Ricinulei

Ricinulei are distinguished from other arachnid orders by the presence of a cucullus—a hinged hood that can be raised or lowered over the prosoma. When lowered, this structure completely conceals the chelicerae and mouth. They possess a heavily sclerotized, thick cuticle on both body and legs. The opisthosoma attaches to the prosoma via a narrow pedicel with a complex locking mechanism unique among arachnids. All extant species lack eyes, though fossil representatives retain evidence of lateral eye structures. Males possess uniquely modified third legs that function as copulatory organs; the shape of these structures is taxonomically diagnostic at the species level.

Extant ricinuleids occupy two primary habitat types: leaf litter of tropical rainforest floors and caves. The African genus Ricinoides inhabits undisturbed, closed canopy, mid-elevation evergreen forests. The American genera Cryptocellus and Pseudocellus occur in both leaf litter and cave systems, with at least one species (Cryptocellus armasi) exhibiting troglomorphic adaptations including elongated appendages. Fossil evidence suggests Carboniferous representatives were surface-dwelling, unlike modern species.

The order has a disjunct distribution across two landmasses. Ricinoides occurs in tropical West Africa from The Gambia and Senegal eastward to Congo and Gabon. Pseudocellus ranges from Panama through Central America and Mexico to southern Texas, USA, including Caribbean islands. Cryptocellus extends from Rondônia in western Brazil northward to Honduras. The fossil record includes Late Carboniferous deposits in Euramerica (Europe and North America) and Cenomanian Burmese amber from Myanmar.

Ricinuleids are predatory, feeding on small arthropods. Specific prey items documented in natural settings remain sparse; the diet is presumed to include various small invertebrates encountered in leaf litter and cave habitats.

Ricinuleids exhibit direct development with maternal care. Females carry eggs under the cucullus until hatching. Newly emerged young are six-legged larvae, a trait shared with Acari. The larvae subsequently molt into eight-legged adult morphology. The six-legged larval stage represents a derived developmental pathway among arachnids.

Ricinuleids are nocturnal. The second pair of legs, which are the longest, function as 'long-range' sensory organs extended forward during locomotion, analogous to antennae. Pedipalps serve as 'short-range' sensory structures and manipulate food items. Males transfer spermatophores to females using their modified third pair of legs. Individuals have been observed in large congregations; the function of these aggregations remains unknown. Non-mating social interactions between individuals have been documented.

As small predators in leaf litter and cave ecosystems, ricinuleids contribute to invertebrate community structure through predation on microarthropods. Their specific ecological functions and population impacts have not been quantified.

Ricinuleids have no documented economic or medical importance to humans. Their obscurity has prevented the establishment of a widely accepted common name, though 'hooded tickspiders' appears in academic literature. The order attracts scientific interest due to its unusual morphology, extensive fossil record, and uncertain phylogenetic relationships.

• Opiliones (harvestmen)Historical misidentification; ricinuleids were originally classified as unusual harvestmen by early workers. Ricinuleids differ in possessing a cucullus, a narrow pedicel with locking mechanism, and six-legged larvae.
• Acari (mites and ticks)Proposed sister group relationship based on shared six-legged larval stage and morphological characters. Ricinuleids are readily distinguished by their large size, heavily sclerotized cuticle, cucullus, and walking leg morphology.
• Trigonotarbida (extinct)Proposed sister group relationship based on shared opisthosomal tergite structure, body locking mechanism, and pedipalp claw morphology. Trigonotarbida is extinct; ricinuleids represent the only living arachnids with this combination of characters.

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• They Created 'New Species' of Spiders | Bug Squad
• A new Amazonian Cryptocellus Westwood (Arachnida, Ricinulei)
• A new ricinuleid of the genus Ricinoides Ewing (Arachnida, Ricinulei) from Ghana
• A new species of Cryptocellus (Arachnida, Ricinulei) from the Kofán Territory in southwestern Colombia
• Cryptocellus armasi, a new troglomorphic hooded tick-spider from Venezuela (Arachnida, Ricinulei)
• Advances in Understanding the Karyotype Evolution of Tetrapulmonata and Two Other Arachnid Taxa, Ricinulei and Solifugae.