or immovable. Appendages of 2nd pair weak, pediform not prehensile. Genital orifice covered by an operculum.
- Families—Phalangiidae (Phalangium, Gagrella).
- Ischyropsalidae (Ischyropsalis, Taracus).
- Nemastomidae (Nemastoma).
- Trogulidae (Trogulus, Anelasmocephalus).
Sub-order c. Cyphophthalmi (Anepignathi).—Orifice of foetid glands opening on a tubercle situated near the lateral border of the carapace above the base of the 5th appendage. Orifice of coxal gland probably situated at base of coxa of 5th appendage; sternal plate of prosema minute or absent; no prosternal element underlying the mouth. Coxae of 5th and 6th, and usually also of 4th appendages immovable. Appendages of 2nd pair weak, pediform, not prehensile. Genital orifice not covered by an operculum.
- Families—Sironidae (Siro, Pettalus).
- Stylocellidae (Stylocellus).
Remarks on the Opiliones.—These include the harvest-men, sometimes called also daddy-long-legs, with round undivided bodies and very long, easily-detached legs. The intromittent organs of the male are remarkable for their complexity and elaboration. The confluence of the regions of the body and the dislocation of apertures from their typical position are results of degeneration. The Opiliones seem to lead on from the Spiders to the Mites. Reference to literature (39).
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Fig. 77.—Stylocellus sumatranus, one of the Opiliones; after Thorell. Enlarged. A, Dorsal view; I to VI, the six prosomatic appendages.
B, Ventral view of the prosoma and of the first somite of the opisthosoma, with the appendages I to VI cut off at the base; a, tracheal stigma; mx, maxillary processes of the coxae of the 3rd pair of
appendages; g, genital aperture. C, Ventral surface of the prosoma and opisthosoma; a, tracheal stigma; b, last somite.
D, Lateral view of the 1st and 2nd pair of appendages.
E, Lateral view of the whole body and two 1st appendages, showing the fusion of the dorsal elements of the prosoma into a single plate, and of those of the opisthosoma into an imperfectly segmented plate continuous with that of the prosoma.
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Apparently related to the Opiliones are two extinct groups, the Anthracomarti and Phalangiotarbi, which are not known to have survived the Carboniferous period. In the Anthracomarti the opisthosoma was movably articulated to the prosoma, and consisted of from eight to ten segments furnished with movable lateral plates, the anal segment being overlapped dorsally by a laminate expansion of the preceding segment. The carapace of the prosoma was unsegmented and often bore a pair of eyes. The appendages of the 2nd pair were slender and pediform; those of the 3rd, 4th, 5th and 6th pairs were similar in form and ambulatory in function with their basal segments arranged round a sternal area as in the order Araneae. The best-known genera were Anthracomartus and Eophognus.
In the Phalangiotarbi the appendages resembled those of the Anthracomarti, except that the basal segments of the last four pairs were usually approximated in the middle line leaving a long and narrow sternal area between; and the carapace of the prosoma was unsegmented. The prosoma and opisthosoma were broadly confluent and probably immovably welded together. The opisthosoma consisted of eight or nine segments, whereof the anterior five or six were very short in the dorsal region, and the posterior three exceptionally large with the anal orifice terminal.
Several genera have been established, the best-characterized being Geraphognus and Architarbus.
Order 9. Rhynchostomi = Acari (see fig. 78).—Degenerate Arachnids resembling the Opiliones in many structural points, but chiefly distinguishable from them by the following features:—The basal segments of the appendages of the 2nd pair are 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).
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| 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. III, IV and V, has been cut away.
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Sub-order e. Astigmata.—Degenerate, mostly parasitic forms approaching the Prostigmata in the development of integumental sclerites and the softness of the skin, but with the respiratory system absent.
- Families—Tyroglyphidae (Tyroglyphus, Rhizoglyphus).
- Sarcoptidae (Sarcoptes, Analges).
Sub-order f. Vermiformia.—Degenerate atracheate parasitic forms with the body produced posteriorly into an annulated caudal prolongation, and the 3rd, 4th, 5th and 6th pairs of appendages short and only three-jointed.
- Family—Demodicidae (Demodex).
Sub-order g. Tetrapoda.—Degenerate atracheate gall-mites in which the body is produced posteriorly and annulated, as in Demodex, but in which the appendages of the 3rd and 4th pairs are long and normally segmented and those of the 5th and 6th pairs entirely absent.
- Family—Eriophyidae (Eriophyes, Phyllocoptes).
Remarks on the Rhynchostomi.—The Acari include a number of forms which are of importance and special interest on account of their parasitic habits. The ticks (Ixodes) are not only injurious as blood-suckers, but are now credited with carrying the germs of Texas cattle-fever, just as mosquitoes carry those of malaria. The itch-insect (Sarcoptes scabiei) is a well-known human parasite, so minute that it was not discovered until the end of the 18th century, and “the itch” was treated medicinally as a rash. The female burrows in the epidermis much as the female trap-door spider burrows in turf in order to make a nest in which to rear her young. The male does not burrow, but wanders freely on the surface of the skin. Demodex folliculorum is also a common parasite of the sebaceous
