Archaeidae

Afrarchaea grimaldii, from Penney (2003). Scale bar = 1 mm.

Belongs within: Palpimanoidea.

The strangest of spiders
Published 29 August 2008

Some of the smallest spiders are mind-bogglingly tiny—the smallest known male spider, Patu digua, reaches all of 0.37 mm in length as an adult, but at least one other species known as yet only from females could potentially have a male even smaller. If one of these spiders crawled into your ear while you were sleeping, it could probably slip into your Eustachian tubes and tap on the back of your eyeballs. But even more remarkable than their small size is the bizarre morphologies on show among the micro-spiders. And no group of micro-spiders is more bizarre than the Archaeidae.

Archaeid eating, copyright Jeremy Miller.

Archaeids are a bit bigger than Patu, but still pretty small—the largest examples reach about six millimetres. The name “Archaeidae”, of course, means “old”, and archaeids received their name because they were first described in 1854 from fossils in Baltic amber from northern Europe. In Europe, the archaeids are long gone (they may have disappeared along with the amber forests), but nearly thirty years after their initial description living examples were found in Madagascar. They are also known from Australia, while a specimen from Cretaceous Burmese amber has been placed in a living genus from South Africa and Madagascar (Penney 2003). A species has also been described from the Jurassic of Kazakhstan, but it is uncertain whether this species is an actual archaeid or belongs to another micro-spider family such as Pararchaeidae.

Many micro-spiders show relatively long chelicerae (the fangs and their base) relative to body size, but in Archaeidae this is taken to the extreme. Because the trochanter (base) of the chelicerae is a rigid structure, lengthening them in spiders requires that the carapace as a whole be raised, otherwise the fangs would not be able to get anywhere near the mouth. Archaeids have developed a long “neck” supporting the eyes and chelicerae. The distinct shape of the cephalothorax together with the long chelicerae gives them an unmistakeable profile, and one common name used for the group is “pelican spiders”. Despite their small size, archaeids are active hunters and voracious exclusive predators of other spiders (another common name is “assassin spiders”). It has been suggested that the lengthened chelicerae are directly related to their araneophagous diet, allowing them to strike their prey without getting too close, but as I already noted archaeids are not the only small spiders with lengthened chelicerae (though they are still the most dramatic), and I’d be interested to know if there is a correlation between small size and long chelicerae.

I’d also like to share this diagram from Wood et al. (2007) showing a molecular-derived phylogeny of the endemic Madagascan genus Eriauchenius. As can be seen, there is a fair amount of variation in the thickness of the “neck” (the darkness of the bars reflects the mean carapace height/length ratio for whichever group they subtend), and it had been suggested that those species with a particularly slender neck formed a derived clade. Wood et al. (2007) found that this does not appear to be the case, with at least two extreme narrow-neck groups—E. workmani in one and E. gracilicollis and E. lavatenda in the other—at quite divergent points in the tree. I also looks to me like at least one group—E. tsingyensis and its allies—may have gone the other way. To paraphrase a Rocky Horror Picture Show audience member—that spider has no neck.

Systematics of Archaeidae
<==Archaeidae
    |--Myrmecarchaea Wunderlich 2004SHR08
    |--Saxonarchaea Wunderlich 2004SHR08
    |--Zephyrarchaea robinsi (Harvey 2002)W13
    |--EntomocephalusP92
    |--Archaea Koch & Berendt 1854SHR08
    |    |--A. gracilicollisFF99
    |    `--A. nodosa Forster 1956M72
    |--Patarchaea Selden, Huang & Ren 2008SHR08
    |    `--*P. muralis Selden, Huang & Ren 2008SHR08
    |--Austrarchaea Forster & Platnick 1984SHR08
    |    `--A. robinsi Harvey 2002RRH09
    |--Eoarchaea Forster & Platnick 1984SHR08
    |    `--*E. hyperoptica (Menge in Koch & Berendt 1854) [=Archaea hyperoptica]SHR08
    |--Jurarchaea Eskov 1987S93, SHR08
    |    `--J. zherikhini Eskov 1987SHR08
    |--Eriauchenius Pickard-Cambridge 1881SHR08
    |    |--E. cornutus (Lotz 2003)SHR08
    |    |--E. gracilicollis Millot 1948SHR08
    |    `--E. workmaniR14
    `--Afrarchaea Forster & Platnick 1984SHR08
         |--*A. godfreyi (Hewitt 1919)SHR08
         |--A. grimaldii Penney 2003SHR08
         |--A. royalensis Lotz 2006SHR08
         `--A. woodae Lotz 2006SHR08

*Type species of generic name indicated

References

[FF99] Forster, R., & L. Forster. 1999. Spiders of New Zealand and their World-wide Kin. University of Otago Press: Dunedin (New Zealand).

[M72] Monroe, R. 1972. Chelicerate type-specimens in the Queensland Museum. Memoirs of the Queensland Museum 16 (2): 291–307.

Penney, D. 2003. Afrarchaea grimaldii, a new species of Archaeidae (Araneae) in Cretaceous Burmese amber. Journal of Arachnology 31 (1): 122–130.

[P92] Poinar, G. O., Jr. 1992. Life in Amber. Stanford University Press: Stanford.

[RRH09] Rix, M. G., J. D. Roberts & M. S. Harvey. 2009. The spider families Synotaxidae and Malkaridae (Arachnida: Araneae: Araneoidea) in Western Australia. Records of the Western Australian Museum 25 (3): 295–304.

[S93] Selden, P. A. 1993. Arthropoda (Aglaspidida, Pycnogonida and Chelicerata). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 297–320. Chapman & Hall: London.

[SHR08] Selden, P. A., Huang D. & Ren D. 2008. Palpimanoid spiders from the Jurassic of China. Journal of Arachnology 36 (2): 306–321.

[W13] Waldock, J. M. 2013. A review of the peacock spiders of the Maratus mungaich species-group (Araneae: Salticidae), with descriptions of four new species. Records of the Western Australian Museum 28 (1): 66–81.

Wood, H. M., C. E. Griswold & G. S. Spicer. 2007. Phylogenetic relationships within an endemic group of Malagasy ‘assassin spiders’ (Araneae, Archaeidae): ancestral character reconstruction, convergent evolution and biogeography. Molecular Phylogenetics and Evolution 45 (2): 612–619.

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