 | This article includes alist of references,related reading, orexternal links,but its sources remain unclear because it lacksinline citations. Please helpimprove this article byintroducing more precise citations.(November 2020) (Learn how and when to remove this message) |
Cribellum literally means "little sieve", and in biology the term generally applies to anatomical structures in the form of tiny perforated plates.
In certain groups ofdiatoms it refers to microscopically punctured regions of thefrustule, or outer layer.
In certain groups of spider species, so-called cribellate spiders, thecribellum is asilk spinning organ. Unlike the usualspinnerets of spiders, the cribellum consists of one or more plates covered in thousands of tinyspigots, tiny holes that hardly project from the surface, in contrast to the elongated spigots that project from spinnerets.[1] These minute spigots produce extremely fine fibers, merely tens of nanometres thick, which are combed out by the spider'scalamistrum, producing silk with a woolly texture.
The fibers are so small in diameter that they are strongly subject toVan der Waals forces.[2] In addition, the fibres have a surface that absorbs waxes from theepicuticle of insect prey on contact. This creates a powerful adhesion without any liquid glue that tends to dry out.[3]
The spider cribellum is afunctional homolog of the anterior median spinnerets ofMesothelae andMygalomorphae, which do not have a cribellum.
The presence or absence of a cribellum is used to classifyaraneomorph spiders into thecribellate andecribellate (not cribellate) type. The distinction can be used to study evolutionary relationships. However, in 1967 it was discovered that there are many families with both cribellate and ecribellate members (Lehtinen, 1967). Some species, such asAmaurobius ferox, are also capable of switching between cribellate and ecribellate silk, primarily using cribellate silk for webs and ecribellate silk for trophic eggs.[citation needed]
Today, it is believed that the precursor of allAraneomorphae was cribellate (symplesiomorphy), and that this function was lost in some araneomorph spiders secondarily (Coddington & Levy, 1991). Many of these still retain acolulus, 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). InAustrochilidae, the cribellum is developed only in the second nymphal stage, so the ecribellate and cribellate conditions change during the spiderontogenesis.[4]
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 themarsupials (Coddington & Levy, 1991).
Cribellatetaxa are not very speciose, and for nearly all cribellate-ecribellate sisterclades the cribellate lineage is less diverse (Coddington & Levy, 1991), for example:
22 families ofaraneomorph spiders, namelyAgelenidae,Amaurobiidae,Amphinectidae,Austrochilidae,Ctenidae,Deinopidae,Desidae,Dictynidae,Eresidae,Filistatidae,Gradungulidae,Hypochilidae,Miturgidae,Neolanidae,Nicodamidae,Oecobiidae,Psechridae,Stiphidiidae,Tengellidae,Titanoecidae,Uloboridae andZoropsidae contain at least some cribellate spiders (Griswoldet al. 1999). While some of these families are entirely cribellate, others contain both cribellate and ecribellate species.
The perforated regions of thefrustule, or outside layer of many forms ofdiatom also are called cribella. In such species of diatom the frustule consists of a thin siliceous plate with many small pores.[5]
| Arachnology |  | |
|---|---|---|
| Taxonomy | ||
| Anatomy | ||
| Human interaction | ||
| Webs | ||
