Coelotes terrestris, the ground-living spider, is a spider in the family Agelenidae that stems from the once-assumed affiliation of the genus Coelotes, including this species, with the family Amaurobiidae, which has since been refuted.
The distribution of Coelotes terrestris extends from Europe to Turkey. It occurs in a broad spectrum of habitats, but is particularly found in forests of various types, where it preferably inhabits the ground level. The abundance of the species depends on the diversity of the forest biotope, including various microhabitats, as well as air humidity. The latter is especially relevant in habitat selection because Coelotes terrestris is rather hemihygrophilous (moisture-loving) and thus avoids habitats that are too dry or too wet.
Like all species in the family Agelenidae, Coelotes terrestris constructs a funnel web typical of the family and named after it for catching prey, though its structure can vary in this species. In addition to the actual web sheet for prey capture, the web also features a retreat tube built in a cavity. Using the catching web, the spider captures a wide range of other invertebrates, particularly beetles. Compared to other spiders, the female of Coelotes terrestris exhibits highly developed parental care. The juveniles remain in the web for some time and are fed by their mother before becoming independent.
Description
Females of Coelotes terrestris reach a body length of 10 to 13 mm, males 8 to 10 mm. The body structure corresponds to that of other species in the genus Coelotes.
The prosoma (cephalothorax) of Coelotes terrestris, like other species in Coelotes and unlike other agelenids, is hardly narrowed. The carapace is dark red-brown to black. The sternum appears almost black. The legs are uniformly red-brown to dark red-brown.
The opisthosoma (abdomen) has a dark gray to black-gray base color. Dorsally, there is also a dark median band in the anterior part of the opisthosoma, which narrows posteriorly and transitions into a yellow-gray chevron pattern in the posterior area. Posteriorly on the dorsal surface of the opisthosoma are yellowish-gray lateral stripes. Laterally, the opisthosoma has yellowish-gray patterns. Genital morphology In males of Coelotes terrestris, a single pedipalp, as typical for the genus Coelotes, has an apophysis (chitinized process) on the patella, whose distal edge is distinctly indented. The conductor (sperm duct) of a single bulb (male genital organ) has a narrow, short tip.
The epigyne (external female genital organ) of the species is characterized within the genus particularly by the parallel lateral margins. The anterior margin is strongly pronounced. Between them is a square pit.
Differentiation from similar spiders
Coelotes terrestris resembles many other species in the genus Coelotes, such as Coelotes solitarius. The other representatives of the genus have a smaller distribution area or are generally rarer than Coelotes terrestris.
Coelotes terrestris shares the most similarities with Coelotes atropos, whose opisthosoma is usually lighter and less distinctly patterned. Additionally, in the male of Coelotes atropos, the apophysis on the patella is equipped with three short, rounded processes, while the epigyne of this species has an indistinct bulge anterolaterally.
Furthermore, Coelotes terrestris can easily be confused with the very similar Inermocoelotes inermis, which can also be found together with Coelotes terrestris, albeit usually at lower individual density. Due to the strong similarities between the two species, Inermocoelotes inermis can only be reliably distinguished from Coelotes terrestris based on genital morphological features. The conductor of a single bulb in Inermocoelotes inermis is basally provided with a broad process, and the tip of each bulb runs transverse to the longitudinal axis of the cymbium (third and last sclerite, or hard part of the bulb). The pit of the female's epigyne in this species is twice as long as wide and has a more or less sinuous appearance. Distribution
Coelotes terrestris is present in Europe and Turkey. The distribution extends north to Denmark. In Central Europe, the species is widespread except for the Baltic states and Kaliningrad Oblast as well as Croatia, which also applies to Western Europe. There, Coelotes terrestris is recorded in the Benelux countries, mainland France, and Great Britain.
In Great Britain itself, the species is mainly present in southeastern England and replaces Coelotes atropos there. From other parts of Great Britain, there are only isolated records of Coelotes terrestris. Old records from Yorkshire and Berwickshire are likely based on confusions with Coelotes atropos, which, unlike Coelotes terrestris, mainly inhabits the west and north of Great Britain.
In Eastern Europe, Coelotes terrestris is only present in Ukraine and Belarus, while in Southeast Europe, it has been recorded so far in Romania, Bulgaria, and North Macedonia as well as mainland Greece and the European part of Turkey.
In Germany, the species particularly inhabits mountainous regions. Examples include the Franconian Jura, the Burgberg near Erlangen, and the area of the district Rathsberg near Erlangen.
Habitats Coelotes terrestris preferably inhabits forests, that are not too dry. Among these, especially deciduous and coniferous forests with higher site quality are sought. The species most frequently inhabits mountainous forest areas on loamy soils. However, it also inhabits flat areas with sandy substrate, compensating for the moisture differences in both soil types by settling areas with high groundwater levels.
Coelotes terrestris requires, as typical for hemihygrophilous (moisture-loving) species, higher saturation of water vapor in the surrounding air, but compared to other life forms with the same preference, it has high resistance to relative humidity of 30% to 75%. Because of this, the spider can also inhabit dry areas with raw humus, as this has high water capacity and good insulation against heat too high for the spider.
Since humidity in the preferred habitats of Coelotes terrestris fluctuates greatly, the species is present there in island-like populations. In transition areas of preferred habitats, including dry sparse pine stands, the spider can also occur abundantly. This is due to the fact that it cannot spread into surrounding dry areas.
From forests, Coelotes terrestris also inhabits forest edges, coppices, as well as shrub and hedge landscapes. Occasionally, wood-poor open land habitats such as dry grasslands, fallow lands, ruderal sites, or heathlands are colonized by the species. In Great Britain, it has also been found on moss-covered water banks.
Conservation status The conservation status of Coelotes terrestris varies by country. The species is generally considered very common in its range. In the Red List of Endangered Animals, Plants and Fungi of Germany, Coelotes terrestris is classified as "least concern", as it is also very common in Germany and populations are stable both long- and short-term.
Although much rarer there, Coelotes terrestris is listed in the Red List of Great Britain (1991) according to IUCN criteria in category LC ("Least Concern"). There, the species is considered widespread but rather local and never found in high density. Threats to Coelotes terrestris on the island include loss of natural forest areas through intensive forestry, loss of heathlands, and removal of fallen trunks and deadwood. In the Red List of spiders of Czech Republic (2015), Coelotes terrestris is listed in IUCN category ES ("Ecologically Sustainable").
Biology
The biology of Coelotes terrestris was described in detail by Erwin Tretzel in 1961. Like all species in Coelotes, the spider is terrestrial (ground-dwelling) and, like all Agelenidae, nocturnal. The hidden lifestyle typical of the family is also present in this species. On clay soils, the spider stays in ground cracks under flat-spreading roots of spruces (Picea) and in the litter layer. On sandy substrate, it is found when the raw humus and litter layer sock-like surrounds the base of trunks of pines (Pinus). It is also found under large stones. On sandy soils, Coelotes terrestris also inhabits tall-growing Hypnum, while avoiding peat mosses (Sphagnum).
The need for water is very low in the species and can also be obtained from moist raw humus if possible. For this, the spider bites into the humus. Intake of water droplets was only observed in captivity and then only sporadically during a longer simulated dry period.
Hunting behavior Coelotes terrestris lives predatorily like all spiders and constructs a spider web for hunting purposes. Like all spiders with this hunting method, this species is also considered an ambush predator.
Web construction The web of Coelotes terrestris is basically a funnel web typical and named for Agelenidae. Depending on the nature of the web site, it can be built very differently in this species. The spider works on its web only at night, corresponding to its activity period.
Web location
A common web site for Coelotes terrestris are trunks of pines (Pinus), and on a single pine trunk, several inhabited webs of the spider can be present even with a relatively small diameter of the trunk. Individuals living on these trunks preferably place their webs between the bark and the surrounding raw humus or litter layer. Occasionally, the spider uses surfaces under stones or tree stumps as web sites.
To prevent flooding of the web when built at trunks by runoff water, Coelotes terrestris preferably places its web on the east side, but not the eastern half. Similarly, the species increasingly avoids the west and northwest sides of the trunks for the same reason. In addition, though less so, the north and south sides of the trunks are accepted, but intermediate directions are mostly avoided. This is probably because Coelotes terrestris prefers eastern orientations over western ones through polarization-optical optimization. If colonization of the east surface of a pine trunk is no longer possible due to an already too large population density of the spider, the individuals then maintain a safety distance from those on the east surface of about 90° from the base cross-section of the tree trunk, so the individuals then move to the east and south surface of the trunk.
Construction phase and web variation
Coelotes terrestris is able to adapt its web well to the spatial conditions of the respective web site, making its web highly variable. Constructing a web usually extends over several nights and begins with a large-scale projection of the web by laying a few thick silk threads for sketching the web to be built. Openings suitable for retreat and side tubes are already included in the design. Since the individual threads cannot be drawn in uniform curvature but only angled by intermediate attachment, the layout of a tube opening is initially more or less rectangular. Later, the openings take a round shape by laying new threads at the corner areas and thus bridging the existing ones. If necessary, Coelotes terrestris can later provide its finished fabric with additional openings by biting them into the fabric with the chelicerae and then widening them to the necessary width for passage with its legs. In contrast, unused openings are spanned and closed with a delicate web. These openings serve the spider for escape in case of disturbances and can be reopened with a strike of the front extremities.