202 ROBERTSON, ON A SPECIES OF ACARUS. 
pairs, and are attached to a hard sternum-like mass, situated 
about the middle of the body, in the median line. Each leg 
consists of five short joints, the terminal of which is straight, 
pointed, and slightly hooked. A few hairs project from the 
sides of the body and the outer surface of the legs. The last 
Fig. 2. Fig. 1. 
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Fig. 
1.—Portion of jugular vein of a pigeon, Columba livia, with a large num- 
ber of Acari attached to its walls. Natural size. 
2.—Ventral surface of Acarus. About 100 diameters. 
3.—Ventral surface of the head of the same. About 400 diameters. 
4,—The two posterior pairs of legs, and the sternum-like mass into which 
they are inserted. About 400 diameters. 
joints of the legs have a considerable number of longer hairs, 
which come out all round the insertion of the hooked 
extremity. 
From the above description it will be seen that this 
acarus agrees with sarcoptes in having a considerable 
interval between the second and third pairs of legs, and the 
absence of a furrow between them. 
I have seen during last vear, in the dissecting-room of the 
Museum, three pigeons affected with the’securious parasites, 
one in February and two in June.* It is rather remarkable 
that all the examples which I have examined should have 
eight legs, and the other parts presenting appearances com- 
mon to all. I hope shortly to meet with them in an earlier 
stage of development. 
* T have since examined a considerable number of both the wood and the 
tame pigeon, and have seldom found them free from Acari. 
