Arachnids

Arachnid Facts

Classification:

Kingdom: Animalia

Subkingdom: Bilateria

Phylum: Arthropoda

Subphylum: Chelicerata

Class: Arachnida

Geographical location: All continents

Habitat: Mainly terrestrial, with some few freshwater forms found among spiders

Gestational period: Extremely variable depending on group or individual species

Life span: Variable, but some members may live up to twenty-five years

Special anatomy: All have eight legs as adults; pincer-like mouthparts that may be modified into fangs in some groups; most members are free-living, but parasites are found among the Acarina; includes some of the most venomous animals of the world, such as the African fat-tailed scorpion, the American black widow spider, and the Australian red back and funnel web spiders

Arachnids belong to the phylum Arthropoda and, as such, have the basic characteristics of this assemblage, including jointed appendages, an exoskeleton, an open circulatory system, and a ventral nerve cord. The Arachnida are a class within the subphylum Chelicerata—those arthropods with a pair of primitive pincer-like chelicerae mouthparts. Scientists assert that arachnids have an aquatic origin from their chelicerate ancestors, but most present-day forms are terrestrial.

Arachnids all have the following characteristics: a body that is divided primitively into a prosoma and an opisthosoma; a pair of chelicerae used as mouth parts; a pair of pedipalps that often end in pincer-like claws and are modified for prey manipulation and sperm transfer; four pairs of walking legs in the adult (juveniles may have three pairs) originating from the prosoma; absence of antennae; simple eyes in most; coxal glands at base of the legs and malpighian tubules extending between the hemocoels and gut tube used for excretion and osmoregulation; gut tube with diverticula; breathing accomplished by book gills (aquatic forms), book lungs, or tracheal tubes; and a dorsal heart.

Though some debate concerning their classification exists, the following orders comprise the living Arachnida species according to most sources: Scorpiones (scorpions), Uropygi (sometimes called Thelyphonida) (whip-tailed scorpions), Schizomida (schizomids or micro-whipscorpions), Amblypygi (whip spiders), Palpigradi (palpigrades, microwhip scorpions), Araneae (true spiders), Ricinulei (ricinuleids, hooded tickspiders), Pseudoscorpionida (false scorpions), Solifugida (sun spiders), Opiliones (daddy longlegs, harvestmen), and Acariformes (ticks and mites).

Scorpions

Scorpions are among the earliest land animals, likely derived from water scorpions known as Eurypterids. Scorpions were established in the terrestrial environment by the Carboniferous era but are thought to have invaded the land in the Devonian. Scorpions are mainly nocturnal and are common in desert and tropical regions. Northern climates are free of scorpions due to the extreme cold. Their bodies are segmented but are divided into three portions: an anterior prosoma (carapace) and an opisthosoma divided into mesosoma and a metasoma. The prosoma bears the mouth parts, which are pincer-like chelicerae used to chew the prey and allow copious amounts of digestive juice to be poured externally over the prey. The next pair of appendages are the clawlike pedipalps that act to catch and hold prey. Scorpions have four pairs of walking legs, each usually with eight segments. The mesosoma bears the genital pores and a pair of comblike structures, the pectines, which are chemosensors for tracking prey as well as for digging burrows, along with the legs. Scorpions breathe through ventrally placed book lungs and thus avoid traveling in water. The tail segment or telson bears at least one stinging spine. A number of species are highly poisonous, and their sting may be fatal.

Uropygi, Schizomida, Amblypygi, and Palpigradi

Uropygi, whip-tailed scorpions, look like scorpions, having chelicerae mouthparts and large pedipalps for capturing prey. Some species attain lengths of up to eight centimeters. Like scorpions, these forms are mainly nocturnal. Uropygans earn their name by elongating the telson into a whiplike structure, the flagellum. At the end of this organ are a pair of glands that can spay acetic and caprylic acids at would-be predators. They range from arid climates to the tropics and subtropics of South America and the southern United States. Some species have been introduced into Africa.

The Schizomida are small, under a centimeter in size. Like Uropygi, they possess glands that spray acid, although the telson in these forms is short. The first pair of walking legs is sensory in nature. Schizomids are common in Asia, Africa, and the Americas, inhabiting leaf litter and secreting themselves under rocks and fallen trees. They are tropical and subtropical.

Amblypygi resemble whip-tailed scorpions in body form, although they lack the elongated telson. Internally, they resemble spiders but do not have fangs and use their chelicerae for tearing apart prey. The first pair of legs is sensory in nature and may be quite long in some species (up to twenty-five centimeters in length). These whip spiders are found in tropical areas under tree bark, leaf litter, and in caves.

Palpigradi are small arachnids under three millimeters in length organized into two families and six genera. About 109 species have been described since their discovery in 1885. Not only have they undergone a reduction in size, but their exoskeletons are thin and colorless, facilitating the loss of respiratory organs. They have a whiplike flagellum similar to that of the Uropygi. They are nocturnal and hide in trees or under dry leaves during the day. They eat tiny invertebrates they catch using their pedipalps.

The Aranae

Spiders comprise a large arachnid group with at least 35,000 known species. They inhabit all terrestrial environments, and some species have adapted to freshwater and estuarine areas. The body is normally in two parts: a prosoma and an opisthosoma. Spiders are organized into two suborders, Mesothelae and Opisthothelae. Mesothelae spiders are further organized into the groups Liphistiidae and Heptathelidae. Opisthothelae spiders are further organized into the infraorders Mygalomorphae and Araneomorphae.

Spiders have fang-like chelicerae that inject venom and digestive enzymes into the prey, which liquefies the internal organs so the spider can pump out the contents. Although all spiders are venomous, only a few species are dangerous to humans. Among these are the black widow and brown recluse spiders. Black widows, belonging to the family Theridiidae or cobweb spiders, are capable of inflicting a fatal bite, owing to the neurologic aspects of their venom. Males, who are multicolored and small, are also venomous. The brown recluse spiders are fairly common throughout the southern United States. Their venom is mainly hemolytic; that is, it dissolves tissues and may create large lesions as the result of the digestion of tissue. Their bite is rarely fatal. Other lethal spiders occur in Australia (the funnel web spiders) and in South America (the ctenidae hunting spiders). Another major characteristic of spiders is their ability to spin silk through organs called spinnerets. The spinnerets are connected to silk glands. Silk is used for webs, cocoons for eggs and over-wintering, wrapping prey, and draglines.

Spiders have many sense organs, including hairs that sense vibration and touch. Other hairlike structures are hollow and are chemoreceptors. Slit organs like the lyriform organs are slits in the cuticle that lead to sensory neurons. They are considered to be mechanoreceptors. Vision is variable in spiders depending upon the life habits of the spider, ranging from eight eyes arranged in two rows on the prosoma to a complete lack of eyes. The jumping and hunting spiders have the largest eyes capable of forming an image.

Spiders have interesting reproductive behaviors that range from rhythmic web tapping to flashing iridescent pedipalps. A male will pick up sperm deposited in a sperm web with a specialized structure on one of the pedipalps. If he is successful in approaching a female, he will mate with her by inserting the pedipalp tip into her seminal receptacle and beat a hasty retreat. Otherwise, the female may recycle him. The female will lay up to three thousand eggs sometime after the mating.

Opiliones

Harvestmen are found throughout the world. The carapace is broadly joined with the segmented abdomen without the usual constriction seen in spiders. The legs of most are long and spindly, hence their common name of “daddy longlegs.” They are most abundant in the tropics. They have pincer-like but small pedipalps that are used to feed on small invertebrates and insect eggs. They also are scavengers. For defense, they have two repugnatorial glands that give off noxious chemicals to ward off would-be predators. Unlike other arachnids, these male spiders have a penis for sperm transfer, and females have an ovipositor with which they deposit their eggs in the soil.

Acarina

Both the mites and ticks are included in this group, with around 50,000 species. They may not be a monophyletic group but one that has more than one ancestor. These forms are cosmopolitan in distribution and are either free-living or parasitic on both plants and animals. Mites are the most diverse members of this assemblage. They have opted for small size with a trend toward fusing the prosoma and opisthosoma together. Mites are predatory on other arthropods, including other mites. Many others are parasitic on plants and vertebrates. Some have become aquatic and can be found in many freshwater and ocean environments where they may parasitize mollusks, crustaceans, and aquatic insects; some are suspension feeders. Mites cause various ailments, including skin mange and other irritations, feather loss in birds, as well as subcutaneous tumors. Mites can also act as vectors in disease distribution, including wheat and rye mosaic viruses. Other mites may destroy stored grain products and thus have negative economic impacts. The ticks are parasitic, blood-sucking parasites on vertebrates, although one beetle is also parasitized by this taxon. The chelicerae are modified with teeth for anchoring the tick in the skin of the host, which makes it difficult to remove. Ticks are vectors for many diseases, including protozoan, bacterial, fungal, and viral agents. Ticks transfer Rocky Mountain spotted fever as well as Lyme disease.

Ricinulei

These secretive arachnids are small, ranging from five to ten millimeters in length. There are just over ninety species found in Africa, the southern United States, and Brazil. They have pincer-like pedipalps, and their third legs are modified for sperm transfer in males. There is a hood structure in front of the prosoma where brooding of the single egg of one species occurs. The reproductive habits are largely unknown. These are secretive animals living under leaf litter and in caves. They are predatory on smaller arthropods.

Principal Terms

Arthropoda: Animals having jointed legs, an exoskeleton, and a ventral nerve cord

Book Lungs: A system of blood-filled diverticula that are surrounded by air pockets located in a chamber called the atrium

Chelicerae: Pincer-like mouthparts used in macerating food

Pedipalps: Modified walking legs; these may be clawlike, as in scorpions, or have a modified structure, a palpal organ for sperm transfer, as in male spiders

Podites: The parts of the jointed appendages of arachnids

Sensilla: Hairlike structures associated with nerves that act as mechanoreceptors and chemoreceptors

Tracheas: A system of branched tubes that, in some arachnids, deliver oxygen to the blood

Bibliography

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