, except 
Warburton, appear to have failed to notice them. 
Behind the plastron half a dozen ridges pass across the dorsum and behind 
these there are a number of transverse rows of scales (sc, Figs. 1 and 3). 
These scales cover the greater part of the mid-dorsal surface; they are absent 
from the flanks and from the area in front of the anus. The rows on which 
they are arranged are continuous laterally with the ordinary ridges of the 
integument, and the scales represent a specialization of the ridge. Various 
stages in the perfection of the scale may be seen, from short blunt projections 
to the fully-developed organ, which is not thickly chitinized nor darker in 
colour than the integument from which it springs. In shape the scale is 
nearly equilateral, with a slightly concave base and very slightly convex sides; 
Fig. 3. Sarcoptes scabiei var. e</ui. On left, cones and scales from the notothorax; on right, 
scales and spines from the notogaster of an adult female; co, cone; sc, scale; sp, spine. 
it is never acorn-shaped. It is difficult to count the scales owing to their 
irregular arrangement, but they number a little over one hundred. In Fig. 1 
I have drawn an example in which the transverse rows are relatively regular, 
and in which traces of a longitudinal arrangement can be seen. The precise 
direction in which some of the smaller scales point is also variable, though the 
general direction of all is backwards. The largest scales are those on the lateral 
part of the notogaster. 
In the example figured in Fig. 1 there is a single definite bare area ( ba) in 
the middle of the dorsum. Its margins are ill-defined but from it scales and 
ridges are absent. In other adult females taken from the same horse this bare 
area is divided into two parts by a row or two of small scales passing trans¬ 
versely across it. The fact that this character is variable is of some importance 
because Railliet and also Canestrini and Kramer regard the presence of two 
bare areas as a specific character of Sarcoptes scabiei var. equi. As there is also 
variation in the number of scales present and in the regularity of the rows in 
which they are set I think we are justified in deciding that specific characters 
founded on them are of doubtful value. It is to these bare areas which occur 
