Cribellum

Cribellum

A cribellum is a kind of comb-like device in certain spiders, used to separate fibers of silk drawn from its spinnerets into many extremely fine fibers, giving it a wooly structure. Those fibers are so small in diameter that prey insects easily become entangled in them, without any glue needed. The spiders then bite them before they can get away.

A cribellum is also a portion of diatom exoskeleton.

The cribellum is a functional homolog of the antorior median spinnerets of Mesothelae and Mygalomorphae, which do not have a cribellum.

The presence or absence of a cribellum is used to classify araneomorph spiders into the cribellate and ecribellate (not cribellate) type. The distinction can be used to study evolutionary relationships. However, in 1967 it was found out that there are many families with both cribellate and ecribellate members (Lehtinen, 1967). It is today believed that the precursor of all Araneomorphae was cribellate (symplesiomorphy), and that this function was lost in some araneomorph spiders secondarily (Coddington & Levy, 1991). Many of these still retain a colulus, which is thought to be a reduced cribellum, and is of unknown function. However, some "ecribellate" spiders seem to have evolved independently, without cribellate precursors (Foelix, 1979).

Only about 180 genera in 23 families (1991) still contain cribellate members, although the diverse Australian cribellate fauna is still mostly undescribed. However, that fauna may be an example of high diversity in Australian animals that are only relicts in other regions of the world, like the marsupials (Coddington & Levy, 1991).

Cribellate taxa are not very speciose, and for nearly all cribellate-ecribellate sister clades the cribellate lineage is less diverse (Coddington & Levy, 1991), for example:

Cribellate families

22 families of araneomorph spiders (Agelenidae, Amaurobiidae, Amphinectidae, Austrochilidae, Ctenidae, Deinopidae, Desidae, Dictynidae, Eresidae, Filistatidae, Gradungulidae, Hypochilidae, Miturgidae, Neolanidae, Nicodamidae, Oecobiidae, Psechridae, Stiphidiidae, Tengellidae, Titanoecidae, Uloboridae, Zoropsidae) contain at least some cribellate spiders (Griswold et al. 1999). While some of these families are entirely cribellate, others contain both cribellate and ecribellate species.

References

  • Lehtinen, P.T. (1967). Classification of the Cribellate spiders and some allied families, with notes on the evolution of the suborder Araneomorpha. Ann. Zool. Fennici 4: 199-467.
  • Foelix, Rainer F. 1979. Biologie der Spinnen. Georg Thieme Verlag Stuttgart. ISBN 313575801x
  • Coddington, J.A. & Levi, H.W. (1991). Systematics and Evolution of Spiders (Araneae). Annu. Rev. Ecol. Syst. 22: 565-592.
  • Eberhard, William G. and Pereira, Flory. 1993. Ultrastructure of cribellate silk of nine species in eight families and possible taxonomic implications (Araneae: Amaurobiidae, Deinopidae, Desidae, Dictynidae, Filistatidae, Hypochilidae, Stiphidiidae, Tengellidae). Journal of Arachnology 21(3): 161-174. PDF
  • Huber, B.A. (1994): Spermophore morphology reveals a new synapomorphy of Oecobius and Uroctea (Araneae, Oecobiidae). Journal of Arachnology 22: 73-74. PDF
  • Griswold, C.E., Coddington, J.A., Platnick, N.I. and Forster, R.R. (1999). Towards a Phylogeny of Entelegyne Spiders (Araneae, Araneomorphae, Entelegynae). Journal of Arachnology 27: 53-63. PDF

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