Nemesia arboricola Pocock, 1903

Cassar, Thomas, Mifsud, David & Decae, Arthur E., 2022, The Nemesia trapdoor spider fauna of the Maltese archipelago, with the description of two new species (Araneae, Mygalomorphae, Nemesiidae), European Journal of Taxonomy 806 (1), pp. 90-112 : 93-101

Nemesia arboricola Pocock, 1903

Figs 1–4 , 9–14 , 23, 25 , 27–53

Nemesia arboricola Pocock, 1903: 225–226 (♀).

Nemesia arboricola – Baldacchino et al. 1993: 40. — Kritscher 1994: 49–57, figs 1–18 (♀); 1996: 121 (♀). — Dandria 2001: 103–107, fig. 1 pl. 1 (tree nest). — Le Peru 2011: 77. — Decae 2012: 25, fig. 2 (species groups).

New diagnosis

Female

Nemesia arboricola from Malta is difficult to distinguish from N. macrocephala from nearby Sicily ( Kritscher 1994). Females of the two species are among the largest Nemesia species known (fully grown females in both species range from 26 to 28 mm in total body length; the average in Nemesia is around 17.5 mm), and show only slight differences in their sexual and somatic morphology. Kritscher (1994) used the following three diagnostic characters to distinguish females of N. arboricola from those of N. macrocephala : (1) a different distance between the ALE and PLE, (2) a different shape of the spermathecal receptacles, (3) the presence or absence of labial cuspules. Pocock (1903) also reports the configuration of the eyes and the presence of labial cuspules as distinctive for N. arboricola . We found that, in general, the configuration of eyes and in particular the distance between the ALE and PLE ( Figs 1–2 and 5–6 ) are unsuitable to distinguish N. arboricola from N. macrocephala on grounds of overlapping and highly variable measurements (dis. ALE–PLE av. 0.46, sd. 0.13 and av. 0.52, sd. 0.18, respectively). Although the shape of the receptacles appears to differ between the two species ( Figs 3–4 and 7–8 ) it is difficult to quantify these differences. We therefore regard this character as only marginally reliable as diagnostic. We confirm Kritscher’s third diagnostic character (also noted in Pocock 1903) as truly diagnostic and agree that the presence of labial cuspules in females is distinctive for N. arboricola ( Figs 9–10 cf. Figs 15–16 ). Furthermore, we found the dark coloured and speckled opisthosoma in N. arboricola ( Figs 11–12 ) versus the lighter coloured opisthosoma with chevron stripes in N. macrocephala ( Figs 17–18 ), and the thicker, slightly swollen, PMS ( Figs 13–14 cf. Figs 19–20 ) in N. arboricola as diagnostic characters to distinguish females of the two species.

Male

Because the male of N. macrocephala remains unknown, no characters that might distinguish this species from N. arboricola can presently be given. The male of N. arboricola however differs from nearly all Nemesia species, for which the relevant information is available, by the absence of the PTR (see section ‘terminology’ above). The only other species known in which the PTR is missing is N. simoni O. Pickard- Cambridge, 1874 from southwestern France and northern Spain. To illustrate the presence and/or absence of PTR in different species of Nemesia species, Figs 21–24 show the dorso-distal male palps of four different Nemesia species collected from geographically widely separated locations in the Nemesia distribution range ( N. bacelarae Decae, Cardoso & Selden, 2007 from Portugal ( Fig. 21 ), N. cellicola Audouin, 1826 from the Middle East ( Fig. 22 ), N. simoni from southern France ( Fig. 23 ) and N. arboricola ( Fig. 24 ) from Malta). Note that species from opposite far ends of the distribution range and a new species from centrally located Malta ( Fig. 56 ) feature a pronounced PTR only absent in N. arboricola and N. simoni ( Figs 23–26 ). Nemesia arboricola males can be distinguished from those of N. simoni by their generally larger size (TBL » 13, n = 2 vs TBL » 11, n = 7) and on several aspects of its sexual and somatic morphology, its burrow structure and its distribution (compare the male description below with information given on N. simoni in Moggridge 1873). Figs 25–26 show distinctive differences in male palps (shape of tibia) and palpal organs of N. arboricola and N. simoni .

Holotype

In collection BMNH, London, not examined.See Kritscher(1994) for a detailed description and photographs of the specimen (in very poor and fragile condition), and for conformation that N. arboricola is the common and widely distributed cork-door building Nemesia species found in the Maltese Archipelago.

Material examined

MALTA • 1 ♂; Comino ; 36.02° N, 14.33° E; 23 Mar. 1975; P.J. Schembri leg.; (no. TD.4); NHMR • 1 ♀; Wied Incita ; 35.88° N, 14.43° E; 25 Oct. 1980; P.J. Schembri leg.; (no. TD.1); NHMR • 1 ♀; Siggiewi, Buskett ; 35.86° N, 14.39° E; 7 Sep. 1976; P.J. Schembri leg.; (no. TD.2); NHMR • 1 ♀; St. Paul’s Island ; 35.96° N, 14.40° E; 20 Apr.1975; P.J. Schembri leg.; (no. TD.3); NHMR • 1 ♂; Gudja; 35.85° N, 14.50° E; 7 Nov. 2020; T. Cassar leg.; (no. TC-011); NHMW • 1 ♀; Zebbug; 35.87° N, 14.44° E, 27 Aug. 2020; T. Cassar leg.; (no. TC.014 terrestrial); NHMW • 1 ♀; Buskett ; 35.86° N, 14.39° E; 5 Sep. 2020; T. Cassar leg.; (no. TC.015, arboreal); NHMR • 1 ♀; Valetta, Imsida; 35.89° N, 14.48° E; 20 Dec. 2017; (no. TC.016 arboreal); NHMR • 1 ♀; Comino ; 36.02° N, 14.33° E; 11 Sep. 2020; T. Cassar leg.; (no. TC.017 terrestrial); NHMR .

Description

Male reference specimen (no. TD.4, NHMR)

PRESERVATION AND CONDITION. Specimen 45 years preserved in 70% ethanol, generally in good condition ( Fig. 27 ), right bulb removed for detail study ( Figs 31–35 ).

GENERAL COLORATION. Carapace uniform dark brown, chelicerae slightly darker than carapace, abdomen dorsal dark grey with light grey speckles in two parallel rows in the cardiac region ( Fig. 27 ), abdomen ventral light yellowish brown, palps uniform dark brown, legs dark brown proximally grading to light brown distally, sternum uniform yellowish brown, labium dark brown.

CARAPACE. Longer than wide (CW/CL 0.8), with light grey pubescence cover (partly lost), bristles concentrated along the margins and posterior on the thoracic part, cephalic part slightly elevated, thoracic part bulging slightly up from the fovea before sloping down to the posterior margin ( Fig. 28 ).

EYES. Ocular-tubercle ( Fig. 28 ) dome-shaped, clypeus narrow sloping down from eyes, eye-group rectangular, twice as wide as long (EL/PR 0.51), AME about their diameter apart (dis.AME/dia.AME 0.94), dis.ALE–PLE <dia.ALE (ALE–PLE/ALE 0.60).

CHELICERAE. Rastellum with few very strong apical teeth, teeth of diminishing strength and size along distal prolateral margin of chelicerae, single, prolateral row of furrow teeth, fang proximally bent, ventral serrated ridge.

VENTRAL PROSOMA. Maxillae rounded distal lobe, cuspules absent; labium: wider than long (LW/LL 1.6) distally truncated, cuspules absent, labial furrow wide, with two distinct semi-circular sigilla; sternum: longer than wide (SW/SL 0.8) three pairs oval sigilla, even cover of bristles.

PALPS. Cymbium with distal group of spines, tibia proximally lightly inflated (TibW/PTib 0.4),PTR absent ( Figs 23, 25 ), patella spineless, femur with few dorso-distal spines, longer than tibia (PFem/PTib 1.3); palpal organ: proximal bulbous part simple pyriform, embolus strong, stiff, curved in ventral and dorsal views ( Figs 31, 33 ), straight and distally narrowing in prolateral and retrolateral views ( Figs 32, 34 ), embolus tip blunt, and slightly flattened or slightly scooped ( Fig. 35 ).

LEGS. All tarsi light coloured with fine ventral scopulae, all femora cylindrical with few spiny bristles dorsally, metatarsus I curved, CF with fine, short bristles ( Fig. 29 ), ventro-apical spines and one central prolateral spine, tibia I distally widened, TA distinct, TS distally slightly sigmoid ( Fig. 29 ), central row of three prolateral spines, patella I with a single prolateral spine, metatarsus and tibia II cylindrical with few spines, patella II with two prolateral spines, metatarsus and tibia III as II, patella III with single prolateral spine ( Fig. 30 ), metatarsus IV> femur IV> tibia IV (Fem4/Met4 = 0.97, Tib4/Met4 = 0.83), patella IV 0–1 retrolateral patellar spine, PTC all tarsi double combs of teeth, leg formula: 4123.

OPISTHOSOMA. Ovoid, anterior narrowing, covered with bristles, PMS small, close together and less swollen than in females, PLS short, thick, proximal segment longer than medial + distal segment, spigots restricted to apical spigot field.

MEASUREMENTS. TBL = 13.0; CL = 6.9; CW = 5.7; CP = 3.8; AR = 1.07; PR = 1.07; EL = 0.55; dia.ALE = 0.25; dia.PLE 0.21; dia.AME = 0.18; dia.PME = 0.17; dis.AME–AME = 0.17; dis.ALE–PLE = 0.15; SL = 3.6; SW = 2.9; LL = 0.6; LW = 1.0; Palp = 9.5 (1.3 + 2.8 + 1.8 + 3.6); Leg I = 20.3 (2.9 + 4.3 + 4.1 + 3.1 + 5.9); Leg II = 19.6 (2.6 + 4.6 + 4.2 + 2.9 + 5.3); Leg III = 19.5 (2.7 + 5.0 + 3.9 + 2.6 + 5,3); Leg IV = 25.0 (3.2 + 6.6 + 5.5 + 3.2 + 6.5); BuL = 2.11; BuW = 0.79; EmL = 1.06.

VARIATION MALES (n = 2). TBL = 12.8, 13.0; CL = 5.8, 6.9; CW = 5.0, 5.7; CP = 3.5, 3.8; AR = 1.07, 1.11; PR = 1.07, 1.08; EL = 0.55, 0.56; dia.ALE = 0.25, 0.27; dia.PLE = 0.21, 0.22; dia.AME = 0.18; dia. PME = 0.15, 0.17; dis.AME–AME = 0.17, 0.20; dis.ALE–PLE = 0.10, 0.15; SL = 3.1, 3.6; SW = 2.5, 2.9; LL = 0.5, 0.6; LW = 0.9, 1.0; Palp = 8.6, 9.4; Leg I = 18.0, 20.3; Leg II = 17.3, 19.7; Leg III = 16.9, 19.4; Leg IV = 22.5, 24.9; Bul = 1.88, 2.11; BuW = 0.71, 0.79; EmL = 0.78, 1.06.

Notes on female

Females of N. arboricola have been described in proper detail by Pocock (1903) and Kritscher (1994). Here we compare specimens collected from tree-nests with specimens collected from terrestrial burrows. Figures 36–42 , illustrate the variation in general appearance of the specimens in our study sample. Although individual variation in size, colour patterns and shades are evident, no qualitative morphological differences were found that would distinguish tree dwelling spiders from ground dwelling spiders at the species level. Moreover, all specimens in our sample closely fit the descriptions given by Pocock (1903) and Kritscher (1994). On these grounds we feel confident to state that arboreal and terrestrial specimens studied here are conspecific members of N. arboricola Pocock (1903) . Figures 43–44 , show characters common to all N. arboricola specimens (terrestrial and arboreal) studied, that distinguish N. arboricola from its supposed sister species, N. macrocephala .

Mesurements, variation females (n = 7) TBL = 21.1–26.9 (av. 23.2, sd. 2.5); CL = 7.8–9.1 (av. 8.3, sd. 0.6); CW = 6.4–7.5 (av. 6.8, sd. 0.4); CP = 4.8–5.6 (av. 5.1, sd. 0.3); AR = 1.33–1.51 (av. 1.41, sd. 0.07); PR = 1.29–1.51 (av. 1.38, sd. 0.09); EL = 0.53–0.77 (av. 0.71, sd. 0.09); dia.ALE = 0.26–0.41 (av. 0.35, sd. 0.05); dia.PLE = 0.21–0.34 (av. 0.29, sd. 0.04); dia.AME = 0.13–0.20 (av. 0.18, sd. 0.03); dia.PME = 0.14–0.23 (av. 0.18, sd. 0.03); dis.AME–AME = 0.20–0.34 (av. 0.26, sd. 0.05); dis.ALE– PLE = 0.10–20 (av. 0.16, sd. 0.04); SL = 3.8–5.2 (av. 4.7, sd. 0.5); SW = 3.6–4.1 (av. 3.8, sd. 0.2); LL = 0.9–1.1 (av. 1.0, sd. 0.1); LW = 1.4–1.6 (av. 1.5, sd. 0.1); Palp = 11.9–13.5 (av. 12.5, sd. 0.8); Leg I = 16.1–18.7 (av. 17.1, sd. 1.0); Leg II = 14.6–16.9 (av. 15.4, sd. 0.8); Leg III = 14.5–16.9 (av. 15.5, sd. 0.9); Leg IV = 20.1–23.4 (av. 21.5, sd. 1.2).

Field observations

Nemesia arboricola constructs a simple tube-shaped burrow, capped with a sturdy lid (trapdoor) fitting neatly into the shaft ( Figs 46, 51 ). Apparently older nests have been found with a layer of moss growing on the door surface ( Fig. 45 ). The burrows are constructed in natural settings such as in tree trunks, between rocks in rubble walls and sloping ground ( Figs 45–53 ). Tree-nests have been found between 150 cm and 8 cm above the ground surface in the trunks of ornamental palms ( Phoenix canariensis and P. dactylifera ), carob trees ( Ceratonia siliqua ) and olive trees ( Olea europaea ). In such tree-nests, spiders make use of pre-existing hollows and holes in the tree trunks into which they can snugly fit, or make use of hollows which are filled with woody debris and soil into which they can easily burrow. Burrows have also been found, at similar heights above the ground surface, in old rubble walls which hold back soil at field margins; here the spiders burrow into the clayey soil which fills the cracks between the individual stones. In both tree-nests and wall-nests, the burrow lids are flush with the vertical surface and dorsally hinged so that the trapdoor opens vertically upwards, with the shaft directed into the trunk/ wall perpendicular to the vertical axis. Nemesia arboricola also nests directly in the ground, but always it seems under one of the following conditions: the ground must be steeply sloping or, if the soil is flat, located immediately beneath an overhanging structure such as large rocks or a rubble wall ( Figs 52–53 ). No burrows have been found on level ground in full exposure (‘in plain sight’ as it were). Tree-nests and wall-nests vary from 4.5 cm to 9.5 cm in depth for mature individuals, with burrow width varying according to the size (maturity) of the individual. Ground-nests in deep soil have, on average, deeper shafts than tree or wall-nests. In all cases, the burrow shaft is relatively simple in construction; it has entire, thick lining of silk with no internal doors, plugs or other features; and it is unbranched.

The resident spiders themselves respond to disturbance by holding the hinged door very tightly; if the door is prised open using forceps (which takes considerable force) the spider may either retreat immediately head-up to the bottom of the shaft, or else repeatedly attempt to close the door while striking the forceps aggressively with its chelicerae. Recently hatched juveniles are retained within the mother’s burrow, from which they disperse and apparently establish themselves in close proximity, as N. arboricola populations are always localized and often quite dense. Moulted exoskeletons are discarded from the burrow and may be observed outside near the burrows if they are sheltered from wind. During the aestivation period, the door to the burrow is sealed shut from the inside, but the door still remains exposed as under normal circumstances (i.e., the burrow lid is not concealed with soil during periods of inactivity). Most mature males emerge from their burrows and wander between the months of November and March, though a few individuals may appear outside this timeframe.

Figures

Figs 1–8. Characters reported to distinguish N. arboricola Pocock, 1903 from N. macrocephala Ausserer, 1871 in Kritscher (1994). 1–4. N. arboricola; represented by two specimens (TC.016, NHMR, Figs 1 & 3, specimen collected from an arboreal-nest; TC.017, NHMR, Figs 2 & 4, specimen collected from a terrestrial nest). 5–8. N. macrocephala; represented by two specimens (Isaia.046, Figs 5 & 7 and Isaia.047, Figs 6 & 8), both collected from terrestrial nests near the type locality, Palermo, Sicily. 1–2, 5–6. Comparative eye-formations. Note the overlap and variation in distances between ALE and PLE (ALE–PLE N. arboricola ranges between 0.10–0.20, n = 7; N. macrocephala ranges between 0.14– 0.22, n = 2). Furthermore, note the variation in general configuration of the eyes that renders species recognition on these characters uncertain. 3–4, 7–8. Comparative shapes of spermathecae in ventral view. Note both similarities and differences in shape and differences in distance between receptacles of both species that renders unambiguous species recognition difficult, although spermathecal receptacles in N. arboricola shown here are somewhat smaller and finer built. Scale bars = 1 mm.

Figs 9–20. Characters distinguishing Nemesia arboricola Pocock, 1903 from N. macrocephala Ausserer, 1871. 9–14. N. arboricola; represented by two specimens (TC.016, NHMR, Figs 9, 11, 13, specimen collected from an arboreal-nest; TC.017, NHMR, Figs 10, 12, 14, specimen collected from a terrestrial nest). 15–20. N. macrocephala; represented by two specimens (Isaia.046; Figs 15, 17, 19 and Isaia.047; Figs 16, 18, 20), both collected from terrestrial nests near the type locality, Palermo, Sicily. 9–10, 15–16. Presence or absence of labial cuspules. Note presence of labial cuspules (cu) in N. arboricola (Figs 9–10) versus absence of labial cuspules in N. macrocephala (Figs 15–16). 11–12, 17–18. Colour pattern opisthosoma. Note dark coloured and speckled opisthosoma in N. arboricola (Figs 11–12) versus light coloured ophistosoma with chevron lines (Figs 17–18) in N. macrocephala. 13–14, 19– 20. Differences in structure of PMS. Note thickened and slightly swollen PMS (is) in N. arboricola (Figs 13–14) versus slender, conical PMS (cs) in N. macrocephala (Figs 19–20). Scale bars = 1 mm.

Figs 21–26. Male diagnostic features for species of Nemesia Pocock, 1903. 21–22. Dorsal palp tibia, PTR is present in all species ofNemesia for which information is available, here illustrated by two species from the eastern and western ends of the Mediterranean distribution range of the genus; N. cellicola Audouin, 1826 (21) from the Middle East and N. bacelarae Decae, Cardoso & Selden, 2007 (22) from Portugal. 23–24. Dorsal palp tibia of the only Nemesia species known in which the PTR is absent; N. arboricola Pocock, 1903 (23) from Malta and N. simoni O. Pickard-Cambridge, 1874 (24) from the western Pyrenees. 25–26. Difference in distal palp morphology (retrolateral view) showing differences in tibia and palpal organ that distinguish N. arboricola (25) from N. simoni (26).

Figs 27–35. Nemesia arboricola Pocock, 1903, ♂ reference specimen (TD.4, NHMR). 27. Dorsal habitus. 28. Prosoma lateral, note slight elevation of the cephalic part, ocular process and gradual posterior slope of thoracic part. 29. Metatarsus and tibia leg I, note tibial apophysis (TA), tibia spur (TS), clasper field (CF). 30. Prolateral patella III with single spine (SS). 31–34. Right palpal organ in four different positions rotated clock-wise by 90°. 31. Ventral view, 32. Prolateral view. 33. Dorsal view. 34. Retrolateral view. 35. Embolus tip at high magnification, note slightly scooped tip.

Figs 36–44. Female characters in Nemesia arboricola Pocock, 1903. 36–42. Variation in general appearance of seven female specimens contained in our study sample, with collection site indication and total body length (TBL). 36. TC.015, NHMR, found on treePhoenix canariensis H.Wildpret, TBL = 23.0. 37. TC.016, NHMR, found on tree Ceratonia siliqua L., TBL = 23.3. 38. TC.014, NHMW, terrestrial burrow at Zebbug, TBL = 21.1. 39. TC.017, NHMR, terrestrial burrow on Comino Island, TBL = 26.9. 40. TD.1, NHMR, terrestrial burrow at Wied Incita, TBL = 26.1. 41. TD.2, NHMR, terrestrial burrow at Buskett, TBL = 20.0. 42. TD.3, NHMR, terrestrial burrow at St Paul’s Island, TBL = 22.3. 43. Labial cuspules (cu) were present in all specimens shown. 44. Swollen PMS were present in all specimens shown. The presence of labial cuspules and swollen PMS distinguish N. arboricola from the closely related N. macrocephala Ausserer, 1871.

Figs 45–53. Nemesia arboricola Pocock, 1903 excavates its burrows in various natural settings such as crevices between rocks and roots and pre-existing, soil filled cavities in rocks and tree trunks. 45. Circular outline of a trapdoor in open, moss-covered soil. 46. Same trapdoor opened to show its thick, plug-like structure that snugly fits into the borrow entrance opening. 47. Trapdoor capping a burrow excavated in a soil filled hole near the base of a carob tree. 48. The same trapdoor (47) in closeup. 49. Trapdoor capping a burrow excavated in a soil filled hole in an olive tree. 50. Trapdoor capping a burrow excavated in a soil filled hole in an unidentified tree. 51. The same trapdoor (50) in close-up and opened. 52. Circular outline of a trapdoor in dry soil at the bottom of a rubble wall. 53. Exposed top of a burrow and trapdoor in soil between rocks of a rubble wall.

Figs 54–63. Nemesia maltensis sp. nov. holotype, ♂ (TC.004, NHMR). 54. Dorsal habitus (note distinct dark flanks of cephalic part). 55. Prosoma in lateral view (note dark margin of the carapace and cephalic part not elevated). 56. Dorso-distal right palp (note strong development of PTR). 57. Prolateral left metatarsus and distal tibia leg I (note CF with dense brush of short bristles, short TA and smoothly curved TS). 58. Spinnerets, lateral view showing macula (MAC) on external proximal article of PLS. 59–63. Right palpal organ in four different positions rotated clock-wise by 90°. 59. Ventral view. 60. Prolateral view. 61. Dorsal view. 62. Retrolateral view. 63. Embolus tip at high magnification, note short row of tiny denticles (den) and bevelled tip.