Arachnid

Arachnida is a subphylum of Chelicerata that includes joint-legged invertebrates (arthropods). Spiders (the largest order), ticks, scorpions, harvestmen, mites, and solifugae and other members of the Arachnida. Horseshoe crabs were also classified as Arachnids in a molecular phylogenetic analysis published in 2019. Here, you will get an answer to the question- What is an arachnid? 

Most adult arachnids possess eight legs, but the front pair of legs in certain species has been transformed to a sensory feature, and distinct appendages in many other species may develop large enough to resemble additional pairs of legs. The phrase comes from the Greek word aráchnē, which refers to the story of Arachne, the hubristic human weaver who transformed into a spider.

Most of the extant arachnids are terrestrial, meaning they spend much of their time on land. Some do, furthermore, live in freshwater and, having the exception of the pelagic zone, marine habitats. There are over 100,000 named species in this community.

Arachnids Examples

Below are Some Examples of Arachnids.

• daddy longlegs (order Opiliones)

• ricinuleids (order Ricinulei)

• microscorpions (order Palpigradi)

• schizomids (order Schizomida)

• pseudoscorpions (order Pseudoscorpiones)

• Sun spiders (order Solifugae)

• scorpions (order Scorpiones)

• tailless whip scorpions (order Amblypygi)

About Arachnids Classification

Kingdom: Animalia

Phylum: Arthropoda

Subphylum: Chelicerata

Class: Arachnida

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Class Arachnida Characteristics: Morphology

Arachnids Characteristics 

Arachnid meaning and morphology: Adult arachnids mostly possess eight wings, except adult insects, which all have six. Arachnids, on the other hand, have two additional sets of appendages that have evolved for feeding, defense, and sensory perception. The chelicerae, the first pair, are responsible for feeding and protection. The pedipalps, the next set of appendages, have indeed been designed for locomotion, feeding, and/or reproduction. The palps of Solifugae are very leg-like, giving the impression that such animals possess ten legs. Mite and Ricinulei larvae seem to have just six legs; while they molt into nymphs, a fourth pair emerges. Adult mites, on the other hand, can have six or even four legs, in addition to the eight.

Arachnids are also differentiated from insects by their lack of antennae and wings. The prosoma, or cephalothorax, and the opisthosoma, or abdomen, are the two tagmata that make up their body. Nevertheless, since there is no fossil or embryological evidence of whether arachnids ever had such a separate thorax-like division, the word cephalothorax, which refers to a fused cephalon (head) and thorax, has indeed been called into question. Also, there are reasons against using the term "abdomen" since many arachnids' opisthosoma comprises organs that are not typical of an abdomen, including a heart and respiratory organs.

A single, unsegmented carapace normally covers the cephalothorax or prosoma. The abdomen is segmented in the much more primitive types, although certain groups have different degrees of integration between the segments. It is broadly categorized into a preabdomen and a postabdomen, but this is only apparent in scorpions, and the abdominal parts of certain orders, including the Acari, are entirely fused. A telson can be found in scorpions with a stinger, as well as in the whip scorpions, Schizomida, and Palpigradi.

Arachnids have had an exoskeleton, much like other arthropods, as well as an internal structure of cartilage-like tissue termed as the endosternite, with which some muscle groups are connected. In certain Opiliones, the endosternite is also calcified.

Locomotion: Throughout the distal joints of their appendages, major of the arachnids do not possess extensor muscles. Spiders and whip scorpions use the pressure of their hemolymph to stretch their limbs hydraulically. Solifuges as well as some harvestmen employ good elasticity thickenings in the joint cuticle to stretch their knees. Muscles that stretch two leg joints (patella-tibia joints and the femur-patella) at the same time have developed in pseudoscorpions, scorpions, and some harvestmen. The pedipalps of scorpions' similar joints, on the other hand, are stretched by elastic recoil.

Arachnid Identification

• Colour: The majority of arachnids are darkly coloured to blend in with a sandy or stony backdrop. While they can be present amongst tropical spiders and others that reside between flowers and foliage, conspicuous or bright colours are uncommon.

• Weight: The body of the 'Largest Arachnid' weighs 85 g (3 oz), while the body of the 'Smallest Arachnid' weighs just a few milligrams.

• Size: The 'Largest Arachnid' measures 16-28 cm in length, while the 'Smallest Arachnid' measures about 1,3 cm.

• Diet: Diet - The majority of arachnids are carnivores. Some, such as mites, eat plants, whereas others, such as ticks, are parasites that survive on the blood and tissue fluid of several other animals. Insects, other arachnids, and a number of invertebrates are among their favourite foods. 

Physiology

Internal respiratory surfaces in the style of tracheae, or alteration of the book gill into a book lung, an internal set of vascular lamellae being used for gas exchange with the air, are features that are especially important for arachnids' terrestrial lifestyle. Although tracheae in insects are mostly individual tube structures, pseudoscorpions, ricinuleids, and some spiders have sieve tracheae, which are a set of tubes that emerge from a small chamber attached to the spiracle. This kind of tracheal system quite certainly originated from book lungs, indicating that arachnid tracheae really aren't identical to insect tracheae.

Appendages adapted for more effective locomotion on soil, internal fertilization, unique sensory organs, and water protection improved by efficient excretory frameworks and a waxy surface surrounding the cuticle are all examples of adaptations to a terrestrial existence. Arachnids have up to four sets of coxal glands across the prosoma's side, as well as one or two pairs of Malpighian tubules that empty further into the gut. Most arachnids only have a single excretory gland, but a few have both. Guanine is arachnids' primary nitrogenous waste product.

The composition of arachnid blood varies based on the method of respiration. Arachnids with a well-functioning tracheal system don't need to transport oxygen via the bloodstream, and their circulatory systems may be less effective. Nevertheless, the blood of scorpions and certain spiders comprises haemocyanin, a copper-based pigment that functions similarly to haemoglobin in vertebrates. The heart is found in the front part of the abdomen and might be segmented or might not be. Some mites don't even have a nucleus.

Diet and Digestive System

Arachnids are mainly carnivorous, consuming insects as well as other small creatures that have already been digested. Ingestion of solid food molecules and therefore vulnerability to internal parasites is just found in harvestmen and mites like the house dust mite, while spiders are known to consume their own silk. To destroy prey or rivals, some groups release venom from specialised glands. A few mites and ticks become parasites, and others are disease carriers.

Arachnids contain digestive juices within their stomachs, which they then spill onto their dead prey with their pedipalps and chelicerae. The prey's digestive juices quickly transform it into a food broth, which is thereby sucked by arachnid into a pre-buccal cavity just in front of the mouth. A muscular, sclerotized pharynx sits behind the mouth and serves as a pump, sucking food via the mouth and then into the oesophagus and stomach. The oesophagus also serves as an auxiliary pump in some arachnids.

Multiple diverticula run across the body, giving the stomach a tubular appearance. Both the stomach as well as its diverticula make digestive enzymes and extract nutrients from the food. It runs the length of the body, connecting to a short sclerotized intestine and anus in the abdomen's back.

Senses

The lateral and median ocelli are the two types of eyes found in arachnids. The lateral ocelli are thought to have developed from compound eyes and may have a tapetum that aids in the light collection. There have never been more than three sets of lateral ocelli existing, except for scorpions, that might have up to five pairs. A transverse fold of the ectoderm gives rise to the median ocelli. Modern arachnids' ancestors most likely had both forms, but they often lack one or the other. The cornea of the eye serves as a lens and is connected to the body's cuticle. A translucent vitreous body lies underneath this, followed by the retina and, if existing, the tapetum. The retina of many of these arachnids is likely deficient in light-sensitive cells, preventing the eyes from forming a proper picture.

Most of the arachnids have two kinds of sensory organs in contrast to their eyes. The fine sensory hairs which surround the body and provide the animal with its sense of touch are by far the most essential to most arachnids. Numerous arachnids have much more complex structures termed trichobothria, which could be relatively simple.

Eventually, slit sense organs are pits containing slits in them that are protected by a thin membrane. A tiny hair within the pit senses movement by touching the membrane's underside. Proprioception and probably hearing are thought to be aided by slit sense organs.

Reproduction

The gonads that are found in the abdomen of arachnids, might possess one or two. On the underside of the 2nd abdominal segment, the genital opening is normally found. The male passes sperm to the female in a packet called a spermatophore throughout major species. Numerous arachnids have developed complex courtship rituals to guarantee the safe transmission of sperm to the female. Numerous orders have sexual dimorphism among their members.

Arachnids deposit yolky eggs that hatch towards immatures that look like adults. Scorpions, on the other hand, bear live young and seem to be either ovoviviparous or viviparous, based on the species. Just females offer parental care across most arachnids, with harvestmen as one of the few exceptions.

Taxonomy and Evolution

Phylogeny

For several years, the phylogenetic relationships amongst these major arthropod subdivisions have become the subject of much debate and study. From around 2010 onwards, a consensus emerged, focused on both morphological and molecular evidence. Extant (living) arthropods are classified into three major clades: pancrustaceans (paraphyletic crustaceans plus insects and their allies), chelicerates (including arachnids), and myriapods  (centipedes, millipedes, and allies).

The extant chelicerates are divided into two groups: marine chelicerates such as sea spiders and terrestrial arachnids and horseshoe crabs. (Pycnogonida (sea spiders) can be omitted from the chelicerates, that are then labelled "Euchelicerata.") According to a 2019 study, Xiphosura is nested deep inside Arachnida.

As of March 2016, discovering relationships inside the arachnids have gotten harder, with various studies yielding distinct results. A 2014 study, depending on the biggest collection of molecular data to date, found systematic conflicts in phylogenetic knowledge, especially concerning the orders of Acariformes, Parasitiformes, and Pseudoscorpiones, that have evolved at far faster rates. Analyses of the data employing sets of genes with distinct evolutionary rates created phylogenetic trees that were mutually incompatible. The authors preferred relationships revealed by slower-evolving genes, that illustrated the monophyly of Euchelicerata, Chelicerata, and Arachnida, and even some arachnid clades.

Tetrapulmonata, which includes Amblypygi, Araneae, and Thelyphonida (Schizomida had not been studied), got a lot of encouragement. The inclusion of Scorpiones to form the Arachnopulmonata clade was quite well received. Pseudoscorpiones, the sister of Scorpiones, might even contribute here. Surprisingly, support has been found for a clade that included Ricinulei, Opiliones, and Solifugae, a combination not seen in many other studies.

A molecular phylogenetic study published in early 2019 identified the horseshoe crabs, Xiphosura, as the sister group to Ricinulei. It also lumped pseudoscorpions in with mites and ticks, which the authors believe is due to the appeal of long branches.

Fossil History

The Uraraneida is a spider-like group belonging to the class Arachnida order that lived during the Devonian and Permian periods.

Chimerarachne yingi, a fossil arachnid found in 100 million-year-old (mya) Myanmar amber, have spinnerets (to manufacture silk) and a tail, such as the Palaeozoic Uraraneida, 200 million years after other known fossils including tails. The fossil matches the mesotheles, the far more primitive living spiders.

Taxonomy

Arachnid subdivisions are commonly referred to as orders. Mites and ticks were once classified as a single order, Acari. Molecular phylogenetic studies, on the other hand, indicate that the two groups may not create a single clade and that the morphological correlations are due to convergence. Acariformes, mites, and Parasitiformes, ticks, are now commonly treated as two distinct taxa that can be graded as orders or superorders. The arachnid divisions are described alphabetically below; species numbers are estimates.

Facts on Arachnid

• Arachnids include spiders, though not all arachnids are spiders. Arachnids are a group of animals that comprises spiders, scorpions, mites, and ticks. Four pairs of legs and also no antennae are just what these all have in general, which separates them from insects.

• The Goliath birdeater tarantula is a Goliath in the spider world (Theraphosa leblondi). This spider, which can grow up to the size of a dinner plate and has been reported to steal birds from their nests, is located in the coastal rain forests of northeastern South America.

• A spider may give the Man of Steel, Superman, a run for his money. Orb-weaver spider silk has a tensile strength comparable to steel.

• Venom is carried by almost everything spiders, but it is used to stun or destroy their insect prey, not to harm humans. Just about a quarter of all spider species are believed to have venom that affects humans. In far more than two decades, the two most well-known venomous spiders in the United States, the black widow, and the brown recluse, have not yet been proven to have induced some deaths.