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Cribellum
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 | This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. (November 2020) |
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, or outer layer.
In certain groups of spider species, so-called cribellate spiders, the cribellum is a silk spinning organ. Unlike the usual spinnerets 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.[1] These minute spigots produce extremely fine fibers, merely tens of nanometres thick, which are combed out by the spider's calamistrum, producing silk with a woolly texture.
The fibers are so small in diameter that they are strongly subject to Van der Waals forces.[2] In addition, the fibres have a surface that absorbs waxes from the epicuticle of insect prey on contact. This creates a powerful adhesion without any liquid glue that tends to dry out.[3]
The spider cribellum is a functional homolog of the anterior median spinnerets of Mesothelae and Mygalomorphae, which do not have a cribellum.
Ancestral trait
[edit]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 discovered that there are many families with both cribellate and ecribellate members (Lehtinen, 1967). Some species, such as Amaurobius 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 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). In Austrochilidae, the cribellum is developed only in the second nymphal stage, so the ecribellate and cribellate conditions change during the spider ontogenesis.[4]
Prevalence
[edit]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:
- Haplogynae: cribellate Filistatidae c. 100 species, all others (mostly ecribellate) c. 3,000 species
- Entelegynae: cribellate Deinopoidea c. 320 species, ecribellate Araneoidea c. 11,000 species
Cribellate families
[edit]22 families of araneomorph spiders, namely Agelenidae, Amaurobiidae, Amphinectidae, Austrochilidae, Ctenidae, Deinopidae, Desidae, Dictynidae, Eresidae, Filistatidae, Gradungulidae, Hypochilidae, Miturgidae, Neolanidae, Nicodamidae, Oecobiidae, Psechridae, Stiphidiidae, Tengellidae, Titanoecidae, Uloboridae and 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.
Diatom cribellum
[edit]The perforated regions of the frustule, 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.[5]
References
[edit]- ^ Foelix, Rainer F. (1996). Biology of Spiders (2 ed.). Oxford University Press. pp. 118–122.
- ^ Hawthorn, Anya C.; Opell, Brent D. (2002). "Evolution of adhesive mechanisms in cribellar spider prey capture thread: evidence for van der Waals and hygroscopic forces". Biological Journal of the Linnean Society. 77 (1): 1–8. doi:10.1046/j.1095-8312.2002.00099.x.
- ^ 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. doi:10.1098/rspb.2017.0363
- ^ Hajer, J., Foberová, L. & Řeháková, D. (2017). Silk-producing organs of ecribellate and cribellate nymphal stages in Austrochilus sp. (Araneae: Austrochilidae): Notes on the transformation of the anterior median spinnerets into the cribellum. Israel Journal of Entomology 47: 21–33. [1]
- ^ Gordon, Richard; Dusan Losic; Mary Ann Tiffany; Stephen S. Nagy; Frithjof A.S. Sterrenburg (February 2009). "The Glass Menagerie: diatoms for novel applications in nanotechnology". Trends in Biotechnology. 27 (2): 116–127. doi:10.1016/j.tibtech.2008.11.003. PMID 19167770.
- 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 3-13-575801-X
- Coddington, J.A. & Levi, H.W. (1991). Systematics and Evolution of Spiders (Araneae). Annu. Rev. Ecol. Syst. 22: 565–592. doi:10.1146/annurev.es.22.110191.003025
- 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 Archived 2007-09-30 at the Wayback Machine
- 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 Archived 2006-02-11 at the Wayback Machine
External links
[edit]- The Spider - Web and Silk (description and pictures)
- Cribellate silk vs. sticky silk
- The Evolution of Web Usage in Spiders
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Cribellum
View on Grokipediafrom Grokipedia
The cribellum (from Latin, meaning "little sieve") is a biological structure consisting of a tiny perforated plate. In certain spiders, it is a specialized sieve-like spinning organ located on the abdomen, anterior to the spinnerets, consisting of one or more plates covered by thousands of minute spigots that produce extremely fine cribellar silk fibrils.[1] In diatoms, the cribellum refers to a thin outer layer of the silica frustule featuring a hierarchical array of nanoscale pores.[2]
These spiders, known as cribellates, use the cribellum in conjunction with a comb-like structure called the calamistrum on their fourth leg to card the fibrils onto a core fiber, forming dry, mechanically sticky capture threads that entangle prey without relying on wet glue.[3] This silk production mechanism enables the creation of distinctive cribellate webs, such as sheet-like or orb-shaped structures, where the threads adhere via van der Waals forces and interactions with insect cuticles.[4]
The cribellum's function is primarily tied to prey capture, as the nanofibrils (as thin as 0.00001 mm) tangle with insect hairs, spines, and claws, providing adhesion that is both hygroscopic and potentially electrostatic in nature.[4] Unlike the aqueous viscid silk used by many modern spiders, cribellar silk is produced from specialized cribellar spidroin (CrSp) proteins in acinous glands, resulting in a time-intensive process that can take hours to spin a web compared to minutes for glue-coated alternatives.[1] This organ is exclusive to cribellate families like Uloboridae and Deinopidae, which build webs without aggregate glue glands, relying instead on the physical entanglement properties of the silk.[3]
Evolutionarily, the cribellum is considered ancestral to all araneomorph spiders—the dominant clade comprising over 95% of extant species—and was likely present in their common ancestor, supporting hypotheses of an ancient origin for orb-weaving behaviors.[4] However, it has been lost independently multiple times across spider lineages, particularly in the RTA clade (e.g., wolf spiders, jumping spiders) and advanced orb-weavers (Araneoidea), correlating with bursts in diversification as spiders transitioned to more efficient viscid silk systems and freed themselves from substrate-bound web architectures.[3] The repeated loss of the cribellum, documented in over 90% of araneomorph species, underscores its role as a key innovation in early spider evolution, with conserved CrSp genes among retaining taxa indicating a monophyletic origin for cribellate capture strategies.[1] Today, cribellate spiders represent a minority but ecologically significant group, highlighting the diversity of silk-based adaptations in arachnids.[4]
In Spiders
Definition
The cribellum is a specialized silk-spinning organ present in certain spiders of the suborder Araneomorphae, positioned on the ventral surface of the abdomen and differentiated from the conventional spinnerets by its plate-like form.[5] This structure enables the production of distinctive silk types, setting it apart from the more typical appendage-like spinnerets found in most spider species.[6] The name "cribellum" originates from Late Latin cribellum, a diminutive of cribrum meaning "sieve," alluding to its sieve-like appearance characterized by numerous perforations.[7] In broader biological contexts, the term applies to various perforated plate structures across taxa, but within arachnids, it specifically designates this ventral spinning organ associated with silk extrusion.[8] Historically, the cribellum was first formally described by Philipp Bertkau in 1882, who employed its presence as a key taxonomic character to differentiate spiders into the groups Cribellata (cribellate species) and Ecribellata (non-cribellate species).[9] This classification highlighted the organ's significance in spider systematics, influencing subsequent phylogenetic studies.[10]Anatomical Structure
The cribellum is located on the ventral surface of the spider's abdomen, positioned at the posterior end immediately anterior to the spinnerets and homologous to the fused anterior median spinneret pair.[5] In gross morphology, it appears as a small, transverse, chitinous plate that is typically flat or slightly raised, often oval or rectangular in shape.[11] This plate may be undivided or longitudinally divided into two symmetrical fields by a median carina, as seen in families like the Eresidae.[12] At the ultrastructural level, the cribellum is densely covered with thousands of minute spigots, numbering from about 1,000 to over 10,000 per plate depending on the species and maturity.[12] Each spigot is an elongate, flexible structure approximately 10 μm in length, featuring a segmented shaft with a pagoda-like tiered tip composed of five tubal segments that enable bending and alignment with neighboring spigots.[12] These spigots produce fine nanofibrils with diameters ranging from 10 to 100 nm, contributing to the sieve-like appearance of the organ.[13] Developmentally, the cribellum forms during nymphal stages, often appearing in the second or third instar after an initial ecribellate phase in the first instar, where a precribellum resembling rudimentary spinnerets is present.[14] In species such as Hyptiotes cavatus, the number of spigots increases progressively across instars—approximately 1.5 to 4 times more from third to sixth instar in females—due to cribellar plate growth while spigot density remains constant.[15] This ontogenetic development culminates in a fully formed organ by maturity in females, though males in some species may lose it in the final instar.[15] Variations in cribellar structure occur across families; for instance, Uloboridae exhibit a distinct plate-like form with a high density of spigots, while in Deinopidae and some Dictynidae, the plate is more integrated with surrounding abdominal sclerites and features cylindrical fibrils without nodules.[16] In Filistatidae, the cribellum produces uniquely ribbon-like fibrils, reflecting family-specific morphological adaptations.[16]Associated Calamistrum
The calamistrum is a specialized comb-like structure consisting of a row of curved, twisted setae located on the metatarsus of the fourth legs in cribellate spiders.[17] These setae form a functional comb that interacts directly with the cribellum to process silk nanofibers.[18] Morphologically, the calamistrum features numerous overlapping setae that bend at approximately 90 degrees and flatten to create a smooth, surface-like area, facilitating the passage and alignment of silk fibrils.[17] In some taxa, such as Deinopidae, the setae exhibit serrated edges or teeth that aid in disentangling and handling the silk, while these are absent in Uloboridae, where the structure relies more on the overlapping bases for density.[17] Variations in seta density and arrangement occur across families, with higher overlap reducing gaps in Deinopidae compared to the more spaced rows in Uloboridae.[17] Functionally, the calamistrum is essential for cribellate spiders, as it sweeps over the cribellum to draw out and comb the extruded nanofibers into a puffy, adhesive thread configuration, enhancing capture efficacy; it is entirely absent in ecribellate species that produce viscid silk instead.[18] This structure ensures the nanofibers pass across its surface rather than interlocking between individual setae, preventing tangling during spinning.[17] The calamistrum is particularly prominent in families such as Uloboridae (e.g., Uloborus plumipes and Zosis geniculata) and Deinopidae (e.g., Deinopis subrufa), where it exhibits adaptive variations suited to their web-building behaviors.[17] Evolutionarily, the calamistrum likely co-evolved with the cribellum as an ancestral retention trait in basal araneomorph lineages, enabling the production of dry adhesive capture threads and persisting in about 21 families despite widespread loss in more derived groups.Function
Silk Production
The cribellum, a specialized spinning organ in cribellate spiders, produces silk through a unique biophysical mechanism involving the simultaneous extrusion of thousands of fine nanofibers from its spigots. These spigots, numbering up to 40,000 on the cribellar plate, each connect to individual cribellar glands and release liquid silk that solidifies into a flat ribbon of nanofibrils upon extrusion. The nanofibers, typically 10–100 nm in diameter with many around 20 nm, form without the need for aggregate glue glands, resulting in a dry, adhesive capture silk. This process contrasts with the wet spinning of ecribellate spiders, as the cribellate output relies on mechanical entanglement rather than aqueous coatings.[18][12][19][20] During spinning, the spider rotates its abdomen while using the calamistrum—a comb-like structure on the hind legs—to hackle and align the extruded fibrils, creating a woolly, puffed texture that enhances thread extensibility and surface area. This dynamic combing occurs at rates of about 8 Hz, pulling the nanofibers into a composite thread with a stiff core from pseudoflagelliform glands coated by the cribellate layer. The resulting silk forms thin sheets or spirals for capture threads, with nanofibrils often tens of nanometers thick, enabling high stretchability up to 1,400% in combed configurations. Experimental analyses of spinning kinematics in species like Badumna longinqua reveal coordinated spinneret movements that optimize fiber alignment and puff formation. Recent studies have reported extensibility of at least 1,100% in nanofibrils of species like the southern house spider.[18][20][21][22] Biochemically, cribellate silk is composed primarily of cribellar spidroins (CrSp), specialized proteins synthesized in acinous glands and expressed at high levels (e.g., 100 TPM in transcriptomics). These spidroins feature tandem repetitive domains with β-sheet motifs (Aₙ), flanked by conserved N- and C-terminal domains, differing from flagelliform silks in their amino acid composition and lack of extensive glycine-alanine repeats. Multi-omics profiling in orb-weaving species like Octonoba octonarius confirms CrSp's role in producing extensible nanofibrils, with a balanced hydrophobic (e.g., leucine, isoleucine) and hydrophilic profile contributing to the silk's adhesive properties via van der Waals forces. This composition supports the dry spinning process, where proteins assemble into fibrils without additional glues.[1][19]Role in Capture Threads
The cribellate silk produced by the cribellum forms dry adhesive capture threads that rely on nanofibrils for prey retention, distinct from the aqueous glue found in ecribellate silks. These nanofibrils adhere primarily through van der Waals forces, physical interlocking, hygroscopic interactions, and electrostatic forces, without any wet adhesive component.[20] Epicuticular waxes from insect cuticles further enhance adhesion by infiltrating the nanofibrils via capillary action, forming a fiber-reinforced composite that increases binding strength eightfold compared to wax-free surfaces (260 ± 117 µN versus 31 ± 16 µN on native versus wax-free elytra). This mechanism allows the threads to absorb waxes from diverse prey cuticles, promoting effective capture across varied insect surfaces.[23] In web construction, cribellate capture threads are deployed in diverse architectures, including orb webs by families such as Uloboridae, where they form the sticky spiral; sheet webs, as seen in some amaurobiids; and irregular tangle or funnel webs with cribellate strands in dictynids and others. For instance, in Deinopidae net-casting spiders, the cribellate silk constitutes the sticky spiral within a small, portable web that is thrown onto prey, enabling precise targeting in low-light conditions. The spinning process involves extrusion of nanofibrils from the cribellum, which are then combed by the calamistrum into puffed structures that maximize surface contact. This puffing behavior significantly boosts adhesion by creating multiple attachment points, yielding up to ten times higher adhesive force (approximately 76 nJ work of adhesion) compared to uncombed threads, while also allowing extreme extensibility over 1,400% to absorb prey impacts on dry surfaces.[18][24] Compared to the aggregate silk used by Araneidae orb-weavers, which employs viscous glue droplets for higher stickiness per volume and broader contact area, cribellate threads achieve greater overall material efficiency in gross adhesion but decline more rapidly with increasing thread volume due to diminishing returns per added fibril. This makes cribellate systems particularly advantageous for retaining prey over extended periods on dry substrates, with age-resilient stickiness that persists for up to 18 months without significant loss, unlike glue-based silks that degrade faster. However, cribellate threads exhibit limitations in humid environments, where wetting from dew or fog collapses the nanofibril puffs, reducing adhesion stress by an order of magnitude (from 1–5 µN/µm² to 0.1–0.5 µN/µm²) and strain displacement (from 2–5 mm to 0.10–0.12 mm), thereby impairing capture efficacy compared to more humidity-tolerant glued silks.[25][26][27]Evolution
Ancestral Trait
The cribellum represents a symplesiomorphic trait in the phylogeny of Araneae, originating in the stem lineage of Araneomorphae, the dominant clade of modern spiders. Phylogenetic analyses indicate that the common ancestor of all araneomorph spiders possessed a cribellum, which served as a specialized silk-producing organ for generating cribellate capture threads. This ancestral feature divides araneomorphs into two major groups: Cribellata, which retain the cribellum and associated structures like the calamistrum, and Ecribellata, in which the cribellum has been secondarily lost multiple times across diverse lineages. Recent comprehensive phylogenomic studies confirm the cribellum's position as a basal characteristic in the Araneae tree of life, with losses correlating to shifts in web architecture and hunting strategies.[9][3] Fossil evidence supports the antiquity of the cribellum, with its presence inferred in early araneomorph spiders from the Carboniferous period, approximately 300–310 million years ago; direct observation is challenging due to the organ's small size. Retention of the cribellum is evident in basal extant clades, such as Austrochilidae, which occupy early-diverging positions in molecular phylogenies and produce cribellate threads in sheet-like webs, underscoring the structure's persistence from deep evolutionary history.[28][29] Developmentally, the cribellum is expressed early in ontogeny, reflecting its ancestral role in silk production. In basal families like Austrochilidae, the cribellum emerges in the second nymphal instar, transitioning from an ecribellate first-instar condition to cribellate silk spinning, which highlights conserved developmental pathways across araneomorphs. This early ontogenetic activation suggests that the genetic and morphological machinery for cribellum function is plesiomorphic and readily deployable, even in lineages where it is later modified or suppressed.[30] Molecular evidence further corroborates the ancestral nature of the cribellum through the presence of specialized spidroin genes, such as cribellar spidroin (CrSp), which encode the unique nanofibrillar proteins of cribellate silk. Proteomic profiling of basal cribellate families reveals conserved CrSp variants that predate the divergence of major araneomorph clades, with expression patterns linking these genes to the primitive cribellum organ. These findings indicate that the molecular toolkit for cribellate silk evolved once in the araneomorph stem and has been retained in early-branching lineages, providing a genetic basis for the trait's symplesiomorphy.[1]Evolutionary Loss and Retention
The cribellum, an ancestral trait in araneomorph spiders, has undergone secondary loss in the majority of lineages, particularly within the RTA clade and Orbiculariae. In the RTA clade, which encompasses a diverse array of hunting and web-building spiders, the cribellum has been lost repeatedly, affecting more than 90% of species in this group. This pattern of loss is evident in phylogenetic reconstructions that map the cribellum's absence as a derived state across multiple subclades. Similarly, within Orbiculariae, the cribellum has been supplanted in derived orb-weavers by the evolution of flagelliform glands, which produce the axial fibers and aggregate glue for sticky capture threads in ecribellate species.[3][31][32] Retention of the cribellum persists in approximately 21 families of araneomorph spiders, representing a small fraction of the total diversity but highlighting its conserved role in certain web-building strategies. These retentions are scattered across basal and derived lineages, such as in Deinopoidea and some marronoid groups, where the organ continues to facilitate the production of dry, adhesive cribellate silk. Rare instances suggest possible re-evolution or resurrection of cribellate structures in otherwise ecribellate lineages; for example, the presence of a cribellum in the leptonetid spider Archoleptoneta schusteri, outside the core Dionycha clade, indicates potential regain in some araneomorph groups previously thought to have lost it definitively. Phylogenetic analyses of Dionycha, comprising two-clawed spiders, reinforce that the clade is broadly ecribellate, with losses occurring early in its diversification.[31][33][34] The primary drivers of cribellum loss appear tied to innovations in web architecture and silk composition, particularly the shift from costly cribellate capture silk to more efficient glued silk systems. This transition correlates with two major diversification events in spiders, enabling release from substrate-bound webs and the evolution of aerial or abandoned web strategies. Recent genomic and phylogenomic studies have refined these insights; for instance, analyses of Uloboridae threads reveal conserved spidroin genes underlying cribellate silk, even as losses dominate in related clades. Updated 2023 phylogenies, incorporating broader taxon sampling, provide more precise mapping of losses across the spider tree, underscoring multiple independent events and highlighting gaps in undescribed diversity, such as in Australian marronoid spiders where additional retentions may exist.[3][31][9][35]Taxonomy
Cribellate Families
Cribellate spiders are distributed across 22 families within the Entelegynae clade of araneomorph spiders, with no presence in the more basal Mygalomorphae suborder. These families encompass around 3,800 species in approximately 180-200 genera, based on recent taxonomic data, though phylogenetic revisions continue to refine this distribution.[36] The cribellum is an ancestral trait retained exclusively in these groups, with some families being fully cribellate (all species possessing the structure) and others partially cribellate (only certain genera or species retain it).[37] Fully cribellate families include Hypochilidae (lampshade weavers), Filistatidae (crevice weavers), Deinopidae (ogre-faced spiders), Uloboridae (cribellate orb-weavers), Dictynidae (mesh-weavers), and Eresidae (velvet spiders), among others.[38] Uloboridae species, for example, are notable for building orb webs with hackled cribellate capture threads and lacking venom glands, a secondary loss.[31] Deinopidae construct similarly cribellate but often irregular webs, using specialized forward- and backward-facing eyes for prey detection.[37] Dictynidae produce tangled, sheet-like cribellate webs in vegetation or litter.[39] Partially cribellate families feature the cribellum in select genera, reflecting evolutionary transitions. In Agelenidae (funnel-weavers), some genera retain the cribellum and produce cribellate capture silk in their sheet webs.[40] Amaurobiidae (hackledmesh weavers) includes cribellate genera such as Metaltella, which spins cribellate funnel webs similar to those of ecribellate relatives but with woolly capture threads.[41] Recent phylogenomic analyses have repositioned Psechridae (lace-sheet weavers) as a cribellate family allied with Lycosoidea, emphasizing their web-building behavior and retention of the cribellum in all known species, with body sizes up to 2 cm.[42] Taxonomic surveys highlight incompleteness, particularly in Australia, where undescribed relatives of Zodariidae may represent additional cribellate lineages pending modern molecular and morphological studies.[3]| Category | Example Families | Notes |
|---|---|---|
| Fully cribellate | Hypochilidae, Filistatidae, Deinopidae, Uloboridae, Dictynidae, Eresidae | All species possess cribellum; associated with diverse web architectures including orbs and sheets.[38] |
| Partially cribellate | Agelenidae, Amaurobiidae | Cribellum limited to specific genera; reflects partial evolutionary loss within families.[40][41] |
| Recently confirmed | Psechridae | Web-building lycosoids; all cribellate, integrated into modern phylogenies.[42] |