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Arachnid: the story on HearLore | HearLore
Arachnid
The word arachnid comes from the Greek arachne, meaning spider, a name bestowed upon the class after the tragic myth of Arachne, a human weaver who challenged the goddess Athena to a contest of skill and was transformed into a spider for her hubris. This ancient story provides the linguistic root for a vast and diverse group of creatures that now number over 110,000 described species, with spiders alone accounting for 51,000 of them. Unlike insects, which possess six legs and often wings, adult arachnids are defined by their eight legs attached to a fused head and thorax region known as the cephalothorax. This fundamental anatomical difference separates them from their six-legged cousins, yet it is not the only way they diverge from the insect world. Arachnids lack antennae entirely, relying instead on a complex array of sensory organs to navigate their environment, and their bodies are organized into two distinct tagmata: the cephalothorax and the abdomen. While the term cephalothorax implies a fusion of head and thorax, there is currently no fossil or embryological evidence that arachnids ever possessed a separate thorax-like division, suggesting the name may be a historical misnomer. The abdomen, too, contains organs atypical of a standard abdomen, such as a heart and respiratory organs, blurring the lines between body segments that are so clear in other arthropods. These creatures are almost entirely terrestrial, living on land, though some have adapted to freshwater environments and even the marine zone, excluding only the deep pelagic regions. The sheer diversity of the group is staggering, ranging from the tiny mites that can have as few as four legs in adulthood to the massive camel spiders that appear to have ten legs due to their leg-like pedipalps.
Hydraulic Limbs And Exoskeletons
Most arachnids lack extensor muscles in the distal joints of their appendages, forcing them to rely on hydraulic pressure to extend their limbs. Spiders and whip scorpions extend their legs by pumping hemolymph, their blood, into the joints to create the necessary force for movement. This hydraulic system is a marvel of biological engineering, allowing these creatures to move with speed and precision despite the limitations of their exoskeleton. In contrast, solifuges and some harvestmen extend their knees using highly elastic thickenings in the joint cuticle, while scorpions and pseudoscorpions have evolved muscles that can extend two leg joints at once. The pedipalps of scorpions, however, are extended by elastic recoil, a mechanism that allows for the powerful strike of their pincers. The exoskeleton itself is a rigid structure that protects the internal organs, but it also contains an internal framework of cartilage-like tissue called the endosternite. This endosternite serves as an anchor for certain muscle groups and is even calcified in some opiliones, adding to the structural integrity of the animal. The abdomen is segmented in more primitive forms, but varying degrees of fusion occur in many groups, sometimes dividing the abdomen into a preabdomen and postabdomen, a division clearly visible only in scorpions. In some orders, such as mites, the abdominal sections are completely fused, creating a smooth, unsegmented appearance. A telson is present in scorpions, modified into a stinger, and in whip scorpions, it becomes a flagellum. At the base of this flagellum in whip scorpions and schizomida, glands produce acetic acid as a chemical defense, a unique adaptation that sets them apart from other arachnids. Except for the pectines in scorpions and the spinnerets in spiders, the abdomen has no appendages, leaving the eight legs and the specialized pedipalps as the primary tools for interaction with the world.
The word arachnid comes from the Greek arachne, meaning spider, a name bestowed upon the class after the tragic myth of Arachne, a human weaver who challenged the goddess Athena to a contest of skill and was transformed into a spider for her hubris.
How many species of arachnids have been described?
There are over 110,000 described species of arachnids, with spiders alone accounting for 51,000 of them.
How do arachnids extend their legs without extensor muscles?
Most arachnids lack extensor muscles in the distal joints of their appendages and rely on hydraulic pressure to extend their limbs by pumping hemolymph into the joints to create the necessary force for movement.
What respiratory systems do arachnids use to breathe on land?
Arachnids utilize either tracheae or book lungs to exchange gases with the air, with book lungs functioning as an internal series of vascular lamellae adapted for air breathing.
When did the fossil arachnid Chimerarachne yingi live?
A fossil arachnid in 100 million year old amber from Myanmar, Chimerarachne yingi, lived during the Cretaceous period and possesses spinnerets to produce silk and also has a tail.
How many species of spiders exist compared to other arachnid orders?
The Araneae, or spiders, are the most numerous with 51,000 species, while the Opiliones, or phalangids, harvestmen, or daddy-long-legs, have 6,700 species.
Arachnids have evolved internal respiratory surfaces to thrive on land, utilizing either tracheae or book lungs to exchange gases with the air. Book lungs are an internal series of vascular lamellae that function similarly to gills but are adapted for air breathing, while tracheae are individual systems of tubes similar to those found in insects. Ricinuleids, pseudoscorpions, and some spiders possess sieve tracheae, where several tubes arise in a bundle from a small chamber connected to a spiracle, a type of system that likely evolved from book lungs. This indicates that the tracheae of arachnids are not homologous with those of insects, highlighting a unique evolutionary path. The excretory glands of arachnids include up to four pairs of coxal glands along the side of the prosoma and one or two pairs of Malpighian tubules, which empty into the gut. Many arachnids possess only one or the other type of gland, though several have both, and the primary nitrogenous waste product is guanine. Arachnid blood is variable in composition, depending on the mode of respiration. Arachnids with an efficient tracheal system do not need to transport oxygen in the blood and may have a reduced circulatory system, but scorpions and some spiders contain haemocyanin, a copper-based pigment with a function similar to haemoglobin in vertebrates. The heart is located in the forward part of the abdomen and may or may not be segmented, and some mites have no heart at all. Dietarily, arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Ticks and many mites are parasites, some of which are carriers of disease, while the diet of mites also includes tiny animals, fungi, plant juices, and decomposing matter. Harvestmen are almost as varied, including predators, decomposers, and omnivores that feed on decaying plant and animal matter, droppings, animals, and mushrooms. Scorpions, spiders, and pseudoscorpions secrete venom from specialized glands to kill prey or defend themselves, and their venom also contains pre-digestive enzymes that help break down the prey. The saliva of ticks contains anticoagulants and anticomplements, and several species produce a neurotoxin. Arachnids produce digestive enzymes in their stomachs and use their pedipalps and chelicerae to pour them over their dead prey, turning the prey into a broth of nutrients that the arachnid sucks into a pre-buccal cavity located immediately in front of the mouth. The stomach is tubular in shape, with multiple diverticula extending throughout the body, and the stomach and its diverticula both produce digestive enzymes and absorb nutrients from the food.
Senses And Courtship Rituals
Arachnids possess two kinds of eyes: the lateral and median ocelli. The lateral ocelli evolved from compound eyes and may have a tapetum, which enhances the ability to collect light. With the exception of scorpions, which can have up to five pairs of lateral ocelli, there are never more than three pairs present. The median ocelli develop from a transverse fold of the ectoderm, and while the ancestors of modern arachnids probably had both types, modern ones often lack one type or the other. The cornea of the eye acts as a lens and is continuous with the cuticle of the body, beneath which lies a transparent vitreous body, and then the retina and, if present, the tapetum. In most arachnids, the retina probably does not have enough light-sensitive cells to allow the eyes to form a proper image. In addition to the eyes, almost all arachnids have two other types of sensory organs. The most important to most arachnids are the fine sensory hairs that cover the body and give the animal its sense of touch. These can be relatively simple, but many arachnids also possess more complex structures called trichobothria. Finally, slit sense organs are slit-like pits covered with a thin membrane. Inside the pit, a small hair touches the underside of the membrane and detects its motion. Slit sense organs are believed to be involved in proprioception and possibly also hearing. Reproduction in arachnids is equally complex, with males transferring sperm to the female in a package or spermatophore. The males in harvestmen and some mites have a penis, and complex courtship rituals have evolved in many arachnids to ensure the safe delivery of the sperm to the female. Members of many orders exhibit sexual dimorphism, and arachnids usually lay yolky eggs that hatch into immatures that resemble adults. Scorpions, however, are either ovoviviparous or viviparous, depending on the species, and bear live young. Also, some mites are ovoviviparous and viviparous, even if most lay eggs. In most arachnids, only the females provide parental care, with harvestmen being one of the few exceptions.
The Evolutionary Puzzle
Discovering relationships within the arachnids has proven difficult, with successive studies producing different results. A study in 2014, based on the largest set of molecular data to date, concluded that there were systematic conflicts in the phylogenetic information, particularly affecting the orders Acariformes, Parasitiformes, and Pseudoscorpiones, which have had much faster evolutionary rates. Analyses of the data using sets of genes with different evolutionary rates produced mutually incompatible phylogenetic trees. The authors favored relationships shown by more slowly evolving genes, which demonstrated the monophyly of Chelicerata, Euchelicerata, and Arachnida, as well as of some clades within the arachnids. Tetrapulmonata, here consisting of Araneae, Amblypygi, and Uropygi, received strong support, and somewhat unexpectedly, there was support for a clade comprising Opiliones, Ricinulei, and Solifugae, a combination not found in most other studies. In early 2019, a molecular phylogenetic analysis placed the horseshoe crabs as the sister group to Ricinulei. It also grouped pseudoscorpions with mites and ticks, which the authors considered may be due to long branch attraction. The addition of Scorpiones to produce a clade called Arachnopulmonata was also well supported. Pseudoscorpions was suggested to also belong there, as all six orders share the same ancient whole genome duplication. Recent genetic analyses support pseudoscorpions as the sister group of scorpions, with this clade, Panscorpiones, forming the sister group to Tetrapulmonata within Arachnopulmonata, with analysis of Solifugae genomes indicating that they do not have a whole genome duplication, making a previously suggested close relationship with pseudoscorpions unlikely. Genetic analysis has not yet been done for Ricinulei or Palpigradi, but horseshoe crabs have gone through two whole genome duplications, which gives them five Hox clusters with 34 Hox genes, the highest number found in any invertebrate, yet it is not clear if the oldest genome duplication is related to the one in Arachnopulmonata. More recent phylogenomic analyses that have densely sampled both genomic datasets and morphology have supported horseshoe crabs as nested inside Arachnida, suggesting a complex history of terrestrialization. Morphological analyses including fossils tend to recover the Tetrapulmonata, including the extinct group the Haptopoda, but recover other ordinal relationships with low support.
Fossils And Ancient Lineages
The fossil history of arachnids reveals a deep and complex past, with extinct orders like the Uraraneida appearing in the Devonian and Permian periods. A fossil arachnid in 100 million year old amber from Myanmar, Chimerarachne yingi, has spinnerets to produce silk and also has a tail, like the Palaeozoic Uraraneida, some 200 million years after other known fossils with tails. The fossil resembles the most primitive living spiders, the mesotheles. The Uraraneida are an extinct order of spider-like arachnids from the Devonian and Permian, and the Haptopoda are extinct arachnids apparently part of the Tetrapulmonata, the group including spiders and whip scorpions. The Phalangiotarbida are extinct arachnids of uncertain affinity, and the Trigonotarbida are extinct from the late Silurian to the Early Permian. These extinct groups provide crucial insights into the evolution of modern arachnids, showing how features like spinnerets and tails have been lost or modified over time. The fossil record also shows that the body plan of arachnids has remained relatively stable, with the eight-legged body plan and the division into cephalothorax and abdomen being consistent across millions of years. The presence of spinnerets in Chimerarachne yingi suggests that the ability to produce silk evolved early in the history of arachnids, even before the appearance of modern spiders. The tail of Chimerarachne yingi, which is similar to the tail of the Uraraneida, indicates that the tail was a common feature in early arachnids, but was lost in most modern lineages. The fossil record also shows that the diversity of arachnids has increased over time, with the number of described species reaching over 110,000, and the total number of species potentially exceeding one million. The study of these fossils helps to fill in the gaps in our understanding of arachnid evolution, providing a window into the past that allows us to see how these creatures have adapted to changing environments over millions of years.
Diversity And Modern Classifications
The subdivisions of the arachnids are usually treated as orders, and historically, mites and ticks were treated as a single order, Acari. However, molecular phylogenetic studies suggest that the two groups do not form a single clade, with morphological similarities being due to convergence. They are now usually treated as two separate taxa, Acariformes and Parasitiformes, which may be ranked as orders or superorders. The extant forms include Acariformes, with 32,000 species of mites, and Parasitiformes, with 12,000 species of ticks. The Amblypygi, or blunt rump tail-less whip scorpions, have front legs modified into whip-like sensory structures as long as 25 centimeters or more, and there are 250 species. The Araneae, or spiders, are the most numerous with 51,000 species, while the Opiliones, or phalangids, harvestmen, or daddy-long-legs, have 6,700 species. The Palpigradi, or microwhip scorpions, are rare with only 130 species, and the Pseudoscorpionida, or pseudoscorpions, have 4,000 species. The Ricinulei, or hooded tickspiders, are even rarer with 100 species, and the Schizomida, or split middle whip scorpions, have 350 species. The Scorpiones, or scorpions, have 2,700 species, and the Solifugae, or solpugids, windscorpions, sun spiders, or camel spiders, have 1,200 species. The Uropygi, also called Thelyphonida, are whip scorpions or vinegaroons, with forelegs modified into sensory appendages and a long tail on the abdomen tip, and there are 120 species. The extinct forms include the Haptopoda, with 1 species, the Phalangiotarbida, with 30 species, the Trigonotarbida, and the Uraraneida, with 2 species. It is estimated that 110,000 arachnid species have been described, and that there may be over a million in total. This vast diversity is a testament to the adaptability of arachnids, which have managed to thrive in a wide range of environments, from the deepest caves to the highest mountains, and from the driest deserts to the wettest rainforests. The classification of arachnids continues to evolve, with new molecular data and fossil discoveries constantly reshaping our understanding of their relationships and history.