Sub-order c. Metastigmata.—Integument mostly like that of the Cryptostigmata. Tracheae opening by a pair of stigmata situated above and behind the base of the 4th or 5th or 6th pair of appendages.
Families—Gamasidae (Gamasus, Pteroptus).
Argasidae (Argas, Ornithodoros).
Ixodidae (Ixodes, Rhipicephalus).
Sub-order d. Prostigmata.—Integument soft, strengthened by special sclerites, those on the ventral surface of the prosoma apparently representing the basal segments of the legs embedded in the skin. Tracheae, except in the aquatic species in which they are atrophied, opening by a pair of stigmata situated close to or above the base of the appendages of the 1st pair (mandibles).
Families—Trombidiidae (Trombidium, Tetranychus).
Hydrachnidae (Hydrachna, Atax).
Halacaridae (Halacarus, Leptognathus).
Bdellidae (Bdella, Eupodes).
| Fig. 78.—Holothyrus nitidissimus, one of the Acari; after Thorell. |
| A, Lateral view with appendages III to VI removed; 1, plate covering the whole dorsal area, representing the fused tergal sclerites of the prosoma and opisthosoma; 2, similarly-formed ventral plate; 3, tracheal stigma. B, Dorsal view of the same animal; II to VI, 2nd to 6th pairs of appendages. The 1st pair of appendages both in this and in C are retracted. C, Ventral view of the same; II to VI as in B; a, genital orifice; b, anus; c, united basal segments of the second pair of appendages; d, basal segment of the 6th prosomatic appendage of the right side. The rest of the appendage, as also of app. Ill, IV and V, has been cut away. |