Phthiracarus affinis (Hull)

Parry, B. W., 1979, A revision of the British species of the genus Phthiracarus Perty 1841 (Cryptostigmata: Euptyctima), Bull. British Mus. nat. Hist., Zool. ser. 35, pp. 323-363 : 335-336

Phthiracarus affinis (Hull)

(Figs 1; 2; 4; 5; Pls 2a, b, d; 3a, d)

Hoploderma affine Hull , 1914: 287, pl. C9.

Phthiracarus affine : Turk, 1953: 89.

Adult (Pl. 3a): Small and moderately sclerotized. The aspis (Fig. 1C, D) ranges in length from 223 -253 µm with a greatest width of 162-192 µm. All the dorsal setae are fine and short. Setae (il) are about 1-5 times the length of setae (la) and extend two-thirds of the distance between the bases of setae (il) and (ro). The latter do not reach the anterior limit of the aspis. The sensillus (Fig. 1B; Pl. 3d) is 50 - 60 µm long, lanceolate and serrated. The notogaster (Fig. 1A, E) ranges in length from 406 - 487 µm with a greatest depth of 294 - 345 µm. All the setae are relatively long (equal to the distance c1 - d1), fine and erect. Setae c1 and c3 are inserted on the posterior margin of the collar and seta c2 submarginally. Vestigial f1 is located a short distance posterior to seta h1. The fissures ip and ips are absent. On each anal plate (Fig. 2E) there are three setae; an1 and an2 being much longer than ad3. The chelicerae (Fig. 2B, C) are about 121 µm long. The principal segment carries 14-26 sharply pointed spines on the paraxial surface and 9-21 conical spines antiaxially. The leg chaetotaxy (Figs 4; 5) is of the 'complete type' with the setal formulae: I (1-4-2-5-16-1); II (1-3-2-3-12-1); III (2-2-1-2-10-1) and IV (2-1-1-2-10-1). On tarsus I the distal seta coupled with solenidion omega2 is short; its form can only be seen clearly in scanning electron micrographs (Pl. 2a, b). Seta u' on tarsi I and II is often short and thick and resembles a eupathidium. On femur I seta d is short, serrated and curved distally (Pl. 2d).

Distribution: P. affinis is apparently widely distributed in forest soils with a mor humus formation. This species was recorded in the F (fermentation) and H (humus) layers under both the hardwood and coniferous tree species at each of the four study areas (see p. 324) being particularly abundant in the F layer under beech and oak where it constituted 20 - 30 % of the euptyctimoid population.

Material collected from the following localities was also examined: Wytham Woods Estate, Berkshire, 28.X.70 (T. G. Wood); St. Agnes, Isles of Scilly, Cornwall, 5.iv.57 (K. H. Hyatt); Bramshill Forest, Hampshire, 6.xii.70 (B. W. Parry); Woodwalton Fen, Huntingdonshire, 20.V.65 (P. N. Lawrence); Keston Bog, Kent, 8.i.56 (P. N. Lawrence); Thetford Chase, Norfolk, 23.vi.71 (B. W. Parry); Springwell Forest, Co. Londonderry, 1973 (J. Longworth); as well as material labelled ' Hoploderma affine Hull (Ninebanks)' in the Hull Collection deposited in the BMNH.

Remarks: Hull (1914) recorded affinis in moss in woods and in Sphagnum on moors, West Allendale, Northumberland and Gibside, Durham and later (1916) included it in his key to species of the genus Hoploderma . A holotype does not appear to have been designated and no type material is known to exist. Hull's collection was found to contain one tube labelled Hoploderma affine . Three of the specimens are in good condition, conspecific and the shape of their sensilli corresponds with Hull's description: 'Pseudostigmatic organs prominent, slender, fusiform, curved outward and forward, rather long, with a fme point'. Moreover, the specimens are of a similar size to that given by Hull (about 420 µm). One of these specimens, BMNH reg. no. 1973.28 (now mounted in Berlese's fluid), is hereby designated as the neotype. The fourth specimen, although badly damaged, is clearly another larger species with a differently shaped sensillus. The other British material has been compared with the neotype - no morphological differences apart from size Variation could be detected.

P. affinis appears to be close to P. crenophilus , a species described by Willmann (1951) from the edge of a marsh, Ebreichsdorf, Austria, and earlier identified as P. borealis ( Traegaerdh) (see Willmann, 1923, 1928 & 1931). The syntype series (10 specimens collected in Austria, Germany and Switzerland) has been examined and found to be composed of two species, only one of which resembles Willmann's figure of crenophilus and also the neotype of P. affinis . However, the notogastral setae of the latter are considerably shorter than in P. crenophilus .