Taxonomical history

Westring (1851) first described Troxochrus scabriculus (sub Erigone scabricula) based on male and female specimens from Sweden. A decade later, Pickard-Cambridge (1860) described Troxochrus scabriculus sub Walckenaera aggeris from southern England and provided the following information with regard to the record date and locality (on page 174): “Adult males and females of this species were discovered by myself in abundance, during the summer of 1859, at the roots of grass and underneath rubbish on dry bank-sides, near Church Town, Southport, Lancashire”. Later, Pickard-Cambridge (1871) described a fürther new species, sub Walckenaera cirrifrons, based on a single male specimen, which clearly came from the same material in which he originally found T. scabriculus a decade earlier: “An adult male of this spider was captured, in company with W. aggeris (Camb.), at Southport, Lancashire, at the roots of grass &c., in June 1859.” It is important to note that (i) T. scabriculus and T. cirrifrons came from the same material sampled at the roots of grass in the summer of 1859 near Church Town, Southport, Lancashire; and that (ii) the specimens of T. scabriculus were present in abundant numbers, while only one male specimen of T. cirrifrons was identified in the same samples.

Simon (1884: 645) established the genus Troxochrus and was the first to suspect Troxochrus scabriculus of being dimorphic in the males, consisting of the typical form scabriculus and the second form cirrifrons. Simon stated that: “La forme cirrifrons se trouve toujours mêlée au type, mais elle est partout plus rare” [The form cirrifrons is always mixed with the type, but it is everywhere rarer].

More than a quarter of a century later, Pickard-Cambridge (1911) reopened the case of T. scabriculus/cirrifrons and attempted to negate the statement of Simon (1884) that T. cirrifrons is a dimorphic male form of T. scabriculus. Although Pickard-Cambridge (1911) acknowledged that “the two species remarkably differ in the male sex”, he disagreed with the conclusion of Simon (1884), because “Simon relies chiefly, for the identity of scabriculus and cirrifrons, on the two forms being always found together (i.e., at the same time and place); but this I have by no means myself proved to be the fact”.

In the same work, Pickard-Cambridge (1911) described the female of T. cirrifrons which he believed to be different from the T. scabriculus female by drawing reference to illustrations of the epigynes (T. scabriculus, Plate A, Fig. 18 and T. cirrifrons Plate A, Fig. 19). Curiously, Pickard-Cambridge (1911) in his description to Plate A, placed a question mark in the figure captions before both species names, perhaps indicating uncertainty about any differences between the females. Nevertheless, Pickard-Cambridge (1911) managed to conclude that “on the whole T. cirrifrons still seems to me to be a distinct species from T. scabriculus”.

Oddly, Simon (1926), in a work which was completed by Lucien Berland and Jean-Louis Fage two years after Simon's death in 1924, recanted the original opinion of Simon (1884). Thus, in the identification key for the genus Troxochrus, T. scabriculus and T. cirrifrons were treated as different species (Simon 1926: 369). Nevertheless in our opinion, the drawings of the epigynes in Simon (1926) (T. scabriculus, Fig. 652, T. cirrifrons, Fig. 655) are as inconclusive as those in Pickard-Cambridge (1911).

Although T. scabriculus and T. cirrifrons were henceforth recognized as different species in World Spider Catalogs (see Roewer 1942, Platnick 1989), many arachnologists continued to infer that T. cirrifrons is perhaps a subspecies of the typical form T. scabriculus (e.g. Bristowe 1939: 75), or that it is a variety (e.g. Wiehle 1960: 466, Locket & Millidge 1953: 264), or indeed a dimorphic form (Thaler 1986: 496) or at least a sibling species (Aakra et al. 2016). With regard to the females, Wiehle (1960: 466) stated that female specimens which were found with the two male forms cannot be distinguished from each other, not even with detailed vulva preparation. Consequently, in one of his following works, Wiehle (1961: 183) distinguished between the nominate form Troxochrus scabriculus and the form T. scabriculus cirrifrons.

Almost three decades later, Roberts (1987) briefly summarized the debate surrounding T. scabriculus/cirrifrons and pointed out that the male Troxochrus scabriculus forma cirrifrons (page 74, Fig. 31b) differed from the typical form Troxochrus scabriculus (page 74, Fig. 31a) in the size and shape of the cephalic lobe and by the presence of long bristles lateral to each posterior median eye. According to Roberts (1987) there are, however, no significant differences in the male palps between these two forms, and accompanying females exhibit no significant differences in general appearance or genitalia. Concerning the females, there is only slight but insignificant variation in the epigynum of T. scabriculus. Roberts (1987) stated in his book on the spiders of Great Britain and Ireland that T. cirrifrons is a dimorphic male form of T. scabriculus, and he designated it as T. scabriculus f. cirrifrons. Nonetheless, he declined to synonymize the two species. Instead, he argued that this case would have to be ultimately resolved by breeding experiments.

Recently, Aakra et al. (2016) found two additional differentiating morphological features between the two forms, which they considered as two species with reference to the World Spider Catalog (2018). First, males of T. scabriculus possess an extra set of glandular openings on each side of the field of short hairs above the anterior median eyes (Aakra et al. 2016, Figs. 22A–D), while no such structures are visible in T. cirrifrons. Second, considerable differences were noted in the invaginations of the sulci, which are much larger in T. cirrifrons than in T. scabriculus (Aakra et al. 2016, Fig. 22E).

The case of T. scabriculus/cirrifrons is not the only taxonomic example within the family Linyphiidae of male morphs having different head forms within one species, without there being differences in genital morphology. Recently, Bosmans & Oger (2018) confirmed Diplocephalus cristatus as a dimorphic species, having two male morphs: cristatus and foraminifer. Other examples are Diplocephalus conatus/jacksoni and Oedothorax gibbosus/tuberosus (see Roberts 1987). Wunderlich (2008) added fürther examples of species from the family Linyphiidae, in which the male prosoma has two or more different shapes (i.e. lobes) and/or hairy areas, and he explicitly considered T. cirrifrons as a variation (or “form”) of T. scabriculus [“Troxochrus scabriculus (=cirrifrons)”].

Thus, the phenomenon of dimorphic males in spiders must be considered as an established fact. As a result of these considerations and based on the examination of our own material, we now follow the original assessment of Simon (1884) and conclude that the cirrifrons male is a second male morph of T. scabriculus. Since there are no differences in the genital morphology between male specimens of cirrifrons and scabriculus, and since there are no differences in the females of different populations, we consider Troxochrus cirrifrons (O. Pickard-Cambridge, 1871) to be a junior synonym of Troxochrus scabriculus (Westring, 1851) and consequently, the male morph cirrifrons as a second form of the nominal T. scabriculus.

We are not the first to arrive at this conclusion. For example, Müller (1984) clearly regarded T. scabriculus and cirrifrons as not being distinct species since their male genital morphology is identical, and he also did not consider them as sub-species (as suggested by Heimer 1976) due to their sympatric occurrence.

In several checklists T. scabriculus and T. cirrifrons have been treated as synonyms. Yet in the checklist of spiders of Germany, Platen et al. (1991) listed T. scabriculus and cirrifrons as two distinct species. In the checklist four years later, Platen et al. (1995: 36) considered T. cirrifrons as a synonym of the typical T. scabriculus; and again, in Platen et al. (1999: 25) T. cirrifrons is defined as a synonym of T. scabriculus. Notwithstanding the fact that T. cirrifrons was recorded in Berlin (von Broen 1977), Platen & von Broen (2002) no longer listed T. cirrifrons in the checklist of the spiders of Berlin.

Likewise, in the checklists of the spiders of Russia, Mikhailov (1996: 99; 1997: 102; 2013: 96) consistently mentioned T. cirrifrons (O. Pickard-Cambridge, 1871) as a (junior) synonym of T. scabriculus (Westring, 1851). This is also true in the provisional atlas of the British spiders by Harvey et al. (2002). In the caption to the map of records of Troxochrus scabriculus, Harvey et al. (2002: 116) commented that any record submitted to the scheme as T. cirrifrons (O. Pickard-Cambridge, 1871) under BRC number 15301, is now considered to be a form of male T. scabriculus. Thus, we can summarize that T. cirrifrons (O. Pickard-Cambridge, 1871) has been repeatedly considered to be a form of male T. scabriculus in several national spider checklists.

However, there are spider checklists of other countries or regions where T. scabriculus and T. cirrifrons are still listed as separate species, e.g. in the departments “Nord” and “Pas-de-Calais” of northern France (Lecigne 2016: 56), in Flanders (Maelfait et al. 1998: 136), Belgium (Bosmans 2009: 52, Bosmans & van Keer 2017), the Netherlands (van Helsdingen 1999: 156, 2016: 111) and Bulgaria (Blagoev et al. 2002–2018); and of course, in the World Spider Catalog (2018) T. cirrifrons and scabriculus are still considered two separate species (see also Bosmans & Oger 2018: 52).

Material examined

The epigeic spider fauna of six overgrown gardening plots (study sites A–F) on the rooftop of the “Biozentrum Althanstraße” in Vienna, Austria, was examined from 8 April 2016 to 7 April 2017 by means of one pitfall trap per site (Milasowszky & Hepner 2017).

Troxochrus scabriculus/cirrifrons material: AUSTRIA: 100 ♂♂ (95 scabriculus, 5 cirrifrons), 34 ♀♀, Vienna, Alsergrund, Biocenter Althanstraße, UZA1, rooftop, overgrown gardening plots, ruderal sites, 176 m a.s.l., M. Hepner & N. Milasowszky leg. & det., M. Hepner collection.

After identifying the material of male scabriculus and cirrifrons available to us, we completed drawings of the prosomas in lateral and frontal views from both morphs (Figs 1a–d). Additionally, we provide drawings of the female genitalia, i.e. the epigynum and vulva (Figs 2a–b). For views of the identical-looking palps of scabriculus and cirrifrons males see Figs 3a–b.

Habitat and distribution

Platen et al. (1991) listed T. scabriculus and T. cirrifrons as two distinct species, however, the information provided for both with regard to habitat was identical, i.e., plant formation: subatlantic broom-heathland, sand dry grassland, couch grass-meadows and persistent ruderal areas. Fürthermore, both species or forms were characterized as xerophilic inhabitants of the soil-surface with a peak of activity in the summer months (mid-June to mid-September). Müller & Schenkel (1895) reported T. scabriculus and T. scabriculus cirrifrons adult male forms under the taxon T. scabriculus; both forms were collected from October to December in an alder forest along a meadow riverbank in Switzerland. Also, Thaler (1986) documented that both forms occurred together in a black alder forest near Vienna (Austria), where, 69 males of the form scabriculus and two males of the form cirrifrons were collected. Steinberger & Thaler (1990) collected one cirrifrons male in a small relictual floodplain forest strip on the left riverbank of the river Inn in Tyrol (Austria). Interestingly, T. scabriculus males were present in the same area, but not in the same study sites, in contrast to Thaler (1999), who considered the records in the two study sites as evidence of sympatric occurrence.

Fig. 1:

Troxochrus scabriculus (Westring, 1851), male prosoma: a. forma scabriculus, lateral view, b. frontal view, c. forma cirrifrons, lateral view, d. frontal view. Scale bar 0.2 mm

Fig. 2:

Troxochrus scabriculus (Westring, 1851), female genitalia: a. epigynum, ventral view, b. vulva, dorsal view. Scale bar 0.1 mm

Fig. 3:

Troxochrus scabriculus (Westring, 1851), male palps: a. forma scabriculus, lateral view, b. forma cirrifrons, lateral view (photos taken from Oger 2018)

Fig. 4:

Phenology of Troxochrus scabriculus males (forma scabriculus and forma cirrifrons) and females in 13 study periods between 8 April 2016 and 7 April 2017 on the rooftop of the “Biozentrum Althanstraße” in Vienna (Austria). Black bars: T. scabriculus f. cirrifrons males; Grey bars: T. scabriculus f. scabriculus males; White bars: T. scabriculus females.

In Denmark, Larsen & Bøggild (1970) noted sympatric occurrences of T. scabriculus and T. cirrifrons from sand dunes and marram grass. Wiehle (1960) related that one male of the form cirrifrons was on the southern slope of the “Kyffhäuser” mountains in Germany, 4 October 1958, and another male in a salt meadow (Salicornietum) near Hecklingen-Anhalt, 12 June 1958. Von Broen (1977) reported T. scabriculus/cirrifrons from a “Ligusterhecke” [privet hedge] in Berlin, Germany. In the “Niederrhein”-Lowland, Otrembnik (1978) secured one T. scabriculus f. cirrifrons male and one female in a riparian grassland and seven T. scabriculus f. cirrifrons males and one female in a fertilized nutrient-rich meadow; however, no records were made of the typical male form. In Aldenhoven, Germany, Esser (1997) collected two cirrifrons males together with 64 T. scabriculus specimens (probably mostly males) in a small ryegrass-plantain field margin strip (300 m length, 3 m width) situated between an arable field and an asphalt road. In Renkum, a town in the eastern Netherlands near Arnhem, van Helsdingen & IJland (2008) discovered a single male in a former arable grassland field between 1 April and 31 October 2008. Dekkers-Scheutjens (2010) obtained 31 T. scabriculus males in a study site within a nature reserve southwest of Tilburg, together with three cirrifrons forms.

According to Harvey et al. (2002), T. scabriculus is restricted to dry habitats in the British Isles, such as calcareous grassland, quarries, river shingle, haystacks and bare ground. The spider is a typical inhabitant of sand dunes where it prefers dense clumps of marram grass; in gardens the species can be encountered on gravel paths (Harvey et al. 2002). Mikhailov & Trishina (2013) observed one T. cirrifrons male form cooccurring with one male and two females of T. scabriculus in a birch and lime tree plantation in the vicinity of Pushta (Mordovian Republic, Russia) on 19 August 2011. In Norway, T. cirrifrons inhabited the same type of habitat as T. scabriculus, i.e. open sand and shingle (= gravel) dominated localities near rivers, streams and the seashore (Aakra et al. 2016). Entling et al. (2007) compared 224 Central European spider communities along two major environmental gradients, i.e. shading and moisture. Within the shading gradient from open habitats to forests, T. scabriculus and T. cirrifrons had very similar average niche positions in open habitats (Entling et al. 2007, Appendix S2), while within the moisture gradient their niche positions slightly differed; T. scabriculus could thus be considered as an inhabitant of mesic moist habitats, while T. cirrifrons as one of mesic dry habitats. However, niche width values indicate a wide niche range in both species/forms, i.e. a great niche overlap within the moisture gradient.

In the distribution maps of the “Nord” and “Pas-de-Calais” departments of northern France, six records exist of T. cirrifrons that overlap with the records of T. scabriculus to 100 % (Lecigne 2016). Of interest here is that fact that 26 of the overall 33 records of T. scabriculus were located on the coast (Lecigne 2016: 212). Fürthermore, in Bulgaria, where T. scabriculus also occur, the first record of T. cirrifrons was made by Deltshev (2004) in a pine forest near Sandansky, where he uncovered three cirrifrons and no scabriculus males. Perhaps this exclusive occurrence of the form cirrifrons is the reason for its entry in the spider checklist of Bulgaria. Deltshev (2004) commented that the locality in Bulgaria is at the southeastern border of its range. However, Mikhailov (1996, 1997) previously documented T. cirrifrons [as a junior synonym of T. scabriculus] from Russia (e.g. Russian Plains, Urals, and Middle Siberia) and the Ukraine. Mikhailov & Trishina (2013) noticed one T. cirrifrons male form co-occurring with one male and two females of T. scabriculus in a birch and lime tree plantation in the vicinity of Pushta (Mordovian Republic, Russia) on 19 August 2011. According to Roberts (1987), T. scabriculus and T. scabriculus f. cirrifrons have a similar distribution throughout the British Isles. However, both forms do not always occur together, rather they are locally common in dry habitats. Thus, T. scabriculus and its form cirrifrons have a widespread but patchy distribution in much of Britain. In summary, both forms are extensive in western and central Europe, and their distribution range extends east to Russia and the Far East.

Phenology

Simon (1884) stated that the form cirrifrons is “commun au premier printemps dans les détritus humides” [common in early spring in moist litter]. Wiehle (1960) reported one male of the form cirrifrons from Germany, 4 October 1958, and another male, 12 June 1958. Larsen & Bøggild (1970) registered T. scabriculus males in June, July, August and October, and T. cirrifrons in June and August. Females were present in April, May, June, July, August and October. Thaler (1986) recorded one cirrifrons male in the period from 24 April to 5 May, and one cirrifrons male in the period from 5 to 19 May. Von Broen (1977) reported T. scabriculus/cirrifrons from urban ruderal areas in Berlin, Germany, and presented the phenology data of both forms over one calendar year. According to this data, T. scabriculus males occurred from the beginning of March until the end of November. Nine specimens of T. cirrifrons were collected in April, May and June. According to von Broen (1977), the activity peak of adult males was in April. Since the species could be found nearly all year round, von Broen characterized it as eurychronus. Adult females may be present throughout the year but there is a peak in recorded number of adults of both sexes in late spring and early summer, and again in the autumn (Harvey et al. 2002). Mikhailov & Trishina (2013) discovered one T. cirrifrons male form cooccurring with one male and two females of T. scabriculus on 19 August 2011. In our study, we noted that the highest peak of activity of T. scabriculus was in the months February and March (Fig. 3), which is about a month earlier than in the study from Berlin where the maximum lies between March and April (von Broen 1977). According to the phenology figure in Harvey et al. (2002), the activity peak in the United Kingdom seems to be in May.

Male morph ratio within populations

Simon (1884: 645) stated: “La forme cirrifrons se trouve toujours mêlée au type, mais elle est partout plus rare” [the form cirrifrons is always mixed with the type, but it is everywhere rarer]. Based on their findings, von Broen & Moritz (1965) arrived at the same conclusion that the variety T. scabriculus cirrifrons occurs in all populations of T. scabriculus. In general, this is true, although there are many exceptions (e.g. see the T. scabriculus/cirrifrons maps of Lecigne 2016). Roberts (1987) summarized the up-to-date data and concluded that T. scabriculus f. cirrifrons was rather less common than the typical form T. scabriculus. In general, this is true, but there are exceptions or even populations of T. scabriculus that consist purely of cirrifrons males. To give an example, Deltshev (2004) documented three cirrifrons males in a pine forest in Bulgaria without any scabriculus morphs.

Pickard-Cambridge (1860, 1871) was the first to quantify the ratio between the male form of T. scabriculus and cirrifrons as “abundant” to “one”. In our data set we obtained a ratio of 95:5 = 19:1. Thaler (1986) documented 69 males of the form scabriculus and two males of the form cirrifrons in a floodplain area near Vienna, Austria; hence, the ratio between the typical form and “cirrifrons” was about 35:1. Interestingly, in two relictual floodplain forests along the Inn River in Tyrol, Austria, Steinberger & Thaler (1990) counted about 45 T. scabriculus males in the study site “Kufstein” (large floodplain forest on the right river bank) and one “cirrifrons” male in the study site “Langkampfen” (small floodplain forest strip on the left river bank), i.e. both forms seemed co-occur in the same study area, but not in the same study sites. In Aldenhoven, Germany, Esser (1997) identified two cirrifrons males together with 64 T. scabriculus specimens (probably mostly males) in a small ryegrass-plantain field margin strip (300 m length, 3 m width) situated between an arable field and an asphalt road; hence the ratio of scabriculus and cirrifrons was at a maximum of 32:1, but probably a little lower due to the unknown number of females. Von Broen (1977) accounted for both T. scabriculus and cirrifrons from three urban ruderal areas in Berlin, Germany, (i) a “Ligusterhecke” [privet hedge], (ii) an “Erdbeerbeet” [strawberry patch] and (iii) a “Holzmehlfläche zwischen Ziegelbau und Holzschuppen” [an area covered with saw dust between a brick building and a woodshed]. In the privet hedge, the ratio between T. scabriculus and T. cirrifrons males was about 14:1 (164 T. scabriculus, 12 T. cirrifrons), while in the strawberry patch and the sawdust site only T. scabriculus specimens were captured. Von Broen (1977) also provided unpublished data on the ratio of the two forms from Greifswald, where he captured 10 T. scabriculus males and two T. cirrifrons males; hence, there the ratio was 5:1. Dekkers-Scheutjens (2010) collected 31 T. scabriculus males in a study site within a nature reserve southwest of Tilburg (Netherlands) together with three cirrifrons forms, i.e. the ratio was about 10:1.

Conclusion

It is evident that the synonymy of T. scabriculus/cirrifrons is not a taxonomically problematic case to resolve. Since both males and females from populations with co-occurring male forms are identical in their genital morphology, it is highly plausible that the different male morphs must represent two forms of the same species. It is also apparent that Troxochrus scabriculus (Westring 1851) is the typical form, while the later described form cirrifrons is a second male morph that must be designated Troxochrus scabriculus forma cirrifrons (O. Pickard-Cambridge, 1871). Based on fürther evidence from distribution maps, habitat preferences, phenology and appearance in populations (see above), it is obvious that Simon (1884) made the correct judgement from the very start.

Although breeding experiments would be desirable to clarify the status of the two forms of the male spider, as once recommended, we consider the taxonomic case of the dimorphic males in T. scabriculus to be closed and quote the famous fictional figure Sherlock Holmes: “We must fall back upon the old axiom that when all other contingencies fail, whatever remains, however improbable, must be the truth” (Doyle 1908).