Almost anywhere one travels on Earth, some type of arachnid can be found. From deserts to tropical rainforests to deep-sea hydrothermal vents, if you look closely enough, you are bound to find a member of the Arachnida. Spiders have even been collected ballooning through the stratosphere! Not only are they everywhere, arachnids are also incredibly diverse, both in their number of species and their ecology and behavior. So who are these creatures and why aren't more people familiar with them? Below is a brief introduction to the major groups of arachnids, with links to our research aimed at discovering how these groups are related to one another.
Arachnids are one of the four major groups of arthropods. The other three major groups are insects, crustaceans (crabs and their kin), and myriapods (centipedes and millipedes). Insects are well known for their species diversity, but it is now recognized that the Acari (mites and ticks) may contain up to a million species (although most are not yet described). Spiders, too, have a rich fauna, with greater than 40,000 described species (Platnick 2008). The other nine orders of arachnids contain many fewer species, but harbor a great diversity of body forms, ecologies, behaviors, physiologies, and life histories.
Below, we follow the nomenclatural system for arachnid orders proposed by T. Savory (1972) in Systematic Zoology 21:122-125. A nice review of the less species-rich arachnid orders is: Harvey, M. 2002. The neglected cousins: What do we know about the smaller arachnid orders? Journal of Arachnology. 30:375-372.
(*Disclaimer* Any errors in the text are entirely our own, and comments/corrections should be sent to smasta@pdx.edu)
Taxonomy of Arachnida:
Phylum Arthropoda
Chelicerata
Pycnogonida – Sea Spiders (not agreed upon if these are truly chelicerates)
Xiphosura – Horseshoe crabs
Arachnida
Acari – mites and ticks
Amblypygi – whip-spiders
Araneae – spiders
Opiliones – harvestmen
Palpigradi
Pseudoscorpiones
Ricinulei
Schizomida
Scorpiones
Solifugae – camel or sun-spiders
Uropygi or Thelyphonida – vinegaroons
Below is a stylized body plan of a spider. Anatomical features shared among arachnids include:
4 pairs of legs originating on the prosoma or cephalothorax
1 pair of pedipalps
1 pair of chelicerae (from the Greek “cheli” meaning claw)
Put simply, the Acari (ticks, mites, and allies) exemplify diversity. Most species are tiny, even microscopic, and often overlooked. The largest species (such as velvet mites and ticks) are generally not longer than 10 mm, while the smallest are tiny enough to live in such bizarre places as the human hair follicle. The Acari are commonly parasites of other organisms. For example, Locustacarus buchneri is a parasitic mite that lives in the tracheae (breathing pores) of Arctic bumblebees, whereas other species specialize on marine gastropods. The three lineages of Acari (the Acariformia, Opilioacariformia, and Parasitiformia) have uncertain relationships, and some researchers even doubt they are very closely related (See Lindquist 1984). Some features that many Acari share include:
(1) A narrow anterior (gnathosoma or capitulum) with oral opening and appendages. (2) Larvae have six legs (3) Anterior legs may be used in sensory behavior.
The lineage Acariformes is the most species-rich arachnid group, with over 30,000 currently described species (OConnor 1984). The lineage Parasitiformes includes the ticks and their close relatives, and these are rather more conspicuous. The Acari have a worldwide distribution, living on plants and animals, and inhabiting both freshwater and marine ecosystems. In fact, mites can even be found at deep-sea hydrothermal vents. In addition to being diverse and filling many ecological niches, the Acari are unparalleled among arachnids in their vast reproductive output (Walter and Proctor 1999).
Lindquist, E.E. 1984. Current theories on the evolution of major groups of Acari and on their relationships with other groups of Arachnida, with consequent implications for their classification. In: Acarology VI (Volume 1), D.A. Griffiths and C.E. Bowman (eds), John Wiley & Sons, New York, pp. 28-62
OConnor, B.M. 1984. Phylogenetic relationships among higher taxa in the Acariformes, with particular reference to the Astigmata. pp. 19-27 in D.A. Griffiths and C.E. Bowman. 1984. Acarology VI, Vol. I. Ellis-Horwood Ltd., Chichester.
Krantz, G.W. 1978. A Manual of Acarology. 2nd Edition. Oregon State University, Corvallis.
Proctor, H.C. and I. Owens. 2000. Mites and birds: diversity, parasitism and coevolution. Trends in Ecology and Evolution 15: 358-364
Walter, D.E. and H.C. Proctor. 1999. Mites: Ecology, Evolution and Behaviour. University of New South Wales Press and CABI Publishing, UK
Read more about Acari at the Tree of Life website
"Two features of the Ricinulei combine to make them the most romantic order of the Arachnida, and might even support a claim to be placed among the most absorbing orders of the animal kingdom...."
Theodore Savory 1977 Arachnida, Academic Press, p 219.
The two features that most fascinated Savory about ricinuleids were their reputation for being extremely rare and the fact that they were discovered in fossil form before a living specimen was found. However, ricinuleids also have a number of anatomical and behavioral features that make them quite intriguing:
(1) They are eyeless. (2) The anterior carapace forms a movable hood (a “cucullus”). When the hood is lowered, it covers the mouth and the chelicerae, which terminate in pincers. The female carries her single egg under this hood, displaying a type of maternal care. The young hatch into six-legged "larvae" similar to mites and ticks. (3) The pedipalps are small, and also end in tiny pincers. (4) In males, the legs of the third pair are modified into copulatory organs, used in transferring a spermatophore (see the illustration of the male ricinuleid above, in which the third leg is bent forward to show these organs).
Ricinuleids are a small order of arachnids with about 55 known species worldwide. They have been found only in a few scattered tropical regions, and all extant species are currently placed into three genera, Cryptocellus from Neotropical America (Westwood 1874), Pseudocellus from the Neotropics and Nearctic (Platnick 1980), and Ricinoides from Africa (Ewing 1929). Ricinuleids are heavy-bodied arachnids with a thick cuticle, and range in size from 5 to 10 mm. In fact, they have been described as “the armored tanks of the arachnid world” (Platnick 2002). They feed on arthropods, and have a digestive tract similar to other arachnids. The first ricinuleid was described from a paleontological specimen by W. Buckland in 1837 (apparently he thought it very similar to a weevil, as the abdomen is covered by a hard unsegmented shield), the year before F.E. Guérin-Méneville (1938) described the first extant specimen from Guinea (and erroneously attributed the creature to the arachnid order Opiliones).
Bonaldo, A. B. and R. Pinto-de-Rocha. (2003). On a new species of Cryptocellus from the Brazilian Amazon (Arachnida, Ricinulei). Revista Ibérica de Aracnología, 7: 103-108
Buckland, W. 1837. Geology and Minerology considered with reference to natural theology. Vol. 2. William Pickering, London.
Ewing, H. E. (1929). A syopsis of the American arachnids of the primitive order Ricinulei. Ann. Soc. Ent. Amer. xxii, 583-600.
Guérin-Méneville, F. E. (1838). Note sur l'Acanthodon et sur le Cryptostemme, nouveaux genres d'Arachnides. Revue Zoologique. 1:10-14.
Platnick, N. I. (2002). Ricinulei: In Amazonian arachnids and Myriapoda. Identification keys for all classes, orders, families, some genera, and a list of know terrestrial species. J. Adis (ed.) Pensoft Ed, Sofia-Moscow. P.381-386.
Platnick, N. I. (1980) On the phylogeny of Ricinulei. In Verhandlungen des 8. Internationalen Arachnologen-Kongress,Wien. J. Gruber (ed.) H. Egermann, Wien. Pp. 349-353.
Harvestmen are often mistaken for spiders, given their superficial eight-legged resemblance. However, they differ in a number of ways. Harvestmen (also commonly called daddy-longlegs) lack the silk glands that spiders possess, and therefore cannot spin silk webs. They also lack the venom glands that spiders possess, so even if you were to provoke a harvestman enough for it to bite, it would result in little more than a pinch.
While usually drab in color, opilionids are quite rich in form. Their stout bodies may be variously adorned with spines and contours while, depending on the species, the legs may be short and robust or long and graceful. Like most arachnids, harvestmen are nearly blind. In general, opilionids have a pair of simple eyes set atop a stalk (eye turret) on the top of their prosoma. These eyes, however, are little more than light sensors and are probably unable to focus or form images. Most of an opilionid’s information about its surroundings comes from touch and chemosensory organs. Unique among arachnids, harvestmen have a pair of exocrine glands and can secrete a variety of compounds including quinones, ketones, and phenols (Cokendolpher 1993; Gnaspini 1998). These and other chemicals are used by opilionids for communication, defense, and even as antibacterial agents.
Harvestmen can be found in the dark and damp places of the world. Living among leaf litter and within stony crevices, opilionids are predators of many pest invertebrates such as mites, aphids, and caterpillars (Adams 1984; Allard 2005). Researchers are still discovering surprising things about harvestmen. In addition to their unique chemical secretions and their role in pest control, the sheer diversity and abundance of these animals invites further research.
Adams, J. 1984. The habitat and feeding ecology of woodland harvestmen (Opiliones) in England, Oikos 42: 361-370.
Allard, C. and Yeargan, K. 2005. Effect of diet on development and reproduction of the harvestmen Phalangium opilio (Opiliones: Phalangiidae). Environmental Ecology 34(1): 6-13.
Cokendolpher J. 1993. Pathogens and Parasites of Opiliones (Arthropoda: Arachnida). Journal of Arachnology 21: 120-146.
Gnaspini, P. 1998. Chemical and Behavioral Defenses of a Neotropical Cavernicolous Harvestman: Goniosoma spelaeum (Opiliones, Laniatores, Gonyleptidae). Journal of Arachnology 26: 81-90.
For further reading, see:
Pinto-da-Rocha, R., G. Machado, and G. Giribet. 2007. Harvestmen: The Biology of Opiliones. Harvard University Press, Cambridge, Massachusetts.
(This piece was written largely by Robert Richardson, a graduate student in the Biology Department at PSU.)
These tiny arachnids (less than 3 mm) generally live in soil underneath rocks and stones where humidity is high. Very little is known about their biology. Although little is known about their reproductive behavior, it has been noted that only a few large eggs are laid at a time. Other traits palpigrades exhibit include:
(1) Eyes are absent. (2) Integument is thin and pale. (3) Carapace is divided into two plates between the 3rd and 4th legs ("anterior carapace” and “posterior carapace”). (4) Pedipalps are adapted for locomotion only. (5) Segmented abdomen is broadly connected to the prosoma. (6) A mobile flagellum extends from the end of the abdomen. (7) Some species have three pairs of book lungs; others lack discrete gas exchange organs and probably respire cutaneously.
About 80 species are described, from southern Europe, Africa, southeast Asia, and the Americas. Most species have been described quite recently (see Condé 1996). It has been suggested that palpigrades are not completely terrestrial because the species Leptokoenenia scurra from littoral (seashore) regions is able to swim in seawater (Monniot 1966). However, this conclusion is debatable, as palpigrades have sensory trichobothria (hair-like structures) that function only in air (Walter and Procter 1999).
Condé, B. 1996. Les Palpigrades, 1885-1995: Acquisitions et lacunes. Rev. Suisse Zool., hors sér. 1: 87-106.
Hansen, H. J., and W. Sorensen . 1897 . The order Palpigradi Thorell (Koenenia mirabilis Grassi) and its relationships to other Arachnida . Entomol. Tidskr. 18: 223-240 .
Monniot, F. 1966. Un palpigrade interstitiel : Leptokoenenia scurra, n. sp. Rev. Ecol. Biol. Sol. 3:41-64.
Pseudoscorpiones (pseudoscorpions)
These small arachnids (typically less than 5 mm in length), otherwise known as “false scorpions,” are superficially similar to the larger true scorpion, but lack the elongate metasoma (tail) and telson (stinger). They are generally quite drab in color, ranging from reddish brown to black. Many are dorsoventrally compressed (flattened), which enables them to hide in small crevices, holding their long grasping pedipalps outward awaiting the arrival of their arthropod prey. Some of their features include:
(1) Chelate pedipalps, with modified tibia and tarsus. (2) Silk-spinning apparatus in a cheliceral finger. (3) Medial eyes absent. (4) A cheliceral flagellum. (5) Dorsal cephalothorax covered by the carapace, with little or no evidence of segmentation. (6) Eggs laid in brood sac attached to female operculum.
There are just over 3,000 described species of Pseudoscorpiones, and they inhabit most regions of the world (Harvey 1990, 1992). Pseudoscorpions show highly developed parental care, in which the female "nurses" the offspring. Pseudoscorpions also have silk glands, located on the chelicerae. These distinctive arachnids use their silk to create over-wintering nests, and males also use it for sperm transfer. In many species, the courtship routine is highly complex. Males attach a stalked spermatophore (sperm sac) to the substrate (Weygoldt 1969), which the female takes up as she passes over. A typical female produces 20-40 eggs, and carries them on her underside until the young emerge. She then cares for them prior to their dispersal.
A few species (like the neotropical Cordylochernes scorpiodes) have been found to display an interesting ‘hitchhiking’ behavior, grasping onto other larger arthropods (like harlequin beetles) and catching a ride for free. It has been suggested that this behavior, called phoresy, may enable individuals to move easily into new areas of habitat (Zeh and Zeh 1992). Another species (Paratemoides elongatus) displays some degree of sociality, showing cooperative predation and together constructing communal molting nests.
Chamberlin, J.C. 1931. The arachnid order Chelonethida. Stanford University Publication of Biological Science 7: 1-284.
Harvey M.S. 1990. Catalogue of the Pseudoscorpionida (Mahnert V, Ed.). Manchester University Press.
Harvey, M.S. 1992. The phylogeny and classification of the Pseudoscorpionida (Chelicerate: Arachnida). Invertebrate Taxonomy 6: 1373-1435.
Weygoldt, P. 1969. The Biology of Pseudoscorpions. Harvard University Press, Cambridge, Massachusetts.
Zeh, D.W., and J.A. Zeh. 1992. Dispersal-generated sexual selection in a beetle-riding pseudoscorpion. Behavioral Ecology and Sociobiology 30: 135-142.
Zeh, J.A. Zeh, D.W. 1990. Cooperative foraging for large prey by Paratemnus elongatus (Pseudoscorpionida, Atemnidae). Journal of Arachnology. 18 3:307-311.
Solifugae (sun-spiders, camel-spiders, or wind scorpions)
“it may be true to say that Solifugae have the most formidable pair of jaws in the animal world”
(Savory 1964)
Solifugids (also called solpugids) are fast-moving arachnids with a fearsome appearance resulting from their enlarged chelicerae (jaws), which may be as large as the rest of the animal’s prosoma. Most species are opportunistic generalist predators, although interestingly, a few species specialize on termites. The name of the order ‘Solifugae’ is derived from Latin — “those that flee from the sun” — and most species are nocturnal. The smallest species measure only a few millimeters in length, while the largest are about 10 cm (4 inches). Some species are fast runners, attaining speeds up to 10 mph. Many species construct somewhat permanent burrows, remaining inside for much of the year, dependent on rainfall patterns (Punzo 1998). Characters include:
(1) Enlarged articulated chelicerae, often bearing teeth-like serrations. (2) A rostrum, or beak-like mouth. (3) Long pedipalps, used as sensory organs. (4) A reduced first pair of legs. (5) Mallet-shaped sense organs called “malleoli” on the basal segments of leg 4, more pronounced in males. (6) In males, heavily modified bristles (flagella) on the fixed chelicerae-finger (except in members of the family Eremobatidae).
There are just over 1,000 described species of solpugids, which are found mostly in dry, warm tropical and sub-tropical regions worldwide (except Australia). The only European species are found in southeast Spain, while American species are mostly confined to portions of the west coast of the United States, and the north and west coasts of South America.
Brownell, P.H., & Farley, R.D. 1974. The organization of the malleolar sensory system in the solpugid Chanbria sp. Tissue and Cell 6(3): 471–485
Punzo, F. 1998. The Biology of Camel-Spiders (Aachnida, Solifugae). Kluwer Academic Publishers: Boston.
Savory, T. H. 1964. Arachnida. Academic Press, London
Uropygi or Thelyphonida (whip-scorpions or vinegaroons)
The common name “vinegaroon” comes from the vinegar-smelling acetic acid these animals secrete from glands when they are disturbed. One species lives in the southern United States, and happens to be quite large (up to 85 mm!), truly living up to its name, Mastigoproctus giganteus. General features of uropygids include:
(1) One pair of eyes at the prosoma, and three on each side. (2) A long whip-like flagellum on the pygidium, a small plate made up of the last three segments. The function of this structure is unclear, but it is possible that it may be used to detect air movement, and/or may have a chemosensory function. (3) Only six of the eight legs are used for walking, as the first legs are elongated and held out horizontally as the animal walks. These first legs function as sensory organs (this similarity is shared by Schizomida, Amblypygi, and Solifugae). (4) Most species have robust pedipalps, which are raptorial and move on a horizontal (lateral) plane.
Over 100 species, both tropical and subtropical, occur in two separate American regions (southern North America and northeastern South America), as well as West Africa and eastern Asia. Males secrete a sperm sac, which is transferred to the female. The burrowing female ceases eating and lays multiple eggs within a mucous membrane. In the initial stages, the young attach themselves to the mother’s back. In Mastigoproctus, the young disperse after their first molt, and the mother dies soon after.
Dunlop, J. and C. A. Horrocks. 1996. A new upper Carboniferous whip scorpion (Arachnida: Uropygi: Thelyphonida) with a revision of the British Carboniferous Uropygi. Zoologischer Anzeiger 234:293-306.
Haupt, J. and D. Song. 1996. Revision of East Asian whip scorpions (Arachnida Uropygi Thelyphonida) I. China and Japan. Arthropoda Selecta. 5:43-52.
Rowland, J. M. and J. A. L. Cooke. 1973. Systematics of the arachnid order Uropygida (=Thelyphonida). Journal of Arachnology 1:55-71.
These small animals (less than 5mm) inhabit leaf litter and soil in the tropics. They are not very well studied, but most systematic studies place Schizomida as sister group to Uropygi. Some of their features:
(1) Eyeless (2) Pedipalps are well developed and raptorial, but unlike Uropygi move on a vertical plane. (2) Prosoma divided into two plates. (3) The first legs are modified as sensory organs.
Araneae (spiders)
Under Construction!
Learn more about spider relationships at the Spider Assembling the Tree of Life website
Amblypygi (whip spiders)
Under Construction!
Scorpiones (scorpions)
Under Construction!
PSU Biology Masta Homepage Masta Research Arachnid Mitogenomics Arachnid Primers