Progradungula otwayensis immature male with catching ladder

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 of diatoms it refers to microscopically punctured regions of the

frustule A frustule is the hard and porous cell wall or external layer of diatoms. The frustule is composed almost purely of silica, made from silicic acid, and is coated with a layer of organic substance, which was referred to in the early literature on ...

, or outer layer. In certain groups of spider species, so-called cribellate spiders, the cribellum is a

silk Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and is produced by certain insect larvae to form cocoons. The best-known silk is obtained from the ...

spinning organ. Unlike the usual

spinnerets A spinneret is a silk-spinning organ of a spider or the larva of an insect. Some adult insects also have spinnerets, such as those borne on the forelegs of Embioptera. Spinnerets are usually on the underside of a spider's opisthosoma, and are ...

of spiders, the cribellum consists of one or more plates covered in thousands of tiny spigots, tiny holes that hardly project from the surface, in contrast to the elongated spigots that project from spinnerets. These minute spigots produce extremely fine fibers, merely tens of nanometres thick, which are combed out by the spider's

calamistrum In spiders, the calamistrum is a row of specialized leg bristles used to comb out fine bands of silk. It is only found on cribellate spiders, that is, spiders that possess the spinning organ known as the cribellum. The calamistrum and cribellum ar ...

, producing silk with a woolly texture. The fibers are so small in diameter that they are strongly subject to

Van der Waals force In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and th ...

s. In addition, the fibres have a surface that absorbs waxes from the

epicuticle The cuticle forms the major part of the integument of the Arthropoda. It includes most of the material of the exoskeleton of the insects, Crustacea, Arachnida, and Myriapoda. Morphology In arthropods, the integument, the external "skin", or ...

of insect prey on contact. This creates a powerful adhesion without any liquid glue that tends to dry out. Bott, Raya A. Baumgartner, Werner Bräunig, Peter Menzel, Florian Joel, Anna-Christin. Adhesion enhancement of cribellate capture threads by epicuticular waxes of the insect prey sheds new light on spider web evolution. Proceedings of the Royal Society B. 31 May 2017. The spider cribellum is a functional homolog of the anterior median spinnerets of

Mesothelae The Mesothelae are a suborder of spiders (order Araneae) that includes a single extant family, Liphistiidae, and a number of extinct families. This suborder is thought to form the sister group to all other living spiders, and to retain ancestral ...

and

Mygalomorphae The Mygalomorphae, or mygalomorphs, are an infraorder of spiders, and comprise one of three major groups of living spiders with over 3000 species, found on all continents except Antarctica. Many members are known as trapdoor spiders due to the ...

, which do not have a cribellum.

Ancestral trait

The presence or absence of a cribellum is used to classify

araneomorph The Araneomorphae (also called the Labidognatha) are an infraorder of spiders. They are distinguishable by chelicerae (fangs) that point diagonally forward and cross in a pinching action, in contrast to the Mygalomorphae (tarantulas and their ...

spiders into the cribellate and ecribellate (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 as ''

Amaurobius ferox ''Amaurobius ferox'', sometimes known as the black lace-weaver, is a common nocturnal spider belonging to the family ''Amaurobiidae'' and genus '' Amaurobius''. Its genus includes the permanently social species ''A. socialis'', and three subsoci ...

'', are also capable of switching between cribellate and ecribellate silk, primarily using cribellate silk for webs and ecribellate silk for trophic eggs. Today, it is believed that the precursor of all

Araneomorphae The Araneomorphae (also called the Labidognatha) are an infraorder of spiders. They are distinguishable by chelicerae (fangs) that point diagonally forward and cross in a pinching action, in contrast to the Mygalomorphae (tarantulas and their ...

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). In

Austrochilidae Austrochilidae is a small spider family with nine species in two genera. ''Austrochilus'' and ''Thaida'' are endemic to the Andean forest of central and southern Chile and adjacent Argentina. Taxonomy , two genera are placed in the family Austr ...

, the cribellum is developed only in the second nymphal stage, so the ecribellate and cribellate conditions change during the spider

ontogenesis Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the st ...

Prevalence

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

marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a ...

s (Coddington & Levy, 1991). Cribellate

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

are not very speciose, and for nearly all cribellate-ecribellate sister

clades A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

the cribellate lineage is less diverse (Coddington & Levy, 1991), for example: *

Haplogynae The Haplogynae or haplogynes are one of the two main groups into which araneomorph spiders have traditionally been divided, the other being the Entelegynae. Morphological phylogenetic studies suggested that the Haplogynae formed a clade; more recen ...

: cribellate Filistatidae c. 100 species, all others (mostly ecribellate) c. 3,000 species *

Entelegynae The Entelegynae or entelegynes are a subgroup of araneomorph spiders, the largest of the two main groups into which the araneomorphs were traditionally divided. Females have a genital plate ( epigynum) and a "flow through" fertilization system; m ...

: cribellate

Deinopoidea The Deinopoidea or deinopoids are group of cribellate araneomorph spiders that may be treated as a superfamily. As usually circumscribed, the group contains two families: Deinopidae and Uloboridae. Some studies have produced cladograms in whi ...

c. 320 species, ecribellate

Araneoidea Araneoidea is a taxon of araneomorph spiders, termed "araneoids", treated as a superfamily. As with many such groups, its circumscription has varied; in particular some families that had at one time moved to the Palpimanoidea have more recently ...

c. 11,000 species

Cribellate families

22 families of

spiders, namely Agelenidae,

Amaurobiidae ''Amaurobiidae'' is a family of three-clawed cribellate or ecribellate spiders found in crevices and hollows or under stones where they build retreats, and are often collected in pitfall traps. Unlidded burrows are sometimes quite obvious in crust ...

Amphinectidae Desidae is a family of spiders, some of which are known as intertidal spiders. The family is named for the genus '' Desis'', members of which live in a very unusual location — between the tides. The family has been reevaluated in recent years an ...

Ctenidae Wandering spiders (''Ctenidae'') are a family of spiders that includes the Brazilian wandering spiders. These spiders have a distinctive longitudinal groove on the top-rear of their oval carapace similar to those of the Amaurobiidae. They are hig ...

Deinopidae Deinopidae, also known as net casting spiders, is a family of cribellate spiders first described by Carl Ludwig Koch in 1850. It consists of stick-like elongated spiders that catch prey by stretching a web across their front legs before propelling ...

Desidae Desidae is a family of spiders, some of which are known as intertidal spiders. The family is named for the genus '' Desis'', members of which live in a very unusual location — between the tides. The family has been reevaluated in recent years an ...

Dictynidae Dictynidae is a family of cribellate, hackled band-producing spiders first described by Octavius Pickard-Cambridge in 1871. Most build irregular webs on or near the ground, creating a tangle of silken fibers among several branches or stems of one ...

, Eresidae, Filistatidae, Gradungulidae,

Hypochilidae Lampshade spiders, family Hypochilidae, are among the most primitive of araneomorph spiders. There are two genera and twelve species currently recognized. Like mygalomorphs, most hypochilids have two pairs of book lungs, but like araneomorphs the ...

Miturgidae Miturgidae is a family of araneomorph spiders that includes nearly 170 species in 29 genera worldwide. First described by Eugène Simon in 1886, it has been substantially revised, and includes the previous family Zoridae as a synonym, and exclud ...

, Neolanidae,

Nicodamidae Nicodamidae is a family of araneomorph spiders with about thirty species in seven genera. They are small to medium-sized spiders found near the ground of eucalypt forest in small sheet webs. The species of this family are only present in Australia ...

Oecobiidae Oecobiidae, also called disc web spiders, is a family of araneomorph spiders, including about 100 described species. They are small to moderate sized spiders (about long combined head and body length, depending on the species. Larger ones tend to ...

, Psechridae,

Stiphidiidae Stiphidiidae, also called sheetweb spiders, is a family of araneomorph spiders first described in 1917. Most species are medium size ('' Stiphidion facetum'' is about long) and speckled brown with long legs. All members of this family occur in ...

, Tengellidae,

Titanoecidae Titanoecidae is a family of araneomorph spiders first described by Pekka T. Lehtinen in 1967. It is fairly widespread in the New World and Eurasia with five genera and more than 50 species worldwide. These are mostly dark-colored builders of "wo ...

Uloboridae Uloboridae is a family of non-venomous spiders, known as cribellate orb weavers or hackled orb weavers. Their lack of venom glands is a secondarily evolved trait. Instead, they wrap their prey thoroughly in silk, cover it in regurgitated digestive ...

and

Zoropsidae Zoropsidae, also known as false wolf spiders for their physical similarity to wolf spiders, is a family of cribellate araneomorph spiders first described by Philipp Bertkau in 1882. They can be distinguished from wolf spiders by their two rows of ...

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.

Diatom cribellum

The perforated regions of the

, or outside layer of many forms of diatom also are called cribella. In such species of diatom the frustule consists of a thin siliceous plate with many small pores.

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. * 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
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* Huber, B.A. (1994): Spermophore morphology reveals a new synapomorphy of ''Oecobius'' and ''Uroctea'' (Araneae, Oecobiidae). ''Journal of Arachnology'' 22: 73–74
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* 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
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External links

(description and pictures)

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