united in the middle
line behind the mouth, those of the 3rd, 4th, 5th and 6th pairs are
widely separated and not provided with sterno-coxal (maxillary) lobes,
and take no share in mastication; the respiratory stigmata, when
present, belong to the prosoma, and the primitive segmentation of the
opisthosoma has entirely or almost entirely disappeared.

Sub-order a. _Notostigmata._--Opisthosoma consisting of ten segments
defined by integumental grooves, each of the anterior four of these
furnished with a single pair of dorsally-placed spiracles or tracheal
stigmata.

Family--Opilioacaridae (_Opilioacarus_).

Sub-order b. _Cryptostigmata._--Integument hard, strengthened by a
continuously chitinized dorsal and ventral sclerite. Tracheae
typically opening by stigmata situated in the articular sockets
(acetabula) of the 3rd, 4th, 5th and 6th pairs of appendages.

Family--Oribatidae (_Oribata, Nothrus, Hoplophora_).

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_).

[Illustration: 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