'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: These obviously are the primary reason for the existence of this website and their anatomical and
behavioural characteristics are described in detail on other pages. However, as a prelude to a description of each of the other arachnid Orders it is
appropriate to summarize the fundamental features of a typical true spider. A spider's body has a prosoma (cephalothorax) and an opisthosoma
(abdomen). The prosoma has 8 eyes (usually) on its 'head' region, four pairs of legs, two chelicerae that carry the fangs, and a pair of leg-like
pedipalps (often just called palps) which on adult males are modified for mating. Unlike the relatively rigid prosoma the abdomen is soft and easily
damaged. On its underside are one or two pairs of book lungs and perhaps a pair of respiratory spiracles, as well as up to three pairs of silk-secreting spinnerets,
and (at least on adult female araneomorph species) an epigynum into which the male injects his sperms. The silk glands are used to create insect-trapping
webs, retreats and egg sacs. Spiders do not have conventional jaws and therefore digest their food, which is almost always insects or other spiders,
extracorporeally and ingest it in liquid form.|
||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 make a form of silk from their mouthparts.
Because ticks and mites are of both medical and agricultural importance they will be discussed in more detail than any of the other arachnid Orders listed
The total number of tick species present in Australia is uncertain but at least one author claims there are about 75 different species in this country. Most if not
all of these are parasitic, feeding on the blood of other animals, usually vertebrates', but only four species deserve special mention on this page:
1. The scrub or paralysis tick, Ixodes holocyclus, which is found in moist bush settings along the east coast of Australia;
2. The so-called bush tick, Haemaphysalis longicornis, common along both the east and west coasts of Australia;
3. The cattle tick, Rhipicephalus microplus, mostly found in Northern Australia;
4. The brown dog tick, Rhipicephalus sanguineus, found in all coastal regions except those of South-west Australia.
The paralysis tick is by far the species of greatest concern in Eastern Australia. It possesses a glycoprotein neurotoxin that inhibits the actions of
acetylcholine at neuromuscular synapses and hence causes flaccid (relaxed) paralysis of the voluntary muscles and especially the breathing muscles. There
can also be other adverse effects, including allergic reactions, and ticks can also carry a Rickettsia that causes a form of typhus and, at least in some
countries, the pathogen for a debilitating disorder called Lyme disease. A
specific tick venom antitoxin has been prepared by the Commonwealth Serum Laboratories for human use so it is now rare for people to die from tick bites.
However, the same is not true for domesticated animals such as family pets because the presence of a tick on the animal's body is often not detected until
the envenomation is severe.
The following diagram summarizes the life cycle of the paralysis tick. This takes approximately a year to complete with the adult stage being most common
in spring and early summer. The environmental conditions play a significant role in determining how fast the cycle can proceed, warm moist weather
favouring surges in tick numbers. Adult females lay approximately 3000 eggs in leaf litter but only a small percentage of these hatch out as
viable six-legged larvae. These are very small but can still climb up low vegetation and then get brushed onto the bodies of passing animals such as bandicoots and
rats. They acquire a blood meal from this first host then drop off and moult to become eight-legged nymphs. These are still very small but are
capable of latching onto a second passing host and even of causing a degree of paralysis in it. After another blood meal the larva again leaves its host
and transforms into a male or female adult. Both sexes try to find a third vertebrate host, the female feeding so voraciously that it becomes grossly
distended. It then drops off and lays its eggs in leaf litter to commence the next cycle. Curiously, the male rarely feeds on this third host. Instead, it
mates with the female and then sometimes parasitizes her.
The above diagram also shows a typical mite life cycle. This is somewhat less complex than that of ticks because mites are not obligate blood feeders
and do not need a blood meal before they can moult. Of the species that have been studied so far the majority have a larva and two nymph stages
between the egg and the adult, though it seems some manage with only one nymph and others have at least three of them. The anatomy of a
'typical' mite is quite similar to that of a spider but one curious difference is that while they do not possess spinnerets at least some species can
secrete silk. The so-called spider mites that are pests of many cultivated plants and fruit trees spin small webs using silk glands that are associated
with the palps rather than the abdomen.
The extent to which mites are present in the Australian environment is largely unknown by the general public, probably because all common mite species
are very small (less than 2 mm) and only a few of them cause us any serious problems. There are many different species in this country and it is
likely that a large percentage of them have not even been formally named and classified as yet. In general, mites will feed on most kinds of organic
matter with the possible exception of relatively 'pure' substances like the cellulose of wood, but each species has its own preferred source(s) of
nutrition. Thus, anyone who sifts through leaf litter from a forest is very likely to find mites among the creatures living there. They can be thought of as part
of nature's system for recycling 'used' organic matter. For the sake of brevity the following paragraphs will be restricted to some Australian mite
species that are of agricultural, veterinary or medical significance.
Many cultivated plants are subject to attack by mites, including fruit trees, grape vines, strawberries, vegetable crops and
ornamental plants. Such mites are usually sap suckers and serious infestations can therefore lead to impairment of growth and hence a reduced yield.
Another potential problem is that mites can act as vectors for transfer of
plant pathogens such as viruses that can severely damage plant growth. It is for this reason a variety of synthetic miticides are now available for
use on fruit trees and some other important crops to control mite infestations. There are even a few predatory mites that are deliberately added to crops to reduce
the numbers of plant-feeding mite species. For example, it is possible to purchase colonies of the predatory mite, Phytoseiulus persimilis, to disperse through
crops that are being attacked by the red spider mite, Tetranychus urticae.
Most of the mite species that are of medical or veterinary importance in Australia are a problem because they feed on skin cells and thereby cause
itch, allergic reactions and sometimes systemic illness. Some grass mite species such as Acomatacarus australiensis inhabit inadequately mown areas of
lawn or footpath and are readily acquired by simply walking through the grass. Their presence is noticed only when the victim develops itch sensations
in the lower legs (or the undersides of small animals like cats and dogs).
Bird mites are often a problem for people who raise poultry, breed pigeons, or have birds as pets. These mites irritate the skin of the birds themselves
but often attack their owners as well. A somewhat similar situation can develop when sparrows or other birds take up residence in the eaves
of houses, the mites being added to the dust that is inevitably present to some extent within the house. Dust mites such as Dermatophagoides pteronyssinus
are present in many Australian houses and are a particularly important problem because they feed on shed human skin scales and then can be
inhaled to cause asthma attacks in people susceptible to this disorder. It is for this reason there are frequent reports in the popular media
about the need to replace pillows regularly because the skin scales that accumulate in them inevitably lead to a build up of dust mite numbers within the
Mites are a cause for concern for both humans and smaller domesticated animals and even for some small animals such as the burrow-dwelling
mygalomorph spiders. There are even some mite species that are adapted to feed on aquatic animals and mites can be extremely irritating in the ears of dogs. However, by far the most important example is the scabies mite, Sarcoptes scabei, which burrows just
under the human skin (mainly of the hands) and there causes an intense rash and itch which is at least partly secondary to an allergic
reaction. The scabies mite is easily transferred from person to person anbd hence it is a common problem for hospital/nursing home staff who acquire the mites from their patients.
||Scorpiones, the true scorpions: These are up to 12 cm (but typically about 6 cm for Australian species) in body length and, like
spiders, have 4 pairs of legs plus a pair of palps.
However, the latter appendages are larger than on any spider and have strong terminal pincers that the scorpion uses to tear off pieces of its prey and place them next to
its mouth so the juices can be sucked out. The prosoma and opisthosoma are fused together and the latter has a number of segments and a long
tail (called a telson) which arches forward over the abdomen and has a venom gland and a sharp stinging barb at its end. There are two large eyes in the middle of the head and up to 5 sets of small lateral eyes but
scorpions are largely nocturnal hunters and do not have very good vision. Instead they depend more on sensory hairs and other proprioceptors as well as
odour detectors called pectines. There apparently are no recorded human deaths from stings by Australian scorpions although people who are stung can expect local
pain and swelling. On the other hand, there are some overseas scorpion species that have potent neurotoxins that act on calcium or potassium ion channels
and are known to be potentially lethal to humans. Fortunately, scorpions are not naturally aggressive towards large creatures like us, their normal prey
being insects and other small arthropods. They lack silk-secreting glands so they do not use webs when catching their prey.
Like spiders scorpions use abdominal book lungs (four pairs) to obtain oxygen but they do not mate in the same way. Instead the male deposits a packet
of sperms on the ground near the female, who then moves over it to become inseminated. Scorpions also differ from spiders in that they produce living
young instead of laying eggs, the young scorpions passing through several moults to reach adulthood and riding on the back of the adult female while
very young. According to the available published reports there are 29-53 Australian scorpion species and these normally live under logs or loose rocks
especially on rocky hillsides, desert species such as Urodacus yashenkoi preferring burrows in the ground. A few species are quite often found in
domestic backyards, the brown scorpion (Urodacus manicatus) and marbled scorpion (Lychas marmoreus) probably being the species most often seen there.
||Pseudoscorpiones, the false scorpions: In many respects these arachnids resemble true scorpions but are considerably
smaller in overall size, most species being only 3-12 mm long. They also have a fused prosoma and opisthosoma, the latter being very obviously
segmented, as well as an enlarged pair of palps with strong pincers that include venom glands. Pseudoscorpions are predators of other small invertebrates
(and, as the following image shows, also larger ones killed by other predators)
which they grasp in their pincers, envenomate, tear open, and digest extracorporeally in a manner similar to that for true spiders. The prosoma has
0-2 pairs of primitive eyes. They resemble all arachnids other than the true spiders in lacking abdominal spinnerets but like the spider mites they can secrete silk from glands in
their chelicerae. This silk is not used for catching prey but for building small retreats and for wrapping packets of sperms. Some male pseudoscorpions
then physically position the female over the sperm packages. After fertilization is complete the female retains the eggs in a brood
sac attached to her genitalia until the first of three juvenile stages hatches out. It is said that some pseudoscorpion species then promote their dispersal by
clinging to the bodies of insects.
Pseudoscorpions 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. Some live in caves; others have even been found within older buildings. There are many Australian species,
the actual number being at least 82 species (some authors claim twice that number exist here) belonging to ten different families, yet few Australians have ever seen a pseudoscorpion. How can this be? The
main reasons are that all pseudoscorpion species are very small and do not bite people or cause any kind of medical problem and they also are
not known to damage cultivated crops or domesticated animals.
||Opiliones, the harvestmen: These typically have bodies less than 10 mm in length but relatively long, slender and hairless legs,
the fourth pair of which are sometimes comparatively robust and bizarre in shape. Some people have applied the trivial name 'daddy-long-legs' to the opilionids but
this is inappropriate because that name is more widely used to refer to true spiders of the Family Pholcidae. The prosoma is so closely fused with
the somewhat larger, visibly segmented opisthosoma that for some species there is no distinct join between the two. The opisthosoma may be oval or
grossly distorted in both shape and
markings, presumably for camouflage purposes. There are no spinnerets or book lungs, opilionids using a tracheal system to obtain oxygen.
The chelicerae are very short and lack venom glands while the palps vary greatly from species to species
and are often quite long and equipped with large spiny processes. Curiously, male opilionids do not use their palps for sperm transfer as true spiders do
and neither do they follow the scorpion and pseudoscorpion practice of depositing small sperm packages for the female to pick up. Instead they have a
long penis that allows direct insemination of the female. Another curious fact is that while the female lays the eggs, sometimes in a
small 'nest', the male tends to do most of the guarding of the eggs until they hatch. Moults lead to 4-8 nymphal instars between the egg and adult stages.
And another unusual characteristic of the Opilionidae is their willingness to live in close colonies of up to 200 individuals.
Opilionids have just one pair of eyes (sometimes reduced in size or entirely absent in the case of some cave-dwelling species) oriented to face
sideways. However, most species are nocturnal foragers and use the second pair of legs as antennae. Many species are omnivores and while some ambush insects
and other small creatures others are happy to feed on dead organisms, fungi, plant debris, and even decomposing vertebrate carcases. Unlike most arachnids
they can ingest small pieces of solid food rather than just predigested, liquified prey. Their defences are also rather unusual. Some species use noxious
quinone-containing secretions while others vibrate strongly or mimic some small lizards in detaching a leg, which continues to move and thus distract a
predator for some time. Recent surveys indicate there are approximately 200 Australian opilionid species. They gained the trivial name of 'harvestman' because they are common
in fields in late summer and autumn but they also occur in moist leaf litter of forests or under logs, rocks or loose bark. A few species have even
managed to adapt to the arid conditions of Central Australia. No opilionid is known to be harmful to humans.
||Thelyphonida, whip scorpions: Also known as the Uropygidae the Thelyphonidae resemble scorpions in having very robust palps that are
superficially claw-like and useful for holding and crushing their prey. They are found in several tropical and subtropical countries but there apparently
are none in Australia. Their overall body length is 25-85 mm. The whip scorpion name is based on the fact that they have a thin, whip-like
telson at the end of their segmented abdomen but this 'tail' does not have stinging terminal structure. The prosoma is a 'conventional' arachnid one with
4 pairs of legs, though the first pair of legs are longer and thinner than the rest and serve as antennae. There are also one pair of prominent eyes
plus three pairs of smaller lateral eyes. The purpose of these eyes is uncertain since thelyphonids are normally nocturnal hunters and prefer humid dark
places. They lack venom glands but have abdominal glands that spray an irritant mixture of caprylic and acetic acids (for this reason they are sometimes
called vinegaroons). They don't secrete silk but use their palps to capture small arthropods. Males seek out females and transfer small packages of sperms. The
females then seal themselves in burrows in the ground and lay up to 40 eggs in a membranous sac. After hatching the juvenile stages tend to attach themselves
to their mother's back and pass through 4-5 moults before reaching adulthood.|
Amblypygi, tailless whip scorpions: The creatures in this arachnid Order are quite similar in general appearance to the thelyphonids
but an obvious difference is the lack of the whip-like telson at the end of the abdomen. The amblypygid body is up to 75 mm long (sometimes much longer) and
comprises distinct prosoma and opisthosoma segments. Once again, the first pair of legs are much longer and thinner than the other legs and function as
antennae. It is common for an amblypygid to move sideways while the 'antenna' legs probe the immediate environment. The palps are flexed and quite robust
with sharp spines on their inside edges, giving the general appearance of folded combs. These facilitate grasping and holding their prey, which they tear
into small pieces for easy ingestion. They lack venom glands and do not spin silk. Once again there is one pair of larger eyes and three pairs of smaller ones but amblypygids are typically
nocturnal predators of insects.
The Amblypygi can be found in tropical and sub-tropical locations of many countries. There are at least 4 Australian species all of which are in Northern Australia.
Most species live in leaf litter or under stones, fallen logs, and loose bark, their relatively flat bodies allowing them to slide into narrow crevices.
Their reproduction is like that of the Thelyphonidae. The male deposits sperm packages, the female picks them up, then carries a sac of fertile eggs under her abdomen
until they hatch. To survive the hatchlings must then ride on the female's back and subsequently pass through several moults before reaching adulthood. As
is true for the opilionids, the Amblypygi will sometimes group together especially in threatening circumstances.
||Solifugae: Commonly referred to as camel spiders, sun spiders or wind scorpions this type of arachnid became well known during
the 1990s because of the image shown below. 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. There may be as many as 1000 species
worldwide but none in Australia. They seem to prefer relatively dry environments but do also occur in open grasslands and even some forest areas. Despite
the 'sun spider' trivial name solifugids are largely nocturnal hunters. They have a prosoma tightly joined to a larger segmented abdomen but no tail is present. There
is normally one central pair of eyes that work efficiently and sometimes lateral eyes that are rudimentary. Four pairs of legs are present but no
spinnerets or book lungs (they use a tracheal system instead).
It is said the first pair of legs are mainly used as sensory organs rather than for locomotion but solifugids have a pair of palps that function
as antennae and also have adhesive properties. An 'average' solifugid species has a body
length of approximatrely 7 cm but some are much bigger and all of them have relatively long legs. This makes them very fast runners, allegedly capable of
moving at up to 16 km/hour and able to outrun most prey species, even including small lizards. They therefore have no need for fangs or spinnerets but they
possess very large, beak-like chelicerae containing teeth that allow them to grasp and tear apart their prey, though they still digest their food outside
the body in the same manner as true spiders do. Lacking venom, solifugids cannot cause serious harm to a human but their bite can still be painful. The
chelicerae are also able to stridulate, producing a 'rattling' noise that presumably serves as a warning signal. The way a solifugid reproduces varies a
bit from species to species, sperms being delivered either by copulation or by transer of packages. After mating the female lays her eggs in a burrow.
There are about ten juvenile stages before newly hatched solifugids reach adulthood.
||Palpigradi, the microwhip scorpions: This Order consists of a small group of minute (less than 3 mm long) arachnids that
live in caves, soil or leaf litter and so are rarely noticed. They are close relatives of the Thelyphonida and Amblypygi but are somewhat more primitive
anatomically. They have a reasonably big prosoma and a larger opisthosoma at the end of which is a long, segmented flagellum (telson) with a ring of
stiff bristles between pairs of segments. They do not have eyes yet instinctively avoid light and obtain their nutrition by sucking the juices of
miniscule insects. Four pairs of conventional legs are present, the first being the largest with a probable sensory role. The palps are leg-like and the
chelicerae are quite robust. There are no venom glands or spinnerets and only primitive equivalents of book lungs (three pairs or none at all). Almost
nothing is known of their mating habits but the females lay only a few relatively large eggs at one time. The number of moults the hatchlings must
pass through before becoming adults is also unknown. Microwhip scorpions are said to be present on all continents with the probable exception of
Antarctica and there are three described Australian examples, all Eukoenenia species.|
Schizomida: These are also called 'cleaved middle' arachnids because the prosoma is covered by a shell that is divided into a larger
and a smaller plate. The abdomen, which is visibly segmented, is larger than the prosoma and terminates in a short flagellum (telson). For this reason
schizomids are also called 'short-tailed whip scorpions'. The overall body length is usually less than 5 mm so this Order of arachnids are able to
burrow into moist earth in caves or under rocks and logs. They therefore have no need for eyes and the first of the four pairs of legs are relatively long and
modified as a sensory organs. The palps are quite large pincer-like structures, no venom glands or silk being used to catch their prey. Robust
chelicerae allow them to suck the juices of insects and other small arthropods. Schizomids are most common in tropical locations including the northern
parts of Australia. A recent Government survey recorded 53 Australian schizomid species.|
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 about 10 mm in body length and have a distinct prosoma joined by an unusual locking arrangement to a
somewhat larger and distinctively patterned abdomen. On the female the genital opening is under the locking area so the abdomen has to be unlocked from
the prosoma before fertilisation can occur. The 'hooded' part of their trivial name refers to a shell-like hood that normally covers the prosoma
and chelicerae. This is raised only when the creature is feeding. Not surprisingly, there are no eyes although a couple of light-sensitive areas
on the prosoma are said to exist. The female carries her fertilised eggs under her hood until they hatch into a six-legged larva, which then moults at least
once to form the eight-legged larval stage that precedes adulthood.
Four pairs of legs are present. The second pair are the longest and seem to have an
antenna-like sensory role while the ends of the third pair of males have a role in sperm transfer. The palps are short and have pincer-like outer ends.
There are no book lungs, oxygen being obtained by a system of tracheal tubules. Ricinulids require a moist environment and therefore live in caves, leaf
litter and under rocks, usually in tropical locations. They obtain their nutrition from other small arthropods but use neither venom nor silk to catch
them. At the present time this arachnid Order has living representatives in Africa and both Central and South America but there are no known Australian
Pycnogonida: These are not included in the list of spider 'cousins' presented at the top of this page but there is some justification for
listing them as primitive arachnids and they are referred to by the trivial name of 'sea spiders'. They belong in the Phylum Arthropoda because they have
appendages that are segmented and the fact that they have 8 legs links them with the other members of the Class Arachnida. However, at the present time
most authors consider them to be an ancient sister group of the arthropods that are living today and therefore prefer to place them in a Class or Order of
their own. Several hundred species have been described, especially from the Mediterranean and Caribbean Seas and from Arctic and especially Antarctic
waters (where some species are almost a metre in leg span). There are a number of described Australian species, the Great Barrier Reef being perhaps
the best place to find them.|
Pycnogonids possess a very small body (sometimes only 1 mm long), consisting of a cephalothorax and a greatly reduced
abdomen, plus four (usually) to six pairs of legs and often other leg-like appendages, some of which 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 are usually 1-2 pairs 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 conventional arachnids are in the body itself. These
creatures are so slender a respiratory system is not needed. They are usually carnivores and their proboscis allows them to suck nutrients from
soft-bodied invertebrates, especially sessile creatures like sponges, corals, and marine worms. They have no need for special respiratory organs such
as book lungs or tracheae or for fangs or spinnerets. For almost all species the sexes are separate and fertilisation is carried out externally. The
males nurture the newly hatched individuals which pass through about four larval stages before becoming adults.
Email Ron Atkinson for more information.
Last updated 19 December 2015.