by Jason A. Dunlop*1
Arachnida is one of the major arthropod groups. It includes spiders (Araneae), scorpions (Scorpiones), mites (Acari) and harvestmen (Opiliones), as well as a number of rarer and less familiar groups (Fig 1). The name Arachnida was introduced by the French zoologist Jean-Baptise Lamarck and is derived from Greek mythology: in one story, the maiden Arachne challenged the goddess Athene to a weaving contest, and was subsequently transformed into a spider — condemned to weave for evermore. There are about 100,000 living species of arachnids, with mites and spiders representing the most diverse and species-rich groups. Fossil arachnids are considerably rarer, with more than 1,700 described species (well over half of which are spiders) and a record that extends back to the Silurian Period. These oldest arachnids were probably among the first animals to move from water onto land.
Arachnids are instantly recognizable by their eight legs, although it should be mentioned that both mites and the rare order Ricinulei hatch as six-legged larvae, and acquire the last pair of legs later in development. The eight legs of arachnids are in fact part of a wider body plan in which the front half of the body — the prosoma or cephalothorax — bears six pairs of limbs. These are the chelicerae (or mouthparts), the pedipalps and four pairs of walking legs. This division of the body into a prosoma at the front and an opisthosoma (or abdomen) at the back is another textbook character of arachnids (Fig 2). It is very obvious in animals such as spiders, but in groups such as harvestmen the two halves of the body may be broadly joined together. Mites are also problematic, in that their body plan does not always conform to a neat prosoma/opisthosoma division.
The arachnid prosoma is covered from above by a plate that is technically called the prosomal dorsal shield, but is widely known as the carapace. It is not precisely the same thing as a carapace in crustaceans, hence the technical name to avoid confusion. The arachnid carapace is usually a single plate, although in some groups it is divided by furrows or even split into a number of separate parts. The carapace carries the eyes, and in arachnids there may be a pair of median (central) eyes and/or a pair of lateral (side) eyes made up of multiple lenses. Lateral eyes are probably the remnants of multi-faceted compound eyes, like those seen in horseshoe crabs or crustaceans. Different arachnid groups show different combinations of median and lateral eyes. In some spiders, for example, the median and lateral eyes converge together onto a single eye protuberance, or tubercle, at the front of the carapace.
The chelicerae form the mouthparts, and are built up from either two or three segments. In groups such as scorpions and harvestmen they are still shaped like little claws or pincers — probably the primitive condition (the first form to evolve) — whereas in spiders they are shaped more like a pocketknife, with a fang that can be opened out to stab prey and deliver venom. The mouth is located between the chelicerae, and usually has at least two simple lips. The next pair of limbs is the pedipalps. In their basic form these resemble small legs, but in many ways pedipalps are utility appendages. They have been modified in various arachnids into claws or pincers for prey capture (in scorpions, for example), or in the case of male spiders into sperm-transfer devices used in mating. Like the chelicerae and pedipalps, the legs attach to the prosoma and may surround a sternum, although in some arachnids the sternum is reduced or absent. In general, the segments of the legs are called (starting closest to the body): coxa, trochanter, femur, patella, tibia, metatarsus and tarsus. Individual groups may show slight deviations from this pattern, such as extra divisions of some leg segments. In most cases the legs end in one or two curving claws. In whip spiders (Amblypygi, Fig 3) and whip scorpions (Thelyphonida, Schizomida) the first pair of legs is long and thin; these limbs are used as sensory devices similar to antennae, rather than for walking.
The arachnid body is essentially divided along functional lines. The prosoma has the limbs, eyes and feeding apparatus, as well as much of the ‘brain’, whereas the opisthosoma (at the back) contains the heart and in most cases the openings of the respiratory organs, as well as the digestive regions of the gut and the reproductive organs. In most arachnids the opisthosoma is segmented, with plates called tergites across the top and sternites across the bottom, although the majority of spiders and many mites no longer show external segmentation. The reproductive organs always open on the second segment of the opisthosoma. The respiratory organs consist of ‘book lungs’ — in which the many sheets of cuticle in the lung resemble the pages of a book — and/or of tube-like trachea. Such tracheae probably evolved numerous times in different arachnid groups as a more efficient way of breathing on land. In some arachnids the opisthosoma ends in a tail, or telson; the most notorious example is the sting of the scorpions. In general, the arachnid opisthosoma does not show appendages, although remnants of the limbs may still be present, associated with the genital opening or as the spinnerets of spiders. The plates covering the lungs in those arachnids with book lungs may well be highly modified limbs that originally carried the gills in their aquatic ancestors.
Arachnids are clearly part of the Chelicerata group, sharing the diagnostic character of chelicerae: the claw- (or fang-) shaped mouthparts. Relationships among the arachnids are rather less certain, and a number of different proposals can be found in the literature. One of the main sticking points is the position of the scorpions. Palaeontologists in particular have drawn attention to similarities between scorpions and eurypterids (the aptly named sea scorpions). If scorpions were mostly closely related to sea scorpions, then Arachnida in its traditional sense would no longer be a natural group. This idea has been criticized, with other specialists pointing out features shared by arachnids, but absent from the sea scorpions. An example would be the book lungs, which are found in both spiders and scorpions, but not in eurypterids and horseshoe crabs, both of which have gills instead. If book lungs evolved only once, it suggests that arachnids had a single common ancestor, and indeed that this ancestor probably lived on land. This fundamental question of whether arachnids had a single, terrestrial ancestor — or whether different groups moved separately onto land — is intimately tied to the overall question of their relationships.
The most comprehensive study so far (Fig 4) recognized four main groups of arachnids. Stethostomata includes scorpions and harvestmen and is largely defined by similarities of the mouthparts, including a small chamber in front of the mouth. However, it should be noted that this feature is not seen in the oldest fossil scorpions. Haplocnemata groups the pseudoscorpions (Pseudoscorpiones) with the mostly tropical but desert-living camel spiders (Solifugae), on the basis of similarities between their mouthparts and the construction of their legs: unlike other arachnids, their femur is short but their patella is long. Acaromorpha comprises mites and the rare ricinuleids, on the basis that the larva hatchings of both have only six legs. There may also be similarities in the mouthparts. Note that there is some debate about whether mites themselves are a natural group with a single common ancestor. The final group is probably the least controversial. Pantetrapulmonata is defined as all those arachnids with two pairs of lungs (scorpions have four pairs) and some similarities in the shape of the mouthparts. It includes the spiders, whip spiders and whip scorpions, as well as the extinct orders Trigonotarbida and Haptopoda. Note that in most of the more advanced spiders, two of the lungs have been replaced by tracheae.
Alternative ideas in the current literature are challenging the Acaromorpha hypothesis, suggesting that ricinuleids may be most closely related to the extinct trigonotarbids, and thus could also belong in the Pantetrapulmonata group. There is also emerging evidence that camel spiders could be closest to one of the two main mite groups, specifically those known as acariform mites.
The majority of the arachnids are terrestrial. Exceptions are some mites — the so-called water mites (Hydrachnida) in particular — and a handful of spiders, such as the water spider, which in fact breathes air. Arachnids are found across a wide range of environments and habitats, from Arctic regions through to deserts. Mites and spiders show the widest distributions, whereas the rarer orders tend to be restricted to tropical or subtropical zones. Arachnids can occur from deep soil (some mites) up through all levels of vegetation to the tree tops. Most tend to be found on or near the surface of the soil and may construct burrows as a retreat or, in the case of spiders, may build webs in low to moderately high vegetation.
The majority of the arachnids are predators, and it is reckoned that they have a vital ecological role in regulating the number of insects, which make up the bulk of their prey. The largest arachnids, such as tarantula spiders, some scorpions or camel spiders, are capable of catching and eating small vertebrates. Prey is usually caught using the chelicerae and/or the pedipalps and forelegs. Many spiders also rely on silken traps. In the case of spiders, scorpions and some pseudoscorpions, the victim may be further subdued through the use of venom. Venom glands evolved separately in all three groups, as shown by the fact that the glands occur in different parts of the body.
Many arachnids are liquid feeders, and practise ‘preoral digestion’ (digestion in front of the mouth). Here, enzymes are regurgitated onto the prey — or in some spiders, into the bite wound — and the liquefied tissues are subsequently sucked back in. In fact, many arachnids have complex filter systems in and around their mouthparts to prevent solid particles of food entering the body. Harvestmen and some mites can eat solid food, and this may well be the primitive condition for arachnids. Harvestmen and mites also have rather broad tastes, and are not restricted to animal prey. Harvestmen can eat a range of foods, from rotting fruit to bird droppings, whereas mites (as a group) express an extraordinary range of feeding ecologies.
Some mites are simple free-living predators, but others show increasing trends towards parasitism, cumulating in the bloodsucking ticks (Ixodida). Some mites can eat spores, but others have become entirely vegetarian and in many cases represent serious crop pests (gall mites, spider mites, and so on). Another major group, the moss mites (Oribatida), specializes in feeding on decaying matter or the fungi that arise as part of the decay process, and thus plays an important part in the decomposer community in soil and leaf litter.
The oldest arachnids of which we can be certain are found in the Silurian Period. There is a report of a mite from the earlier Ordovician Period, but this is controversial, because it is thought to belong to a highly evolved group. There is nothing in the fossil record that we could confidently call a ‘proto-arachnid’; the oldest arachnids are scorpions, found in a recognizable form from the mid-Silurian onwards (Fig 5). There is some controversy about whether scorpions were terrestrial or aquatic at this time. In the late Silurian we see the first example of the trigonotarbid arachnids: a group that almost certainly did live on land.
The fossil record is patchy, but the first clear examples of mites and harvestmen appear in the early Devonian Period, as does an enigmatic extinct group called Phalangiotarbida. In the mid-Devonian we have the first pseudoscorpions, as well as another extinct group called Uraraneida, which is probably very close to the origins of spiders. In the coal measures dating from the late Carboniferous Period, almost all of the arachnid groups are present and we have examples of spiders, whip spiders, whip scorpions, harvestmen (Fig 6) and camel spiders. The only ‘missing’ groups are the schizomids (Schizomida) and palpigrades (Palpigradi), both of which are small, rather soft arachnids that are unlikely to fossilize easily.
After the Carboniferous, the fossil record across the Permian and Triassic periods becomes rather sparse. This is a shame, because it seems that this was an important time in arachnid evolution. A number of groups died out completely, including the trigonotarbids. Many arachnids first appear in their modern forms in the Mesozoic Era: numerous families of spiders can be recognized that are still alive today. Moving from the late Mesozoic across to the Cenozoic Era, the fossil record becomes much better as we begin to pick up records from amber: the single largest source of fossil arachnid species. There is some evidence that many of the more advanced spider families evolved either in the late Mesozoic — perhaps associated with the rise of the flowering plants and an increase in the number of pollinating insects — or even in the early Cenozoic. Thus, a particular area of interest is determining how ‘modern’ the amber faunas are, and how many of these species belong to living families and genera.
Suggestions for further reading:
Beccaloni, J. 2009. Arachnids. London: The Natural History Museum. ISBN 9780565092207
Dunlop, J. A. 2010. Geological history and phylogeny of Chelicerata. Arthropod Struct. Dev. 39, 124–142. doi:10.1016/j.asd.2010.01.003
Pepato, A. R., Rocha, C. E. F. & Dunlop, J. A. 2010. Phylogenetic position of the actinotrichid mites: sensitivity to homology assessment under total evidence. BMC Evol. 10, 235. doi:10.1186/1471-2148-10-235
Shultz, J. W. 2007. A phylogenetic analysis of the arachnid orders based on morphological characters. Zool. J. Linn. Soc. 150, 221–265. doi:10.1111/j.1096-3642.2007.00284
1 Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity at the Humboldt University Berlin, 10115 Berlin, Germany