Ricinulei

Ricinulei is a small order of arachnids. They are predatory animals that hunt small invertebrates. Because they are quite obscure, they do not have a common name. Some scientists call them hooded tickspiders. Today, there are 103 known species in the family Ricinoididae. These living species are found in two main areas. One group, the genus Ricinoides, lives in tropical West-central Africa. Another group lives in the Americas. This includes the genus Pseudocellus in North and Central America and the Caribbean. The genus Cryptocellus lives from Brazil north to Honduras.

The anatomy of a ricinuleid is very specialized. They have a thick outer shell called a cuticle. Their most famous part is the cucullus, or hood. This hood can be raised or lowered over the head. When it is lowered, it covers the mouth and the chelicerae. The chelicerae are the mouthparts, which consist of fixed and moveable digits. Living ricinuleids are completely blind. They lack eyes, though fossils show they once had lateral eyes. They also have a heavy abdomen called an opisthosoma. This abdomen attaches to the front section, the prosoma, at a narrow waist called a pedicel.

There is a very unusual connection between the prosoma and the opisthosoma. The front edge of the abdomen tucks into a fold on the carapace. This creates a complex coupling mechanism. Scientists do not fully understand why this system exists. One reason it is important is for mating. The genital opening is located on the pedicel. Because of this, the animals must 'unlock' their bodies to mate. The abdomen also features large plates called tergites. Each tergite is divided into a median plate and lateral plates.

Their appendages help them navigate and eat. The pedipalps are complex limbs used to move food. They also act as short-range sensory organs. These pedipalps end in small pincers. The second pair of legs is the longest on the body. These legs act like long-range sensors. They feel the environment ahead, much like antennae. The third pair of legs is different in males. In males, these legs are modified into copulatory organs. The specific shape of these organs helps scientists with taxonomy. This means they use the shape to tell different species apart.

The history of Ricinulei spans over 300 million years. The first fossil was found in 1837 by William Buckland. He actually thought it was a beetle. Later, many scientists like Alexander Petrunkevitch studied them. Fifteen fossil species come from the Late Carboniferous period. These fossils are divided into four extinct families. One family, the Curculioididae, included a giant species. This species was named Curculioides bohemondi. It had a body length of 30 millimeters. Other fossils were found in Cretaceous Burmese amber. These ancient species, called Primoricinulei, may have lived on tree bark.

Researchers study how Ricinulei relate to other animals. Many studies suggest they are closely related to Acari, which are mites and ticks. One reason for this is their development. Ricinuleids hatch as six-legged larvae. They only grow their full eight legs later. This is a trait they share with mites. Some scientists have also looked at the extinct order Trigonotarbida. They noticed that both groups have a similar locking mechanism. They also both have small claws on their pedipalps. However, newer studies called phylogenomics suggest different connections. Some evidence suggests they might be related to Xiphosura, or horseshoe crabs.

Life for a ricinuleid involves living in specific habitats. They usually stay in leaf litter on rainforest floors or in caves. They are often nocturnal, meaning they are active at night. Some species live in Brazil and have a plastron. A plastron can help an animal breathe even if it is under water. Interestingly, ricinuleids are often found in large groups. Scientists do not yet know why they congregate like this. They continue to hunt for small prey in the dark. Every discovery helps us understand these tiny, ancient predators better.