Arachnids are joint-legged arthropods of the class Arachnida, belonging to the subphylum Chelicerata. They include spiders, scorpions, ticks, mites, pseudoscorpions, camel spiders, whip scorpions, and tailless whip scorpions.
The name ‘Arachnida’ is derived from the Greek word ‘Arachne,’ referring to the myth of the egotistic human weaver Arachne, who turned into a spider.
Unlike insects (subphylum: Insecta), almost all adult arachnids possess eight true appendages attached to their cephalothorax, which characteristically have six legs. While most arachnids, like spiders, scorpions, and harvestmen, are strictly terrestrial, sea spiders and water mites are fully aquatic.
About 1,50,000 arachnid species are found worldwide (of which about 51,000 are spiders) and are grouped into 16 extant orders.
Arachnids exhibit a wide range of sizes, from minuscule mites measuring 0.08 mm (0.003 inches) to relatively large species like the scorpion Hadogenes troglodytes, which may be up to 21 cm long.
The largest arachnid, Heterometrus swammerdami, discovered during World War II in Krishnarajapuram, India, measured a remarkable 29.2 cm (11.49 inches) in body length, extending from the tips of its pedipalps to the end of its sting.
Like all arthropods, arachnids have segmented bodies with jointed appendages covered by a thick, chitinous exoskeleton. Except for harvestmen, mites, and ticks, most arachnids have a body divided into two distinct regions: the cephalothorax (or prosoma) and the abdomen (or opisthosoma). The cephalothorax is dorsally covered with a hard carapace, typically bearing six pairs of appendages, including the chelicerae, pedipalps, and the walking legs.
Chelicerae are the first pair of appendages (though not true appendages) in front of the mouth. They serve various functions depending on the species, including feeding and defense. Among spiders, the basal segment of the chelicerae typically houses venom sacs, while the second segment, known as the fang, is responsible for injecting venom into prey and potential threats.
The second pair of appendages are the pedipalps, which mostly function as a sensory organ in arachnids. In scorpions, pseudoscorpions, and ricinuleids, the pedipalps terminate in a pair of pincers, whereas in whip scorpions and most harvestmen, these appendages are raptorial, adapted for seizing and immobilizing prey. In contrast, in Solifugae, the pedipalps are strikingly leg-like, giving these creatures the appearance of having ten legs.
The remaining four pairs of appendages in arachnids typically function as walking legs, aiding in locomotion. However, among tailless whip scorpions, the first pair of these appendages serve primarily as tactile organs, assisting in sensing the environment rather than for walking. In the spider-like ricinuleids, specialized copulatory organs are situated on the third pair of legs. Additionally, some mites, like those in the family Eriophyoidea (especially immature individuals), may possess only two or three pairs of legs.
This segment in arachnids shows segmentation, particularly in less evolved forms, with varying levels of fusion observed across different groups. Typically, it is divided into preabdomen and postabdomen (pygidium), a distinction most pronounced in scorpions. However, in certain orders, such as the Acari, which includes mites and ticks, the abdominal sections are entirely fused, lacking clear segmentation. The abdomen usually lacks appendages, except for a pair of pectins in scorpions and the spinnerets (appendage-like silk-weaving organs) found in spiders.
Among all arachnids, the final segment of the body (telson) is only found in scorpions and short-tailed whip scorpions.
Arachnids have an open circulatory system where hemolymph circulates in tissue sinuses rather than within blood vessels, as found in vertebrates. Specialized venous channels conduct hemolymph from the tissues to the heart, from where it is pumped through a network of blood vessels back to the tissue sinuses.
The heart is typically located in the anterior part of the abdomen and may or may not be segmented depending on the species. While some arachnids, like certain mites, lack a heart altogether, others, like a few spiders and scorpions, possess segmented hearts. Hemolymph serves as the medium for oxygen transport, with the respiratory pigment usually being ‘hemocyanin,’ a copper-based pigment dissolved in the hemolymph. Cells in the hemolymph do not carry oxygen but perform other functions, such as immune defense or nutrient transport.
These arthropods perform respiration through two main respiratory organs: the book lungs and the trachea.
Book Lungs
They are specialized respiratory organs found in scorpions and whip scorpions, typically on the underside of the abdomen. Diffusion of gases takes place between the hemolymph circulating within thin leaflike structures called lamellae (arranged like pages in a book within these pockets) and the air in the spaces between these structures.
Trachea
It is a network of tubes found in pseudoscorpions, mites, ticks, and camel spiders that open to the exterior through paired respiratory pores known as spiracles. In this system, the gases diffuse within small fluid-filled tubes that branch across the internal organs.
The nervous system of arachnids shares similarities with that of other arthropods, comprising a brain and a chain of paired ganglia or nerve bundles. However, it has undergone significant modification through ganglionic fusion and migration towards the head region. The brain, located above the oesophagus, is a large ganglion giving rise to nerves that innervate the eyes and the first pair of appendages. It is connected to another ganglion positioned below the esophagus, and the nerves originating from the lower ganglion extend to the pedipalps. Additionally, an unpaired nerve runs along the esophagus and stomach, connecting to the brain via paired nerves. This nervous system organization facilitates the coordination of sensory input, motor function, and information integration in arachnids, contributing to their survival and adaptation to various environments.
They have two types of eyes: lateral and median ocelli.
While ancestors likely had both types of eyes, many modern arachnids lack one or the other. The cornea is continuous with the body cuticle and acts as a lens, followed by a transparent vitreous body and the retina. The retina usually lacks enough light-sensitive cells to form a clear image and relies on the tapetum for clear vision.
In addition to eyes, most arachnids possess two other types of sensory organs: trichobothria and slit sense organs.
Arachnids have an elaborate mechanism for digesting their food, beginning with external digestion in the pre-oral cavity and culminating in nutrient absorption through the stomach.
Steps of Digestion
Arachnids possess two main types of excretory organs: coxal glands and Malpighian tubules.
Arachnids are guanotelic, with their primary waste product being the amino acid guanine, which is excreted along with their other metabolic wastes. While some arachnids may have coxal glands and Malpighian tubules, others may have only one.
Although phylogenetic relationships among the diverse groups of arthropods have been intensely debated, it is now widely accepted that all living arthropods fall into a single, monophyletic group, which consists of chelicerates (spiders, scorpions and their kin), pancrustaceans (group that combines crustaceans with insects and their close relatives), and myriapods (centipedes, millipedes, and their allies). Chelicerates have been further subdivided into three classes – Arachnida, Merostomata, and Pycnogonida.
Understanding the evolutionary relationships within Class Arachnida has been excruciatingly difficult, with molecular studies often providing conflicting results regarding their phylogenetic interrelations.
A notable study in 2014, utilizing a comprehensive molecular dataset, highlighted systematic conflicts in phylogenetic information. Scientists observed discrepancies in phylogenetic trees, particularly in orders with higher evolutionary rates, like Acariformes, Parasitiformes, and Pseudoscorpiones. However, they favored relationships depicted by genes that evolved at slower rates, supporting the monophyly of major groups like Chelicerata, Euchelicerata, and Arachnida, along with certain clades within arachnids.
The study also emphasized the significance of analyzing genes with varying evolutionary rates and found strong evidence suggesting the existence of the clade Tetrapulmonata, comprising Araneae, Amblypygi, and Uropygi. It also identified a clade comprising Opiliones, Ricinulei, and Solifugae, which was a surprise based on previous studies.
However, arachnid phylogenetics continued to evolve, with a molecular phylogenetic analysis in early 2019 introducing new insights. This analysis considered horseshoe crabs (Xiphosura) as a sister group to Ricinuleids and grouped pseudoscorpions along with ticks and mites. Additionally, including Scorpiones in a clade called Arachnopulmonata received strong support, and pseudoscorpions were suggested to belong to this clade based on shared ancient whole-genome duplication events.
In the present classification of arachnids, groups are typically organized into orders. Traditionally, mites and ticks were grouped under a single order called Acari. However, recent molecular phylogenetic research examining species’ DNA and genetic relationships suggests that mites and ticks do not share a close evolutionary relationship. As a result of these findings, mites and ticks are now classified into two distinct groups: Acariformes, which includes mites, and Parasitiformes, encompassing ticks.
According to the current classification, the arachnids are divided into 2 superorders and 16 orders.
Spiders, harvestmen, false scorpions, mites, and ticks are found in various habitats spreading across continents. Scorpions, camel spiders, tailless whip scorpions, and micro whip scorpions are commonly available in subtropical areas and occasionally in temperate climates. In contrast, sunspiders, schizomids, and ricinuleids predominantly inhabit tropical regions.
They range from terrestrial and underground habitats to marine and freshwater bodies. Some parasitic arachnids like ticks and mites seek refuge in the bodies of their hosts.
In temperate regions, most arachnids have a lifespan that lasts only a single season, whereas, in tropical areas, arachnids like whip scorpions, tailless whip scorpions, camel spiders, and certain species of tarantulas often live for more than a year.
While larger arachnids generally have longer lifespans, some smaller arachnids, particularly those with specialized adaptations or unique life histories, may live for relatively long periods.
Most arachnids reproduce through indirect fertilization in which the male does not transfer spermatozoa directly to the female but initiates various courtship rituals to induce the female to accept his sperm capsule (spermatophore). The stored sperm are released from the spermatheca in females and are used to fertilize the eggs as they are laid.
Camel spiders often engage in more active mating behaviors, usually occurring at dusk or during the night. The male initiates mating by seizing the female, massaging her undersurface, opening her genital orifice, and forcefully transferring sperm into her spermatheca. In contrast, male harvestmen and some mites have a penis, making them capable of direct sperm transfer to the genital opening of the female.
Arachnids exhibit various behaviors regarding the deposition and care of eggs. Some species deposit their eggs in the soil at a protected site but provide no further care, while others, particularly tropical species, guard the eggs during development. Some arachnids, like spiders, false scorpions, and whip scorpions, often construct cocoons for their eggs and attach them to their abdomen.
Scorpions display viviparity, where fertilized eggs develop inside the mother, and the young are born alive. The development process varies depending on the amount of yolk in their mothers’ eggs, with some developing within the oviduct and others remaining in their original location in the mother’s body, having a nutrient supply from the maternal intestine. After birth, the young are carried on their backs by their mothers until they undergo the first molt, while some false scorpions carry their eggs in a ‘brood sac’ attached to their genitalia. The embryos develop within this sac and receive nourishment from the female until they are ready to hatch.
Although details of the early development of different arachnid forms remain unknown, the development pattern in egg-laying spiders is considered typical. The two major divisions of an arachnid’s body, the cephalothorax and the abdomen, emerge early, with appendages initially appearing as knobs.
Growth usually occurs by ‘ecdysis’ or molting, with the first molt often happening while still within the egg. Newly hatched arachnids are small, with less hardened (sclerotized) exoskeletons than adults. Most arachnid hatchlings have four pairs of legs except for mites, ticks, and ricinuleids, which have three pairs of legs during hatching. The number of molts needed to reach maturity varies, with larger species potentially molting up to 10 times.
However, the development and growth of mites differ from that of other arachnids. Mite eggs hatch into six-legged larvae, progressing through one or more immature (nymphal) stages before adulthood. Most mites lay eggs, though in some species, eggs develop internally, hatching within the mother’s body (ovoviviparous). Some Acari species, like Oribatid mites, can reproduce from unfertilized eggs through parthenogenesis. Overall, the life cycle of ticks closely resembles that of mites.
Like all organisms, arachnids have various natural predators that help regulate their populations in the wild.
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