Jin, Miao, Ma, Shuangchun & Tu, Lihong, 2018, A review of the Arcuphantes species with the genitalia of arcuatulus - type (Araneae, Linyphiidae), Zootaxa 4425 (3), pp. 498-510: 500-502

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Arcuphantes  species with the genitalia of arcuatulus  -type

Composition. Arcuphantes arcuatulus (Roewer, 1942)  , A. cruciatus  n. sp., A. denticulatus  n. sp. and A. semiorbiculatus  n. sp.

Diagnosis of the Arcuphantes  species with arcuatulus  -type genitalia. The male palp can be distinguished from that of other Arcuphantes  species by a narrow and posteriorly prolonged radix, and an unbranched lamella characteristica with a broom-shaped and strongly sclerotized apex ( Fig. 2I View Figure ); the epigynum with a U-shaped scape ( Fig. 3 View Figure ).

General description for the Arcuphantes  species with arcuatulus  - type genitalia ( Fig. 1 View Figure ): medium in size, total length 2.15–3.05 in male, 1.59–2.15 in female. Carapace yellowish brown to dark brown. Each eye surrounded by a black ring. Anterior eye row recurved in dorsal view, posterior eye row straight. Anterior median eyes subcontiguous, lateral eyes adjacent. Abdomen dark in color, with black and white spots on the dorsal surface. Legs long and slender, with black rings. Chaetotaxy: Ti I-IV: 2-2-2-2; Mt I-IV: 1-1-1-1; Tm I about 0.19–0.23, Tm IV absent. Tracheal system haplotracheate; epiandrous gland spigots present in male A. cruciatus  n. sp. (other species not examined).

Male palp ( Fig. 2 View Figure ). Both patella and tibia with one macroseta ( Fig. 2G View Figure ). Cymbial retrobasal process wide and blunt ( Fig. 2B View Figure ). Paracymbium narrow and somewhat cross-shaped, with distal arm elongated and rolling up, middle and outer arms arising from anterior and posterior margins, respectively, connected by a transverse ridge ( Fig. 2G–H View Figure ). Distal suprategular apophysis stout, pit hook remnant as a small apophysis pointed forward ( Fig. 4A View Figure ). Embolic division ( Fig. 2I View Figure ): radix narrow, posteriorly prolonged with an anterior process and central serrated area, Fickert's gland located anterior part of radix. Embolic membrane with numerous papillae and chitinous basal part. Lamella characteristica about half of the radix in length, unbranched, broom-shaped apex with numerous strong sclerotized teeth. Terminal apophysis as a large sclerite with several processes, covering over embolus. Embolus trunk-like with serrated surface, a large thumb and a pointed apex at each side of embolus proper ( Fig. 7A View Figure ).

Epigynum ( Fig. 3 View Figure ). Protruding out, posteriorly prolonged ( Fig. 1F View Figure ). Epigynal plate in protruding state with paired groove slits extending on dorsal surface ( Fig. 5F View Figure ). Integument of basal part wrinkled making it extensible. Median plate on dorsal surface about three times longer than wide, and then the posterior part of epigynum becoming gradually narrowed, passing into a scape, without epigynal cavity. Spermathecae located at posterior half part of median plate. Two chambers with special glands at boundary ( Fig. 5H View Figure ), basal part spiraling with one chamber turning upwards and one downwards. Fertilization groove extending forwards along lateral margins of median plate, ending outside epigastric furrow. Scape in U-shaped, with copulatory grooves extending along its dorsal surface, without turning point. Distal part of scape with a pair of lateral lobes, each furnished with a pocket, in which hosting copulatory opening. Neither stretcher nor pit present.

Remarks. As stated by Ma et al. (2016), genitalia of the Nearctic Arcuphantes  species can be divided into three types: fragilis  - type, arcuatulus  - type, and pictilis  - type. The epigyna of the three types are easily distinguished by the shape of the scape: U-shaped in arcuatulus  - type ( Fig. 3 View Figure ), straight in fragilis  - type ( Ma et al. 2016: fig. 4) and S-shaped in pictilis  - type ( Chamberlin & Ivie 1943: fig. 36). Furthermore, the epigyna of fragilis  - type have a triangular epigynal cavity, which is absent in other two types. The male palps of arcuatulus  - type ( Fig. 2 View Figure ) differ from those of fragilis  - type ( Ma et al. 2016: fig. 1; the male of A. pictilis  remains unknown) as follows: 1) the cymbial retrobasal process in the arcuatulus  - type is wide and blunt, while it is a thumb-like apophysis in that of the fragilis  - type; 2) the paracymbium in the arcuatulus  - type is narrow and elongated, with a middle and an outer arm, whereas that of the fragilis  - type is wide with two distal apophyses; 3) the radix in the arcuatulus  - type is posteriorly prolonged with a central serrated area, and Fickert's gland is located anteriorly, while in the fragilis  - type it keeps boat-shaped as that in many other micronetine genera ( Saaristo & Tanasevitch 1996), without central serrated area, and with Fickert's gland located posteriorly; 4) the apex of lamella characteristica in the arcuatulus  - type is broomshaped with strong sclerotized teeth, while in the fragilis  - type it is wide and stout with numerous fine denticles; 5) the terminal apophysis in the arcuatulus  - type is a large sclerite with several processes, while that of fragilis  - type is divided into two parts: a tooth-like anterior terminal apophysis and a waved membranous posterior terminal apophysis; 6) the embolus in both types are trunk-like, while in the fragilis  - type it is S-shaped with a middle depression. Accordingly, each genital type consists of a complex of genital characters, and large gaps exist among different genital types. The relationships among the species having different genital types need to be tested in future study.

Four species recognized here having the genitalia of arcuatulus  - type; one of them is based only on two male specimens and one on two females. Interspecific variation is observed mainly in the characters of the palpal paracymbial apophysis ( Fig. 7 View Figure ), the central serrated area of the radix ( Fig. 2D, I View Figure ) and the terminal apophysis in the male ( Fig. 7 View Figure ), and the shape of the epigynal scape in the female ( Fig. 3B, E, H View Figure ). The rigorous matching between the male and female genitalia plays an important role in the copulation success in linyphiids ( Helsdingen 1965, 1969; Ma et al. 2016), especially in Arcuphantes  species, the close coupling of the male palpal paracymbium and the epigynal scape provides support for the compression of an inflated basal haematodocha that is the key for sperm insemination.

Distribution. USA (Washington, Oregon, California).