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The 11 orders of Arachnids

The class Arachnida contains some of the most fascinating and enigmatic arthropods on earth, many of which are completely unknown to the general public. To remedy this, this article provides information covering each of the 11 different groups or orders of arachnids.

The class Arachnida belongs to the subphylum Chelicerata of the phylum Arthropoda, making them close cousins of insects (class Insecta). Arachnids are a very diverse class of eight-legged arthropods that are characterized by the presence of paired chelicerae at the anterior end of aprosoma. They have managed to diversify into virtually every terrestrial environment, with even some fresh water and marine representatives. They represent predators, phytophages and even obligate parasites of plants and animals.

The eleven orders of arachnids we will be covering thus consist of the Palpigradi (microwhipscorpions), Araneae (spiders), Amblypygi (whipspiders), Thelyphonida (whip scorpions), Schizomida (schizomids), Ricinulei (ricinuleids), Acari (ticks and mites), Opiliones (harvestmen), Scorpiones (scorpions), Pseudoscorpiones (false or bookscorpions) and Solifugae (windscorpions or camel spiders).

  1. Araneae – The true spiders

 True spiders (Araneae) are arguably the most recognizable of all the arachnids, representingdiverse land predators that are broadly categorized into web-building and free-living groups.  They are distinguished from other arachnids by three outstanding characteristics.  Firstly their ingenious exploitation of silk, secondly by the modified sperm transfer organ on the pedipalps used by males, and thirdly the use of venom from prosomal glands, opening on the chelicerae, to subdue prey.These arachnids have evolved into a myriad of forms, but the basic arachnid body plan remains unchanged in all spiders. There are more than 45 000 species recognized worldwide, with only a few of medical significance.

The smallest in the world, the Samoan moss spider (Patumarplesi) is just 0.3mm in length, while the largest leg-span goes to the giant huntsman spider (Heteropoda maxima) from the caves of Laos with a leg-span of 30cm. The title of heaviest spider in the world goes to the Goliath birdeater (Theraphosablondi), a tarantula from northern South America, weighing in at 175g and with a leg-span of around 28cm.

  1. Palpigradi – Microwhipscorpions

 Palpigradi are small (1-3mm), elongate, delicate, pale coloured arachnid predators that mostly inhabit the soil layers in moist areas such as forest floors and caves. Their bodies are divided into two areas, the cephalothorax and the opisthosoma. Their appendages are long and thin, their movement rapid and they possess a flagellum at the posterior end of the opisthosoma that is covered in tactile sensilla. The flagellum can even be actively elevated and bent to the front.Fewer than 100 species have been described. Due to their small size and reclusive nature, little is known about these creatures.

  1. Scorpiones – The true scorpions

 Scorpions are an instantly recognizable group. They are characterized by their large pedipalp claws, the presence of metasoma (tail) ending in a telson (sting), and the presence of special sensory organs, called pectines. They are primarily nocturnal and have poor vision. Prey is located by special sensory hairs on the limbs. This usually doesn’t matter to the average Jane and John Doe on the streets though. To them the important thing is that scorpions look scary and some of them are capable of killing you. But of the more than 1500 species of scorpions worldwide only a handful are capable of delivering venom that can kill a person.

A rule of thumb to follow regarding the potential venomosity of a scorpion would be to look at the size ratio between the scorpion’s pedipalps and its tail. When a scorpion has large, robust pedipalps and a thin tail, its venom is usually mild. If, on the other hand, the scorpion possesses narrow pedipalps and a thick, robust tail, you are better off not getting stung. In South Africa, these scorpions mostly belong to the Buthidae family and can range from matchbox sized Uroplectes species that can be found around your home (the sting of which is comparable to a wasp sting) to the large Parabuthus species of the more arid regions. The latter possessing venom potent enough to pose a serious health risk.

  1. Pseudoscorpiones – False scorpions

 Pre-Devonian in origin, the Pseudoscorpiones are one of the oldest extant lineages and over the past 392 Ma have diverged into more than 3400 known species in 26 families.  Due to their small size (1-5mm) and lack of medical and agricultural importance, the study of the Pseudoscorpiones has been carried out by only a small group of dedicated scientists.  This group has mostly consisted of taxonomists, with these arthropods only beginning to gain the interest of a wider audience of researchers during recent decades.

Their unusual appearance and secretive nature has been recorded as far back as Aristotle, though even Linnaeus failed to clearly define the group, instead grouping the first two described species (Acaruscrancroides Linnaeus, 1758 and A. scorpioides Linnaeus, 1758) together with mites and harvestmen.  They superficially resemble true scorpions, but lack the elongated metasoma (tail) and telson (sting). They do, however, share the six-segmented pedipalps, with the tibia and tarsus modified into a chela with a movable finger.

Many species possess venom which they can inject into prey from small venom teeth at the ends of their pincers. This makes Pseudoscorpiones one of only three arachnid orders that possess venom, the other two being true spiders (Araneae) and true scorpions (Scorpiones). Pseudoscorpiones occur in almost every part of the world, though most species are found within the tropics and subtropics. Their unique morphology makes these secretive generalists a very important predatory component of many terrestrial habitats, where they can readily be found among humid soil, leaf litter, and compostpiles, under stones, bark and logs, as well as in harsh environments such as intertidal zones.

  1. Solifugae – Camel spiders

 Solifugids, also known as camel spiders or wind scorpions, are ferocious predators of mostly arid regions of the world. They possess a massive pair of chelicerae that are used to dismember prey. Many have adapted to function diurnally and can be seen running around during the day in search of prey. This high level of exertion can be maintained thanks to an extensive tracheal system. They possess two medial eyes that are situated on a raised ocular tubercle, though they rely extensively on their long tactile hairs for information about their surroundings and thus appear as very hairy organisms. There are more than 1,100 described species, ranging in size from 20-70mm. Unlike true spiders (Araneae), camel spiders do not possess venom or any silk-producing organs.

  1. Ricinulei – Ricinuleids

 Ricinulei is one of the smallest arachnid orders containing only 56 species of which all belong to the family Ricinoididae. These arachnids inhabit the top layers of soil, and although some do occur in caves, most are found in isolated locations in tropical forests. Ricinuleids range in size from 3-10 mm and possess a suite of unique morphological features, including a pre-carapaceal structure known as the cucullusthat covers the mouthparts, as well as an elaborate means of sperm transfer by a special copulatory organ found on the third pair of legs in males.

In ricinuleids the second pair of walking legs, not the first pair as found in many other arachnid orders, are elongated and used as sensory appendages. Although no eyes are present, ricinuleids possess slender, movable pedipalps with tiny chelae used to grasp prey.  As they move, these arachnids periodically touch the surface they are moving on with their pedipalps which thus serve as short range contact chemosensory appendages that complement the long range sensory capabilities of the elongated second pair of walking legs.

  1. Amblypygi – Whip spiders

 Amblypygi, or whip spiders, closely resemble whip scorpions (Thelyphonida) and are related to both these arachnids and to true spiders (Araneae). Although they are called whip spiders they, in fact, do not possess silk or venom. The pedipalps of whip spiders are subchelate, muscular and covered in spines. They are used primarily as mechanisms for catching prey, but also possess some tactile hairs. In Amblypygi only three of the four walking legs are used for propulsion. The first pair are extensively modified into long antennae-form sensory appendages. Whip spiders are nocturnal animals and as such mechanical and chemical signals are extremely important as they provide the majority of information about the organism’s surroundings. These mechanical and chemical stimuli are, for the most part, received by the myriad of sensory sensilla and pits of the elongated front legs. Found in tropical and arid regions, adults range in body of 20-60mm. There are approximately 175 described species.

  1. Schizomida – Schizomids

 Few studies have been conducted on this order. Thus the biology and ecology of the order Schizomida is largely unknown. From the few studies done, the roughly 300 described species of schizomids (ranging in size from 3-13mm) are considered to be hygrophilous, photophobic inhabitants of soils, particularly in the tropics and subtropics.They show a peculiar type of sexual dimorphism where the male’s flagellum is enlarged into an unsegmented bulbous structure which the female, with a normal segmented flagellum, grasps during mating.

Prey is located through the sensory organs located on the long, slender, first pair of legs which are no longer used for walking but as sensory appendages. When the prey item is located, the pedipalps are used to guide the prey item to the chelicerae. Eye spots are only present in the subfamily Schizomina, and are very poorly developed, capable only of differentiating between light and dark. In schizomids, not only are the first legs adapted, but also the fourth pair. These legs have been modified to serve as jumping legs, possibly as a defensive anti-predation mechanism.

  1. Thelyphonida – Whip scorpions

 Whip scorpions superficially resemble true scorpions and even possess a few features in common with true scorpions, though they lack any venom.  They are, in fact, more closely related to spiders, with which they share certain structures of the mouthparts and a number of booklungs. Like the pedipalps of whip spiders (Amblypygi), the pedipalps of thelyphonids are sub-chelate. They are less spiny and more robust than those of amblypygids and are similarly used for grasping prey. Whip scorpions do not possess a sting, but can raise their flagellum in a pose called aggressive posturing. This is possibly used to make them look more like true scorpions and thus avoid predation. When aggressive posturing fails, they can secrete a variety of irritants from glands at the base of their tails including acetic and octanoic acids. These vinegar-like substances give whip scorpions another popular name, vinegaroons.

As carnivorous and nocturnal hunters, they feed mostly on insects but sometimes on worms and slugs. Much like the whip spiders, whip scorpions have adapted the first pair of legs to function as sensory organs which allow them to respond to chemical and tactile stimuli. The attenuated tail is multisegmental and also functions as a sensory appendage. They possess medial and lateral eyes for light detection, but not so much for prey detection and capture. There are currently over 100 species known worldwide, with adults ranging in size from 25 to 85 mm in length.

  1. Opiliones – Harvestmen

 Opiliones, or harvestmen, are spiderlike arachnids that possess neither venom nor silk and are most commonly found in wooded areas. They are heavily sclerotised carnivores that actively hunt for prey. The second pair of walking legs are used as tactile sensory appendages, together with the first pair, and walking is usually accomplished with just leg pairs three and four. Adult body size ranges from less than 1 to 25 mm. More than 6,500 species are described worldwide.

  1. Acari – Mites and Ticks

 This is the most diverse arachnid order with over 55 000 species known so far. Acariform and parasitiform groups are commonly recognized, the latter including the predatory mites and ticks. The abdominal segments are fused and extremely difficult to distinguish, resulting in the appearance of an unsegmented abdomen. They share a characteristic form of arachnid mouthparts called the gnatosoma, in which the chelicerae and the pedipalps articulate together as a movable unit. These arachnids occupy a huge range of habitats and lifestyles encompassing free-living predators, detritivores, saprophages, phytophages and ectoparasites.

Photos:

Bark Spider and Horned Baboon Spider – ©Jan A. Neethling

Microwhipscorpions – ©Robert Deans

Tree Creeper and Karoo Burrower scorpions – ©Jan A. Neethling

False scorpions – ©Jan A. Neethling

Camel Spiders – ©Willem van Zyl&PeterMichalik

Hooded Tickspiders – ©Marshal Hedin and Fernandez &Giribet

Whip Spiders – ©Jan A. Neethling

Schizomids – ©Marshal Hedin

Vinegaroons – ©Alex Hyde and Justin Overholt

Harvestmen – ©Jan A. Neethling

Tick – ©Gettyimages.com

Velvet Mite – ©Animalsake.com

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