Everything about Arachnids totally explained
Arachnids are a class (
Arachnida) of
joint-legged invertebrate animals in the subphylum
Chelicerata. The term arachnid is from the
Greek word άράχνη or
arachne, meaning spider, and also referring to the
mythological figure
Arachne.
Arachnids are chiefly terrestrial
arthropods, but are also found in freshwater and, with the exception of the pelagic zone, in all marine environments. They comprise over 100,000 named
species, including
spiders,
scorpions,
harvestmen,
ticks, and
mites.
It is commonly understood that arachnids have four pairs of legs, and that arachnids may be easily distinguished from
insects by this fact (insects have six legs or three pairs). Interestingly, arachnids generally have a total of 6 pairs of appendages — two pairs of which have become adapted for feeding, defense, and sensory perception. The first pair of appendages, the
chelicerae, serve in feeding and defense. The next pair of appendages, the
pedipalps have been adapted for feeding, locomotion, and/or
reproductive functions. In
Solifugae, the palpi are quite leg-like and make Solifugae appear to have ten legs. The
larvae of mites (and
Ricinulei) have only six legs; the fourth pair appears when they
moult into
nymphs. However, there are also adult mites with six, or even four legs.
Arachnids are further distinguished by the fact they've no
antennae and no
wings. Their body is organized into two tagma called the
prosoma, or
cephalothorax, and the
opisthosoma, or
abdomen. The
cephalothorax (prosoma) is derived from the fusion of the
cephalon (head) and the
thorax. The
abdomen (opisthosoma) can be further divided into the preabdomen and postabdomen in many taxa, although in some orders such as the
Acari the abdominal sections are fused.
There are some important modifications that are particularly important for the terrestrial lifestyle of an arachnid, such as internal respiratory surfaces in the form of
trachea, or modification of the
book gill into a
book lung, an internal series of
vascular lamellae used for
gas exchange with the air. Further adaptations are
appendages modified for more efficient
locomotion on land, internal fertilisation, special sensory organs, and
water conservation enhanced by more efficient
excretory structures (
coxal glands and
Malpighian tubules) as well as a waxy layer covering the cuticle.
Arachnids are mostly
carnivorous, feeding on the pre-digested bodies of insects and other small animals. Only in the
harvestmen and among
mites, such as the
house dust mite, is there ingestion of solid food particles and thus exposure to internal parasites, althougth it isn't unusual for spiders to eat their own silk. Several groups are largely
venomous — they secrete venom from specialized
glands to kill prey or enemies. Several mites are
parasites, some of which are carriers of
disease. Arachnids usually lay
eggs, which hatch into immatures that resemble adults, but scorpions bear live young.
As all arthopods, they've an external skeleton, and they also have an internal structure of
cartilage like tissue called the
endosternite, where certain muscle groups are attached. Calcification in the endosternite has been found in some Opiliones
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Arachnids have two kinds of eyes, the lateral and median
ocelli. The lateral ocelli evolved from
compound eyes and may have a
tapetum, which enhances the efficiency of
photon capture. The median ocelli develop from a transverse fold of the
ectoderm. The ancestors of modern arachnids probably had both types, but modern ones often lack one type or the other..
Acarina
Acarina or
Acari are a
taxon of arachnids that contains
mites and
ticks. Its fossil history goes back to the
Devonian era, although there's also a questionable
Ordovician record. The Devonian era was the time frame in which certain species of animals developed legs. In most modern treatments, the Acari is considered a
subclass of
Arachnida and is composed of 2–3 orders or superorders:
Acariformes,
Parasitiformes, and Opilioacariformes. Most acarines are minute to small (for example 0.080–1.00 mm), but the giants of the Acari (some ticks and red velvet mites) may reach lengths of 10–20 mm. It is estimated that over 50,000 species have been described (as of
1999) and that a million or more species are currently living. The study of mites and ticks is called
acarology.
Only the faintest traces of primary segmentation remain in mites, the prosoma and opisthosoma being insensibly fused, and a region of flexible cuticle (the cirumcapitular furrow) separates the chelicerae and pedipalps from the rest of the body. This anterior body region is called the
capitulum or
gnathosoma and is also found in the
Ricinulei. The remainder of the body is called the
idiosoma and is unique to mites. Most adult mites have four pairs of legs, like other
arachnids, but some have fewer. For example,
gall mites like
Phyllocoptes variabilis (superfamily
Eriophyioidea) have a wormlike body with only two pairs of legs; some parasitic mites have only one or three pairs of legs in the adult stage. Larval and prelarval stages have a maximum of three pairs of legs; adult mites with only three pairs of legs may be called 'larviform'.
Acarine
ontogeny consists of an egg, a prelarval stage (often absent), a larval stage (hexapod except in Eriophyoidea, which have only 2 pairs of legs), and a series of nymphal stages. Larvae (and prelarvae) have a maximum of 3 pairs of legs (legs are often reduced to stubs or absent in prelarvae); legs IV are added at the first nymphal stage.
Acarines live in practically every
habitat, and include aquatic (freshwater and sea water) and terrestrial species. They outnumber other
arthropods in the soil
organic matter and
detritus. Many are
parasitic, and they affect both
vertebrates and
invertebrates. Most parasitic forms are external parasites, while the free living forms are generally
predaceous and may even be used to control undesirable arthropods. Others are
detritivores that help to break down forest
litter and dead organic matter such as
skin cells. Others still are
plant feeders and may damage
crops. Damage to crops is perhaps the most costly economic effect of mites, especially by the spider mites and their relatives (Tetranychoidea), earth mites (
Penthaleidae), thread-footed mites (
Tarsonemidae) and the gall and rust mites (Eriophyoidea). Some parasitic forms affect
humans and other
mammals, causing damage by their feeding, and can even be
vectors of diseases such as
scrub typhus and
rickettsial pox. A well-known effect of mites on humans is their role as an
allergen and the stimulation of
asthma in people affected by the repiratory disease. The use of predatory mites (for example
Phytoseiidae) in
pest control and herbivorous mites that attack
weeds are also of importance. An unquantified, but major positive contribution of the Acari is their normal functioning in
ecosystems, especially their roles in the decomposer subsystem
Araneae
Spiders are the most familiar of the arachnids, and the most numerous, if only described species are counted. All spiders produce
silk, a thin, strong
protein strand extruded by the spider from
spinnerets most commonly found on the end of the abdomen. Many species use it to trap insects in
webs, although there are many species that hunt freely. Silk can be used to aid in climbing, form smooth walls for burrows, build egg sacs, wrap prey, and temporarily hold sperm, even
flying, among other applications.
All spiders except those in the families
Uloboridae and
Holarchaeidae, and in the suborder
Mesothelae (together about 350 species) can inject
venom to protect themselves or to kill and liquefy prey. Only about 200 species, however, have
bites that can pose health problems to humans. Many larger species' bites may be painful, but won't produce lasting health concerns.
Spiders are found all over the world, from the tropics to the Arctic, with some extreme species even living underwater in silken domes they supply with air, and on the tops of the highest mountains.
Haptopoda
Haptopoda is an extinct order known exclusively from a few specimens from the Upper
Carboniferous of the
United Kingdom. It is monotypic, i. e., has only one species,
Plesiosiro madeleyi Pocock 1911. Relationships with other arachnids are obscure, but closest relatives may be the
Amblypygi,
Thelyphonida and
Schizomida of the tetrapulmonate clade
Opiliones
Opiliones (better known as "
harvestmen" or ""daddy longlegs"") are arachnids that are harmless to people and are known for their exceptionally long walking legs, compared to their body size.
As of 2005, over 6,300 species of Phalangids have been discovered worldwide. The order Opiliones can be divided in four suborders:
Cyphophthalmi,
Eupnoi,
Dyspnoi and
Laniatores. Well-preserved fossils have been found in the 400-million year old
Rhynie cherts of Scotland, which look surpringly modern, indicating that the basic structure of the harvestmen hasn't changed much since then. Their closest relatives are probably the mites (
Acari).
The difference between harvestmen and spiders is that in harvestmen the two main body sections (the
abdomen with ten segments and
cephalothorax, or
prosoma and
opisthosoma) are nearly joined, so that they appear to be one
oval structure. In more advanced species, the first five abdominal segments are often fused into a dorsal shield called the
scutum, which is normally fused with the
carapace. Sometimes this shield is only present in males. The two most posterior abdominal segments can be reduced or separated in the middle on the surface to form two plates lying next to each other. The second pair of legs are longer than the others and works as antennae. They have a single pair of eyes in the middle of their heads, orientated sideways. They have a pair of prosomatic
scent glands that secrete a peculiar smelling fluid when disturbed. Harvestmen don't have
silk glands and don't possess poison glands, posing absolutely no danger to humans. They breathe through
tracheae. Between the base of the fourth pair of legs and the abdomen a pair of
spiracles are located, one opening on each side. In more active species, spiracles are also found upon the
tibia of the legs. They have a
gonopore on the ventral
cephalothorax, and the
copulation is direct as the male has a
penis (while the female has an
ovipositor).
Typical body length doesn't exceed 7
mm (about ¼
in) even in the largest species. However, leg span is much larger and can exceed 160 mm (over 6 in). Most species live for a year. Many species are
omnivorous, eating primarily small insects and all kinds of plant material and
fungi; some are
scavengers of the decays of any dead animal, bird dung and other
fecal material.
Mating involves direct
copulation, rather than the deposition of a
spermatophore. They are mostly
nocturnal and coloured in hues of brown, although there are a number of
diurnal species that have vivid patterns in yellow, green and black with varied reddish and blackish mottling and reticulation.
Palpigradi
Palpigradi, commonly known as "microwhip scorpions", are tiny cousins of the
uropygid, or
whip scorpion, no more than 3 mm in length. They have a thin, pale, segmented carapace that terminates in a whip-like flagellum, made up of 15 segments. The carapace is divided into two plates between the third and fourth leg set. They have no eyes. Some species have three pairs of
book lungs, while others have no lungs at all. Approximately 80 species of Palpigradi have been described worldwide, all in the family
Eukoeneniidae, which contains four genera.
They are believed to be predators like their larger relatives, feeding on minuscule insects in their habitat. Their mating habits are unknown, except that they lay only a few relatively large eggs at a time. Microwhip scorpions need a damp environment to survive, and they always hide from light, so they're commonly found in the moist earth under buried stones and rocks. They can be found on every continent, except in Arctic and Antarctic regions.
Phalangiotarbida
Phalangiotarbi (
Haase, 1890) is an extinct arachnid order known exclusively from the Upper
Carboniferous of Europe and North America.
The affinities of phalangiotarbids are obscure, with most authors favouring affinities with Opiliones (harvestmen) and/or Acari (mites and ticks). Phalangiotarbida has been recently proposed to be sister group to (Palpigradi+Tetrapulmonata): the taxon Megoperculata sensu Shultz (1990). (Pollitt et al., 2004).
Pseudoscorpions
Pseudoscorpions are small arthropods with a flat, pear-shaped body and pincers that resemble those of
scorpions. They range from 2 to 8
mm (to 1/3
inch) in length . The opisthosoma is made up of twelve segments, each guarded by plate-like
tergites above and
sternites below. The abdomen is short and rounded at the rear, rather than extending into a segmented tail and stinger like true scorpions. The colour of the body can be yellowish-tan to dark-brown, with the paired claws often a contrasting colour. They may have two, four or no eyes. They have two very long
palpal chelae (
pedipalps or pincers) that strongly resemble the pincers found on a scorpion. The pedipalps generally consist of an immobile "hand" and "finger", with a separate movable finger controlled by an
adductor muscle. A
venom gland and duct are usually located in the mobile finger; the poison is used to capture and immobilise the pseudoscorpion's prey. During digestion, pseudoscorpions pour a mildly corrosive fluid over the prey, then ingest the liquefied remains. Pseudoscorpions spin silk from a gland in their jaws to make disk-shaped
cocoons for mating, molting, or waiting out cold weather. Another trait they share with their closest relatives, the
spiders, is breathing through
spiracles. Most spiders have one pair of spiracles, and one of
book lungs, but pseudoscorpions don't have book lungs.
There are more than 2,000 species of pseudoscorpions recorded. They range worldwide, even in temperate to cold regions, but have their most dense and diverse populations in the
tropics and
subtropics. The fossil record of pseudoscorpions dates back over 380 million years, to the
Devonian period, near the time when the first land-animal fossils appear.
During the elaborate
mating dance, the male of some pseudoscorpion species pulls a female over a
spermatophore previously laid upon a surface . In other species, the male also pushes the sperm into the female genitals using the forelegs .The female carries the fertilised eggs in a
brood pouch attached to her
abdomen, and the young ride on the mother for a short time after they hatch .
Unlike the majority of Arachnida species, scorpions are
viviparous. The young are born one by one, and the brood is carried about on its mother's back until the young have undergone at least one
moult. The young generally resemble their parents, requiring between five and seven moults to reach maturity. Scorpions have quite variable lifespans and the lifespan of most species isn't known. The age range appears to be approximately 4–25 years (25 years being the maximum reported life span in the species
H. arizonensis). They are nocturnal and
fossorial, finding shelter during the day in the relative cool of underground holes or undersides of rocks and coming out at night to hunt and feed. Scorpions prefer to live in areas where the temperatures range from 20
°C to 37 °C (68
°F to 99 °F), but may survive in the temperature range of 14 °C to 45 °C (57 °F to 113 °F).
Scorpions have been found in many fossil records, including coal deposits from the
Carboniferous Period and in marine
Silurian deposits. They are thought to have existed in some form since about 425–450 million years ago. They are believed to have an oceanic origin, with gills and a claw like appendage that enabled them to hold onto rocky shores or seaweed.
Solifugae
Solifugae is a group of 900 species of arachnids, commonly known as
camel spiders,
wind scorpions, and
sun spiders. The name derives from
Latin, and means
those that flee from the sun. Most Solifugae live in tropical or semitropical regions where they inhabit warm and arid habitats, but some species have been known to live in grassland or forest habitats. The most distinctive feature of Solifugae is their large
chelicerae. Each of the two chelicerae are composed of two articles forming a powerful pincer; each article bears a variable number of teeth. Males in all families but
Eremobatidae possess a
flagellum on the basal article of the chelicera. Solifugae also have long
pedipalps, which function as sense organs similar to insects'
antennae and give the appearance of the two extra legs. Pedipalps terminate in eversible adhesive organs.
Solifugae are
carnivorous or
omnivorous, with most species feeding on
termites,
darkling beetles, and other small
arthropods; however, solifugae have been videotaped consuming larger prey such as lizards. Prey is located with the pedipalps and killed and cut into pieces by the chelicerae. The prey is then liquefied and the liquid ingested through the pharynx. Reproduction can involve direct or indirect
sperm transfer; when indirect, the male emits a
spermatophore on the ground and then inserts it with his chelicerae in the female's genital pore.
Trigonotarbida
The
Order Trigonotarbida is an extinct group of arachnids whose
fossil record extends from the
Silurian to the Lower
Permian and are known from several localities in
Europe and
North America. They superficially resemble
spiders, to which they were clearly related. It was once thought that trigonotarbids lacked the
silk-producing
spinnerets that have apparently been crucial to the spider's
evolutionary success, though in recent years at least one fossil find seems to show distinct
microtubercles on its hind legs, akin to those used by spiders to direct and manipulate their silk.
These early arachnids seem to have been adapted to stalking prey on the ground. They have been found within the very structure of ground-dwellings plants, possibly where they hid to await their prey. Trigonotarbids are currently the oldest known land arthropods. They lack
silk glands on the
opisthosoma and cheliceral
poison glands, and most likely represented independent offshoots of the Arachnida.
Uropygi
The
Uropygi, commonly known as
whip scorpions, range from 25 to 85 mm in length; the largest species, of the genus
Mastigoproctus, reaches 85 mm. Like the related orders
Schizomida,
Amblypygi, and
Solifugae, the uropygids use only six legs for walking, having modified their first two legs to serve as antennae-like sensory organs. Many species also have very large
scorpion-like pedipalps (pincers). They have one pair of eyes at the front of the
cephalothorax and three on each side of the head. Whip scorpions have no poison glands, but they do have glands near the rear of their abdomen that can spray a combination of
acetic acid and
octanoic acid when they're bothered. Other species spray
formic acid or
chlorine. As of
2006, over 100 species of uropygids have been described worldwide.
Whip scorpions are
carnivorous, nocturnal hunters feeding mostly on insects but sometimes on worms and slugs. The prey is crushed between special teeth on the inside of the trochanters (the second segment of the leg) of the front legs. They are valuable in controlling the population of roaches and crickets.
Males secrete a sperm sac, which is transferred to the female. Up to 35 eggs are laid in a burrow, within a
mucous membrane that preserves moisture. Mothers stay with the eggs and don't eat. The white young that hatch from the eggs climb onto their mother's back and attach themselves there with special suckers. After the first molt they look like miniature whip scorpions, and leave the burrow; the mother dies soon after. The young grow slowly, going through three molts in about three years before reaching adulthood.
Uropygids are found in tropical and
subtropical areas worldwide, usually in underground burrows that they dig with their pedipalps. They may also burrow under logs, rotting wood, rocks, and other natural debris. They enjoy humid, dark places and avoid the light.
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