Pholcids

21 09 2011

When I was living in Germany, my room was in the cellar/basement of a house. The most abundant macroscopic organisms in my room are the subject of this post: pholcids. Anyone who has a dusty, somewhat dark room (e.g. a basement, or a cave) will recognise them: they’re the spiders with the really long legs and the really tiny bodies (the first leg is at least twice as long as the body), you can’t miss them. For USAnians, the most common one you find at home is Spermophora senoculata (pers. comm. from a friend in New York, maybe different in other areas).

Since I am now starting to work on them for my Cyprus project, I thought I’d write this up to procrastinate.

As is usually the case, the majority of pholcids aren’t human cohabitants; the house species are few and typical. However, pholcids are one of the taxonomically least researched spider groups, owing primarily to the difficulty of collecting them: active collecting is difficult because they’re quick and hard to see, and their long legs make passive traps rarely effective (unless one uses large pitfall cups, which presents its own set of problems). In fact, they are mostly collected by accident; but when collecting spiders specifically, it becomes clear that pholcids are possibly the most abundant spiders in a habitat (e.g. Manhart (1994) sampled the fauna on the bark of Peruvian jungle trees, and found 62% of the spiders are pholcids).

This understudy is reflected by the paucity of classificatory schemes for pholcids. The starting point of spider classifications, Simon (1839), has only been expanded three times with respect to pholcids, by Petrunkevitch (1928, 1939) and Mello-Leitão (1946). A fourth one was started by Brignoli (1972), but remained incomplete. Regional guides exist, but they tend to be incomplete or deliberately and especially left open to revision.

Anyway, onto their biology. Phylogenetically, they are ecribellate and haplogyne spiders. The sister group to the Pholcidae is either the Diguetidae or the Plectreuridae (or, most probably, a clade combing those two). They have five definite autapomorphies:

  • Their cymbium (broad and hollow male palp) has a retrolaterally-attached sclerite called the paracymbium.
  • Sexually modified male chelicerae (with hairs, cones or large depressions); the modifications are for grasping the female during copulation.
  • Height of clypeus ~ equal to length of chelicerae.
  • Pseudosegmented tarsi.
  • Three trichbothriae (elongated setae) on the tibia.

As said, they’re small, their body being between 1 and 10 mm, in exceptional species going up to 15 mm. Besides this small body size and the above characters, pholcids are extremely variable: for example, they can have anything from no eyes, to two eyes, to six, to eight. It’s the same with all other morphological characters, where they vary by genus/species/sex. What follows are the only generalisations that can safely be made.

Male palps tend to be highly-modified, segment-by-segment. The inner side of the coxa can have complex sculpturing. The trochanter can have unique hairs sticking out at the end of it. The femur’s shape is variable.

They spin webs using three sets of spinnerets, each consisting of one large spigot and one small, pointy spigot (although there are some that have more sets, some have only two).

As for noises, they can stridulate: both sexes do it by rubbing modified hairs on the palpal femur, while females can go one step beyond and have specialised structures from the opisthosome that rub on the prosome (or vice versa).

They are ubiquitous, found from sea level to the highest mountains, from jungles to deserts, from leaf litter to tree canopies. On the latter category, the type of web they spin is dependent on their habitat: ground-dwelling pholcids build simple sheets, while others can build veritable three-dimensional networked webs – in the most extreme cases, they can even build agelenid-like funnels.

If you find them in the wild, good luck catching them. They are very agile and fast runners; catch a leg with your tweezers and they will readily give it up (and still run pretty fast); if you catch them on the web, they will shake it violently or simply press themselves against the background so as to blend in and make themselves even harder to see (this is especially effective in leaf-dwelling pholcids). I am still experimenting on ways to catch and collect them efficiently. From the literature, 80% ethanol seems to be the preservation fluid of choice.

Through the actions of one guy especially, the world authority on pholcids Bernhard Huber, pholcids have become a model system for the study of sexual selection and genital evolution (see e.g. Huber, 2003). I will readily admit that I haven’t kept up with all the papers that this program has pumped out, so I can’t summarise everything, but I can give you a couple of the cooler results.

The first is that it was discovered that contrary to what was previously thought, the genitals of haplogynes can be quite complex; see Huber (2002) for a pholcid example.

Pholcids are unique among araneomorphs in that the females lack a receptacula, a specialised structure in which sperm is stored. Instead, pholcids do store sperm, but in a fold in the uterus (Uhl, 1994). This is not a sophisticated structure, just a pouch with no fertilization ducts, meaning the sperm of all the males mixes (Uhl, 1998).

What this means is that in pholcids, we can observe the purest form of sperm competition there is – hence the appeal of pholcids as study organisms, I guess. Only the best sperm can get to the eggs. The females can’t select which sperm to use, as in many other spiders, and males can somewhat affect the outcome by mate-guarding (preventing other males from copulating) – this is an uncommon behaviour in spiders, but widespread in pholcids; I observed this even in my little room in Germany – an individual spider was never found, it was always a female, with a male hanging out in a separate web nearby, to keep other males away. But besides that, nothing but the sperm’s fitness (and random chance) determines the fertilisers.

There may be mate-guarding, but fights over the female will often break out. These are pretty funny to watch: they basically slap each other with their legs, and it looks like a puppet fight orchestrated by 5 year olds with poor dexterity. A more scientific description can be found in Eberhard (1992).

Anyway, having said all that, I have to explain why I’m concentrating especially on the pholcids for Cyprus. Most banally, I need to complete the checklist, where they are completely unrepresented. But I also have an interest in using the pholcids as a study model for my ophiolite → higher speciation rate hypothesis for Cyprus’s biodiversity.

As I said before, pholcids are probably the most abundant and speciose spiders in any given habitat (or at least the ones relevant to Cyprus), meaning they’re definitely waiting to be found. Biogeographical studies have shown that besides human co-inhabitants, pholcids are rather immobile on geographical scales, meaning we can expect isolation from them. That’s corroborated by the fact that it’s rare to have cosmopolitan genera and species; most are confined to one geographical area (broadly-defined, i.e. Neotropics, tropical Africa, etc.).

What all this adds up to is that I expect many species, and endemic ones at that. This is where it gets fancy (and maybe impossible to do). I want to compare the pholcid fauna of Cyprus systematically to that of North Africa and of the Middle East. By systematically, I don’t just mean taxonomy, I mean coding an extensive morphological character matrix and see if I can combine biogeography and cladistics to unravel the evolutionary history of the pholcids of Cyprus.

This is routine stuff when using molecular markers; but there are four reasons I’m not going to do it the “easy” way:

  • I don’t like molecular methods, even though there are no methodological problems at the species level (unlike at higher levels).
  • I don’t have the funding to do this.
  • If I don’t find available pholcid collections from N. Africa and the Middle East, it will give me an excuse to do some fieldwork in those countries ^^
  • This is much more fun and experimental. I can remember a couple of papers where such a thing was attempted, but the results weren’t good. IIRC anyway. I’d like to give it a shot, and the pholcids, with their variability and propensity for sexual selection, make a perfect test case.

I don’t know yet if it’s possible. But even if it turns out to be a bust, the accumulated comparative data will still be valid, and whatever I write to take care of the analysis can always be salvaged for other purposes. But, well, it would be pretty awesome if it does work, if only because it would give me a good publication. I’m told that’s all we’re supposed to care about in this business :)

And in any case, I can always test the ophiolite hypothesis without this procedure. And even if I can’t… at least the checklist will be complete, and all the negative results will teach me how not to fail epically for the next project ^^

References:

Eberhard WG. 1992. Notes on the ecology and behaviour of Physocyclus globosus (Araneae, Pholcidae). Bulletin of the British Arachnological Society 9, 38-42.

Huber BA. 2002. Functional Morphology of the Genitalia in the Spider Spermophora senoculata (Pholcidae, Araneae). Zoologischer Anzeiger 241, 105-116.

Huber BA. 2003. Rapid evolution and species-specificity of arthropod genitalia: fact or artifact?. Organisms Diversity & Evolution 3, 63-71.

Manhart C. 1994. Spiders on bark in a trpical rainforest (Panguana, Peru). Studies on Neotropical Fauna & Environment 29, 49-53.

de Mello-Leitão C. 1946. Notas sobre os Filistatidae e Pholcidae. Annales da Academia Brasileira de Ciencia 18, 39-83.

Petrunkevitch A. 1928. Systema aranearum. Transactions of the Connecticut Academy of Arts and Sciences 29, 1-270.

Petrunkevitch A. 1939. Catalogue of American Spiders. Transactions of the Connecticut Academy of Arts and Sciences 33, 133-338.

Simon E. 1839. Histoire Naturelle des Araignées.

Uhl G. 1994. Genital Morphology and Sperm Storage in Pholcus phalangioides (Fuesslin, 1775) (Pholcidae; Araneae). Acta Zoologica 75, 1-12.

Uhl G. 1998. Mating behaviour in the cellar spider, Pholcus phalangioides, indicates sperm mixing. Animal Behaviour 56, 1155-1159.

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4 responses

18 02 2012
Current Projects: Pholcidae of Cyprus: A bridge between micro- and macroevolution « Doing Biology

[...] Pholcidae are some of the most abundant spiders in any given habitat: members of the family are found on all continents, all altitudes, and all ecosystems. They live anywhere from leaf litter to tree canopies to bedrooms. However, their cryptic behaviour, agility, and effective camouflage makes them hard to collect passively or actively, so their true abundance and diversity is very underestimated. [...]

6 09 2012
Spiders: What is the name of the spiders with thin legs commonly found in Indian households? - Quora

[...] lots of them when staying in India.Very interesting spiders for evolutionary studies too, see here: http://bioteaching.wordpress.com…Embed QuoteVia Abhijeet Sathe.Comment Loading… • Share • Embed • Just now  Add [...]

15 09 2013
Update, plus fieldtrip pictures | Teaching Biology

[…] “oval” with legs. That’s how you tell them apart from my favourite spiders, the pholcids, who also have very elongated legs but a typical spider anatomy with a separated head and […]

30 11 2013
Daily Paperfest: Finding natural selection, damselflies under climate change, spider egg sacs, and more | Teaching Biology

[…] generally not a fan of the model organism approach to research (hypocritical, since I use pholcid spiders as models for my own research). I see much more value in broadly studying a large section of […]

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