For a long time, dimorphic erigonid spiders were not recognised as such and considered separate species. Only when morph differences were small, for example in the size of the cephalic tubercle or the post-ocular sulci, were these sometimes considered variations. Holm (1979: p. 269) wrote about Pelecopsis mengei (Simon, 1884): “The males occur in two different forms, the one, which is the most frequent, with a high cephalic lobe and large sulcal orifice, the other with lower lobe and with much smaller orifice. ... As no intermediate forms have been found and moreover, the two types of males have quite similar palpal tibiae and bulbs and are found together, the males of P. mengei seem to be dimorphic”. Similarly, Bosmans & Abrous (1992) considered the specimens of Pelecopsis oranensis (Simon, 1884) with small and large postocular sulci, but having identical palpal tibiae and bulbs, as morphs of the same species. Diplocephalus marijae Bosmans, 2010 from Spain is another species occurring in two morphs (Bosmans et al. 2010). The decision to recognise species as being dimorphic is not easy or consequent. Roberts (1987) proposed Troxochrus scabriculus (Westring, 1851) and T. cirrifrons (O. Pickard-Cambridge, 1871) to be one, dimorphic species, but this is not followed in the World Spider Catalog (2018) where they are still considered two separate species. On the contrary, when the same author (Roberts 1987) proposed Diplocephalus connatus Bertkau, 1889 and D. jacksoni (O. Pickard-Cambridge, 1904) to be forms of the same species, this opinion was accepted in the World Spider Catalog (2018).

The best documented case of dimorph linyphiid spiders is that of Oedothorax gibbosus (Blackwall, 1841) and O. tuberosus (Blackwall, 1841) having very different cephalic tubercles and because of that considered separate species in older identification books (e.g., Locket & Millidge 1953, Wiehle 1960, Palmgren 1976). After a detailed study of the male palps of several Oedothorax species, Bosmans (1985) concluded that all palpal sclerites of O. gibbosus and O. tuberosus were completely identical and the two species names were synonymized. De Keer & Maelfait (1988) provided further evidence from breeding experiments. They reared spiderlings from the same egg sac and obtained both forms. In our opinion, when palpal sclerites are identical, specimens should be treated as belonging to the same species.

The aim of the present paper is to discuss two more cases of male dimorphism in Linyphiidae.

Material and methods

The material studied was collected by the authors or loaned from museum collections. Species were examined by mean of a Nikon SMZ1270 stereo microscope. Details of male palps and female epigynes were studied with an Olympus CH-2 microscope with a drawing tube. Left palps are illustrated.

Male palps were detached and transferred to glycerol for examination under the microscope. Female epigynes were excised using sharpened needles. These were then transferred to clove oil for examination under the microscope. Later, palps and epigynes were returned to 70% ethanol.

Abbreviations: CAR-S = Personal collection of Antony Russell-Smith (UK), CPO = Personal collection of Pierre Oger (Belgium), CRB = Personal collection of Robert Bosmans (Belgium), CSD = Personal collection of Samuel Danflous (France), MNHN = Muséum National d'Histoire naturelle, Paris, France (curator: C. Rolland).

A forgotten case of dimorphism

Males and females of Diplocehalus cristatus (Blackwall, 1833) were first described by Blackwall (1833) from England as Walckenaeria c. Subsequently, O. Pickard-Cambridge (1875) described Erigone foraminifera Pickard-Cambridge, 1875 from France. Differences between the two species were based on differently shaped cephalic lobes. Pickard-Cambridge (1875: p. 208) stated that “E. foraminifera is also allied to E. cristata, but the very different form of the caput and its cleft ... will distinguish it at once”.

Later, Simon (1884, 1926) also gave considerable importance to the shape of cephalic lobes in describing several (sub) species in the genus Prosoponcus: P. bicephalus Simon, 1884, P. rectilobus Simon, 1884 an P. thyrsiger Simon, 1884. None of these species was matched to a female, except for P. bicephalus bicephalus; yet the female of the latter species appeared to be that of Dicymbium nigrum (Blackwall, 1834). Denis (1948) added one more species to this species group, D. arvernus, from France, Auvergne. He stated that this species was closely related to D. foraminifer, but slightly differed in the shape of cephalic lobe.

Fig. 1:

a-c. Diplocephalus cristatus (Blackwall, 1833) (Belgium), d-f. D. rectilobus (Simon, 1884) (the holotype), g-i. D. foraminifer (O. Pickard-Cambridge, 1875) (Greece), j-l. D. bicephalus (Simon, 1884) (the lectotype), m, o: D. arvernus Denis, 1948 (from Denis 1948, figs 1–8); n. D. foraminifer (O. Pickard-Cambridge, 1875) (from Deltshev, 1985, fig. 7); a, d, j, m. Male prosoma, lateral view; b, e, h, k, n. Male palp, lateral view; c, f, i, I, o. Male palpal tibia, dorsal view.

Georgescu (1969) was the first author to propose that D. cristatus, D. foraminifer, D. bicephalus, D. rectilobus and D. thyrsigeri belong to the same species, occurring in two morphs: cristatus and foraminifer. She also included Diplocephalus crassilobus (Simon, 1884) in that list, but the conformation of the male palp of the latter species is completely different (cf.,Millidge 1979, Pesarini 1996). The suggestion by Georgescu has not been followed and in the World Spider Catalog (2018), these names are currently listed as separate species. We have been able to re-examine the material of all these Diplocephalus species, including the types of D. bicephalus and D. rectilobus, and can confirm Georgescu's proposal.

Fig. 2:

Diplocephalus cristatus morph foraminifer (Pickard-Cambridge, 1875) (Greece, Lefkada). a. Male prosoma, lateral view; b. Male palp, lateral view; c. Male palpal tibia, dorsal view; d. Embolic division, antero-lateral view; e. Epigyne, ventral view; f. Vulva, ventral view.

Fig. 3:

Savignia harmsi Wunderlich, 1980. a. Morph typica, dorsal view; b. Morph cor, dorsal view c. Male palp of morph typica, lateral view, d. Ibid, of forma cor.

Comments on the type material

The only material of Diplocephalus bicephalus (originally as Prosoponcus b.) that is available in MNHNP is the male, which is therefore designated as the lectotype. The two accompanying females belong to Dicymbium nigrum (Blackwall, 1834), and Simon's figure 674 (Simon 1884) obviously shows the epigyne of this species. The only material of Prosoponcus rectilobus available in the MNHNP is the male holotype.

Comments on the synonymy

We first became interested in the Diplocephalus cristatus complex, while studying specimens collected by Antony Russell-Smith from Lefkada, Greece (Fig 2). A number of males and females were collected from near a spring and they are illustrated in Fig. 2. Having tried to identify these specimens, we found out that their palps and epigynes were completely similar to those of the common European species D. cristatus, but the males had very different cephalic lobes. Further research showed a clear match with D. foraminifer and D. arvernus, as illustrated by Deltshev (1985), Denis (1948) and Georgescu (1969) — compare above the section “A forgotten case of dimorphism”.

Figs 1b-c, e-f, h-i, k-1 and n, o show the male palps and palpal tibiae of respectively D. cristatus, D. rectilobus, D. foraminifer, D. bicephalus and D. arvernus. Detailed examinations of all palpal sclerites and palpal tibiae revealed no differences. Simon (1926: p. 495) already wrote about D. rectilobus in a footnote: “Peut-être une forme ou variété de D. cristatus”. Thus, in our opinion, the males of D. cristatus occur in two morphs: viz., Figs la, d show the morph cristatus with a low cephalic lobe, and Figs 1g, j, m and 2a show the morph foraminifer with a high cephalic lobe.

Distribution and habitat

Specimens of Diplocephalus morph cristatus occur all over Europe (Nentwig et al. 2017). On the contrary, the morph foraminifer has a much smaller distribution: northern Spain, southern France, Switzerland and northern Italy in the western part of Europe, and Bosnia and Hercegovina, Macedonia, Montenegro, Bulgaria, Serbia and Romania in the eastern part (Nentwig et al. 2017).

Fig. 4:

Savignia harmsi Wunderlich, 1980. a. Morph typica, lateral view of prosoma; b. Morph cor, lateral view of prosoma; c. Morph typica, dorsal view of prosoma; d. Morph cor, anterior view of prosoma.

Fig. 5:

Savignia harmsi Wunderlich, 1980. a. Morph typica, dorsal view of prosoma; b. Morph typica, lateral view of prosoma; c. Morph cor, dorsal view of prosoma; d. Morph cor, lateral view of prosoma; e. Male palp, retrolateral view; f. Male palpal tibia, dorsal view; g. Embolic division, prolateral view.

The morph cristatus occurs in a variety of habitats: “in grass, straw, moss, etc.” (Locket & Millidge 1953), “auf offenen Flächen, an Waldrändern, in Gärten” (Heimer & Nentwig 1991). The morph foraminifer occurs in a much narrower range of specialized habitats. These spiders are frequently found under stones at high altitudes in the Cantabrian Range, the Pyrénées, the Massif Central and the Alps (Simon 1884, 1926, Bosmans & de Keer 1985, Denis 1953, 1955, Hänggi & Stäubli 2012, Müller 1985). At lower altitudes, these spiders occur in more restricted habitats such as caves, near springs and rivulets and in screes and cracks (Denis 1934, Georgescu 1969, Deltshev 1985), rarely in deciduous woodlands (Grbic & Savic 2010).