The arachnids belonging to the order Ricinulei, or hooded tick spiders, are neither spiders nor ticks. At first glance, these primitive arachnids look a lot like your typical spider. However, if you look closely you will notice that they have segmented abdomens unlike spiders and a complete lack of eyes. It’s like they were trying really hard to cosplay as a spider but missed most of the critical details on their costume. Even so, you’ll probably never have to worry about making this distinction because ricinuleids are rare in comparison to other arthropods. Though locally abundant, only 55 species worldwide have been identified since the order’s discovery in 1838 and specimens are few and far between. A fossil of an extinct Carboniferous species was found in 1837, but the guy who found it thought it kind of looked like a beetle, so it wasn’t identified as a ricinuleid until later. Extant species are found in tropical regions of Central America and western and central Africa where they live in soil and litter.
Very little is known about this animal group even today, but scientists do know that males use their modified third pair of legs for sex. That’s important. Not just because it’s funny, but because looking at ricinuleid junk can be critical to species identification. These legs are used to hold and then transfer seminal fluid into a mounted female’s genital opening. Females store the sperm for later when they are ready to fertilize their eggs. The eggs are laid singly and sometimes carried around by the mother that laid them. Interestingly, baby ricinuleids hatch with only six legs instead of the usual eight that is the signature of arachnids. This and other morphological features is shared with Acari, or mites and ticks, and is believed to be indicative of a close relationship with that order. As the young develop, they grow their final pair of legs and start to look more like proper arachnids.
If you are so inclined, this paper has an even better picture of the male pedipalp: http://www.scielo.br/pdf/zool/v29n5/v29n5a12
Why are they called “hooded” tick spiders? The spider and tick parts of the common name are understandable, as they look a bit like spiders and are related to mites and ticks, but what about them is hooded? As it happens, ricinuleids have a cute little hood on their heads called the cucullus that can be raised or lowered at will. When the hood is down, it covers their mouthparts completely. How polite! The purpose and function of this structure is not yet understood, but it is one of their most defining features.
Ricinuleids may not be as fierce looking as amblypygids or as cool as vinegaroons, but they’ve been around the block and certainly have their own special quirks. So give them a round of applause for just sticking with it and existing all these years. If any order goes unappreciated, it’s the hooded tick spiders.
1. Harvey, Mark S. Catalogue of the smaller arachnid orders of the World: Amblypygi, Uropygi, Schizomida, Palpigradi, Ricinulei and Solifugae. CSIRO publishing, 2003.
2. Harvey, Mark S. “The neglected cousins: what do we know about the smaller arachnid orders?.” Journal of Arachnology 30.2 (2002): 357-372.
3. Adis, Joachim U., et al. “On the abundance and ecology of Ricinulei (Arachnida) from Central Amazonia, Brazil.” Journal of the New York Entomological Society (1989): 133-140.
4. Ewing, H. E. “A synopsis of the American arachnids of the primitive order Ricinulei.” Annals of the Entomological Society of America 22.4 (1929): 583-600.
5. Platnick, Norman I. “A new Cryptocellus (Arachnida: Ricinulei) from Brazil.” Journal of the New York Entomological Society (1988): 363-366.
6. Talarico, G., J. G. Palacios-Vargas, and G. Alberti. “The pedipalp of Pseudocellus pearsei (Ricinulei, Arachnida)–ultrastructure of a multifunctional organ.” Arthropod structure & development 37.6 (2008): 511-521.