Oedothorax

My lovely manly humps
Published 5 December 2023

The Linyphiidae are one of the most spectacularly diverse of all spider families. Representatives of this family can be found all around the world, from the steamy tropics to the frozen tundra. However, they typically escape attention due to their small size, often only a couple of millimetres long. Commonly known as ‘money spiders’ for reasons that don’t seem entirely clear (somehow, finding one on your person and gently releasing it is supposed to presage good fortune), most linyphiids construct tiny sheet webs hidden among leaf litter. They are most commonly encountered when juvenile spiders disperse via ballooning, trailing silk into the air to lift themselves up and get carried away. As a result of this habit, many linyphiid taxa have a wide distribution, and one of the most widespread genera in the Northern Hemisphere is Oedothorax.

Oedothorax fuscus traverses an unfamiliar landscape, copyright Julien Tchilinguirian.

Oedothorax is a genus of the subfamily Erigoninae, the real midgets of the linyphiid world. Over eighty species have been assigned to this genus, distributed over Eurasia, northern Africa and North America. However, like many linyphiid genera, Oedothorax has suffered from poor definition. Historically, it has been recognised by a tibial spine formula of 2.2.1.1, a trichobothrium in the distal half of metatarsus I, and a trichobothrium on metatarsus IV (Tanasevitch 2015). However, the actual evolutionary significance of these features is questionable. A recent revision of Oedothorax by Lin et al. (2022) identified a clade of just ten species that could be confidently associated with the type species O. gibbosus, united by features of the male pedipalp (including the base of the paracymbium being visible in dorsal view, a median group of setae distally on the paracymbium, a horn at the base of the embolus, and a cylindrical embolic membrane). Forty-three species were excluded from Oedothorax and transferred to other genera. However, another 27 species could not be confidently placed and remained hanging onto Oedothorax in the absence of any better idea what to do with them.

Male pedipalp of Oedothorax tingitanus, from Lin et al. (2022).

A number of species of Oedothorax are sexually dimorphic with males exhibiting various modifications of the cephalothorax, such as humps and grooves behind the posterior median eyes, and lateral sulci or pits (Lin et al. 2022). These structures are associated with glandular tissues that play a role in courtship. While the male inseminates the female, she brings her mouthparts into contact with the glandular structures and slurps away at their secretions. Production of such a nuptial gift is thought to increase the male’s appeal to the female and reduce the risk of her just feeding on him entirely. Differences in structures between species may assist the female in selecting suitable mates and/or they may all for different positions in the mating process.

Male Oedothorax apicatus, copyright Donald Hobern.

So why don’t all Oedothorax males exhibit such structures? This question is particularly intriguing for the European O. gibbosus. Males of this species are dimorphic with some individuals possessing prominent cephalothoracic swellings whereas others look little different from females. One possibility is that cephalothoracic modifications are not vital to the process; at least two non-dimorphic species, O. agrestis and O. fuscus, have been found to possess nuptial glands despite lacking a modified cephalothorax (Lin et al. 2022). Maybe some Oedothorax females just like more feminine guys. Maybe the two forms of O. gibbosus represent two gene pools in the process of diverging. Or maybe there is a different reason. One is immediately tempted to try to make comparisons between the O. gibbosus morphs and the ‘harem-guarder’ vs ‘sneaker’ males found in other animals such as salmon or dung beetles. Perhaps a swollen cephalothorax brings you more attention from females, but also invites attention from rival males unhappy to see you wandering their turf. And perhaps while the more butch individuals are squaring against each other, it is ultimately the meek who shall be responsible for the birth.

References

Lin, S.-W., L. Lopardo & G. Uhl. 2022. Evolution of nuptial-gift-related male prosomal structures: taxonomic revision and cladistic analysis of the genus Oedothorax (Araneae: Linyphiidae: Erigoninae). Zoological Journal of the Linnean Society 195: 417–584.

Tanasevitch, A. V. 2015. Notes on the spider genus Oedothorax Bertkau, 1883 with description of eleven new species from India (Linyphiidae: Erigoninae). Revue Suisse de Zoologie 122 (2): 381–398.

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