'Cousins' of the spiders
Creatures that have been placed in the Class Arachnida differ from other arthropods such as crayfish, insects, centipedes and millipedes
in that their bodies are divided into two main segments: the prosoma, which includes the eyes and mouthparts and to which eight legs are
attached; and the opisthosoma or abdomen, which contains the digestive and reproductive systems. Antennae that are so prominent on insects are
not found on arachnids but some of them have adapted other appendages to serve the same purposes as insect antennae.
Spiders are probably the best known of the arachnids but they are not the only members of this Class. This page contains an overview of those
creatures that might be considered as 'cousins' of the true spiders.
Taxonomists have divided the Class Arachnida into the following Orders:
Each of these are described briefly below:
||Araneae, the true spiders: Characteristics of a typical true spider include 8 eyes on the 'head' region of the prosoma,
two chelicerae that carry the fangs, and between them and the first pair of walking legs a pair of leg-like pedipalps (often just
called palps) which on the males are modified for mating. Spiders cannot ingest solid food but use their mouth parts and fangs
to immobilize their prey and predigest it to a liquid form outside the body. Defence is provided by a pair of venom glands in the chelicerae or prosoma. On the underside of the abdomen are one or two pairs of
book lungs then openings for mating and egg laying (if female) and for voiding of waste materials, and at or near the rear end of the abdomen are up to 3 pairs
of spinnerets that extrude silk for web-making or other purposes.|
||Acari, the ticks and mites: These are characterised by the presence of a very small prosoma tightly and broadly joined to a much larger and
usually oval abdomen. The very small 'head' region of the acarine body may have one pair of eyes and the chelicerae and palps are short
appendages that are closely pressed together to form a beak-like structure that is well suited for piercing and sucking. At least on ticks
the fused chelicerae have rows of backwards-pointing barbs that make the tick difficult to dislodge while it is feeding. There are four pairs of conventional
legs but no spinnerets or other abdominal appendages, though some mites are claimed to make a form of silk from their mouthparts.|
are parasitic, generally feeding on the blood of vertebrates. On the other hand, while some mites attack humans and large animals, the
majority feed on smaller creatures or plant materials and mites therefore can be found in a wide range of environments, including
ponds and buildings. In Australia, ticks such as the paralysis tick, Ixodes holocyclus, are significant hazards to humans and many
other vertebrates while mites, though mostly harmless to us, can damage crops and cause skin irritation, sometimes by inducing a secondary allergic reaction. It is
not uncommon for mygalomorph spiders such as the funnel-web and trapdoor species to have ectoparasitic mites on their body surfaces.
||Scorpiones, the true scorpions: These are the size of large spiders and have 4 pairs of legs plus a very large pair of palps with
strong terminal pincers. There are two large eyes in the middle of the head and up to 5 sets of small lateral eyes. The prosoma and
opisthosoma are fused together and the latter has a number of segments and a long
tail (called a telson) which curves forward and has a venom gland and a sharp stinging barb.
Like spiders, scorpions only ingest liquid food and use 4 pairs of abdominal book lungs to obtain oxygen. They produce living young instead
of laying eggs and also lack silk-secreting spinnerets. In Australia most are found under logs or rocks. |
||Pseudoscorpiones, the false scorpions: In many respects these arachnids resemble true scorpions but are somewhat
smaller in overall size, most species being only a few millimetres long. They also have a combined prosoma and opisthosoma, the latter being visibly segmented, as well as an enlarged
pair of palps with strong pincers. Typically, 1-2 pairs of primitive eyes are present. They lack the curved tail of a scorpion and therefore the capacity to sting like a
scorpion. They do not use silken snares or webs to catch their food but they do possess silk glands associated with their
chelicerae and these are used to build small retreats. They are most often found in leaf litter and under rocks and loose bark but are also known to survive in a wide range
of other habitats, even including seashore ones. There are many Australian species.|
||Opiliones, the harvestmen: These typically have quite small bodies but very long, slender legs. A prosoma with
just a single pair of eyes (sometimes reduced in size) is closely attached to a somewhat larger, visibly segmented opisthosoma and there are also short cheliceae and
palps of varying length and diameter. Each palp has a small claw and the chelicerae have pincers rather than fangs with attached venom glands.
Harvestmen do not have book lungs but instead use a network of fine air tubes that are scattered around the body like our blood vessels. Like
the other arachnids harvestmen ingest only liquid food, mostly squeezed from other small animals that are already dead. Their preferred habitat is
long grass or damp vegetation. They are said to have acquired the harvestmen name because they are most common in fields in late summer and autumn. A number of
Australian species have been described but few people ever see them in the wild.
||Solifugae, sun spiders or wind scorpions: Though not found in Australia these becaue well known during the 1990s
because of the image shown here. This was taken by an unknown photographer (probably in Iraq by a US serviceman) and was part of a widely
circulated hoax email that implied this creature is much larger and more dangerous than is actually the case. Solifugids have a large segmented abdomen without
a telson and a smaller prosoma with a single pair of eyes and very strong, toothed, beak-like chelicerae. The palps and 4 pairs of walking
legs are also rather robust and allow these creatures to run fast. The overall body length can be up to
50 mm. Some species live in burrows and most are nocturnal (their name means "flee from the sun").
||Uropygi, whip scorpions: Some authorities now claim the Order Uropygi should now be known as the Order Thelyphonida.
In many respects uropygids/thelyphonids resemble scorpions except that they have a thin, whip-like telson at
the end of their lightly segmented opisthosoma rather than a robust, curved structure with a terminal bulb and sting. Whip scorpions have a
'conventional' arachnid prosoma and 4 pairs of legs, though the first pair of legs are much longer and thinner than the rest and seem to
have a sensory role like the antennae of insects. In addition, they have strong chelicerae and a pair of palps that are very robust and superficially claw-like
and are used to hold and then crush the whip scorpion's prey. They have one pair of prominent eyes plus three pairs of smaller lateral eyes.
They lack venom glands but have irritant-secreting abdominal glands which are used for defence purposes. Uropygids are ground-dwelling creatures, the males
normally wandering on the surface though the females of at least some species live in burrows while laying their eggs. They are found in several
tropical and subtropical countries but there apparently are none in northern Australia. Photos of some members of the Order Uropygi can be
found at the following website:|
Amblypygi, tailless whip scorpions: The creatures in this arachnid Order are quite similar in general appearance to the uropygids
but an obvious difference is the lack of the whip-like telson at the end of the abdomen. Once again, the first pair of legs overlie the second pair
and are much longer and thinner than the three pairs of 'conventional' legs. They are said to serve a sensory role similar to that for a uropygid.
Also notably different are the palps, which extend to the sides of the prosoma and have the appearance of folded combs. Amblypygids can be
found in many of the same high-humidity places as the uropygids although they probably have a greater overall distribution. Most are nocturnal and hide in leaf
litter or under stones and fallen logs, but some are subterranean. There is at least one Australian species and since this is also a creature that
survives best in tropical habitats it is normally found in the northern parts of Australia.
||Palpigradi, the microwhip scorpions: This Order consists of a small group of minute (less than 3 mm long) arachnids that
live in soil and leaf litter and so are rarely noticed. They are close relatives of the Uropygi and Amblypygi but are somewhat more primitive
anatomically. They have a reasonably large prosoma attached by a stalk to a larger, segmented opisthosoma at the end of which is a long,
strongly segmented telson (usually called a flagellum for this group of animals). Four pairs of conventional legs (the first pair of which are believed to have a sensory role), a pair of leg-like palps,
and two quite robust chelicerae are present but these creatures have no eyes and only primitive equivalents of book lungs (or none at all). Their
nutrition is obtained by sucking the juices of miniscule insects. The females are egg layers. Microwhip scorpions are claimed to be present on all
continents with the probable exception of Antarctica. A photo of a palpigradid specimen can be found on the following website:|
Schizomida, the 'cleaved middle' arachnids: These were given their trivial name because the upper surface of the prosoma is divided
into front and rear plates. The abdomen is larger than the prosoma and has a short telson (often called a flagellum). Schizomids are generally
small creatures although there is at least one species that has a body approximately 10 mm long.
As is the case for the members of several other arachnid Orders the three hindmost pairs of legs being used for walking but the first pair
are relatively longer and are modified as a sensory organs. The palps are leglike but with their ends modified for a clasping role. Robust
chelicerae have the usual role of sucking juices from their prey which include insects and other small arthropods.
Schizomids live in moist leaf litter or soil and are most common in tropical locations, including the northern parts of Australia. They are illustrated on the following website page:|
Ricinulei: These are sometimes referred to by the misleading trivial name of 'hooded tickspiders' but they are very different from
both ticks and spiders. They can grow up to 15 mm in body length and have an overall appearance reminiscent of a huntsman spider. The prosoma
and chelicerae are hooded by a carapace that can be raised or lowered. There are no eyes. Four pairs of legs are present, the ends of the
third pair of males having a role in mating. The palps are leg-like and have small pincers on their ends, these also being present on the chelicerae. The abdomen is segmented and larger
than the prosoma, to which it is joined tightly by a narrow pedicel. There are no book lungs, oxygen being obtained by diffusion or perhaps by
a system of tracheal tubules. Ricinulids require a moist environment and therefore live in leaf litter and under rocks, usually in tropical locations.
They obtain their nutrition from other small arthropods. At the present time this arachnod Order has living representatives only in Africa
and both Central and South America. Ricinulid illustrations can be found on the following website:|
Note: In some books and websites the sea spiders or Pycnogonida are also listed as being examples of arachnids. However, present evidence
leads to the conclusion that they do not belong in the Class Arachnida at all but should be placed in an arthropod Class of their own, this
perhaps being a primitive group that gradually evolved into the more advanced arthropods such as the crustacea, insects, and spiders.
Anatomically, pycnogonids possess a very small body, consisting of a cephalothorax and a greatly reduced abdomen, plus four to six pairs
of legs and often other leg-like appendages that are combined into a proboscis. This may consist of primitive chelicerae and palps and a pair of egg-carrying appendages is located next to it.
There is usually one pair of eyes present but some species have no eyes at all. The legs form the largest part of a pycnogonid and contain parts of organs that in
true arachnids are in the body itself. These creatures are so small a respiratory system is not needed. At the front of the body is a
proboscis that allows them to suck nutrients from soft-bodied invertebrates. Most sea spider species are carnivorous predators or scavengers.
For almost all species the sexes are separate and fertilisation is carried out externally. Pycnogonids have been found in the waters around
many countries, including Australia. Most live under rocks or in algal masses in shallow marine and estuarine habitats but some do occur in
deep oceans. An illustration of a pycnogonid can be seen at the following website:|
Email Ron Atkinson for more information.
Last updated 23 June 2012.