114 
Guide to Arachnida. 
Table-case 
No. 26. 
Sub -order VI. — VERMIEORMIA. 
The Acari belonging to this sub-order are degenerate, parasitic 
forms without tracheae, and with the posterior portion of the 
body produced into an annulated caudal prolongation. The third, 
fourth, fifth, and sixth pairs of appendages are short and three- 
jointed. 
The sub-order includes the single family Demodicidae, the 
members of which live in the sebaceous glands of the skin of man 
and other mammals. A drawing of Demodex caninus, a species 
which gives rise to follicular mange in dogs, is exhibited in Table- 
case 26. This mite is about one-eightieth of an inch in size. 
Fig. 76. 
Demodex caninus , ven- 
tral view of female. 
Greatly magnified 
(after Canestrini). 
Fig. 77. 
Ventral view of a gall- 
mite, Drio'phyes sil- 
vicola, x 135 (after 
Canestrini). 
Sub-order VII. — TETRAPODA. 
These mites are degenerate forms, which resemble the Vermi- 
formia in being without tracheae and in having the body prolonged 
and annulated. The legs of the first two pairs are long and 
provided with the normal number of segments, but those of the 
third and fourth pairs are absent. 
To this sub-order belong the gall-mites, which form a single 
family, Eriophyidae ( Phytoptidae ). They are of very small size and 
are exclusively parasitic on plants of various kinds : many of them 
give rise to pathological conditions resulting in scars, galls, or 
