Chelicerata Heymons, 1901
* Dunlop, J.A.; Selden, P.A. 2009: Calibrating the chelicerate clock: a paleontological reply to Jeyaprakash and Hoy. Experimental and applied acarology, 48: 183–197. doi: 10.1007/s10493-009-9247-1 PDF
The subphylum (or phylum) Chelicerata constitutes one of the major subdivisions of the phylum (or superphylum) Arthropoda, and includes horseshoe crabs, scorpions, spiders and mites. They originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from 445 million years ago in the Late Ordovician period. The surviving marine species are the four Xiphosura (horseshoe crabs), and possibly the 500 species of Pycnogonida (sea spiders) if these are chelicerates. On the other there are over 77,000 well-identified species of air-breathing chelicerates, and there may be about 500,000 unidentified species.
Like all arthropods, chelicerates have segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins. The chelicerate bauplan consists of two tagmata, the cephalothorax and abdomen, except that mites have no visible division between these sections. The chelicerae that give the group its name are the only appendages that appear before the mouth, and in most sub-groups are modest pincers used in feeding; however spiders' chelicerae form fangs that most species use to inject venom into their prey. The group has the typical arthropod open circulatory system, in which a tube-like heart pumps blood through the hemocoel, which is the major body cavity. Marine chelicerates have gills while the air-breathing forms generally have both book lungs and tracheae. In general the ganglia of living chelicerates' central nervous systems fuse into large masses in the cephalothorax, but there are wide variations and this fusion is very limited in the Mesothelae, which are regarded as the oldest and most primitive group of spiders. Most chelicerates rely on modified bristles for information about chemical changes and about touch, vibrations and air currents. However the most active hunting spiders have very acute eyesight.
Chelicerates were originally predators but the group has diversified to use all the major feeding strategies: predation, parasitism, herbivory, scavenging and eating decaying organic matter. Although harvestmen can digest solid food, the guts of most modern chelicerates are too narrow for this and they generally liquidize their food by grinding it with their chelicerae and pedipalps and flooding it with digestive enzymes. To conserve water, air-breathing chelicerates excrete waste as solids that are removed from their blood by Malpighian tubules, structures which also evolved independently in insects. While the marine horseshoe crabs rely on external fertilization, air-breathing chelicerates use internal but usually indirect fertilization. Predatory species generally use elaborate courtship rituals to prevent males from being eaten before they can mate. Most lay eggs that hatch as what look like miniature adults, but all scorpions and a few species of mites keep the eggs inside their bodies until the young emerge. In most chelicerate species the young have to fend for themselves, but in scorpions and some species of spider the females protect and feed their young.
The evolutionary origins of chelicerates from the early arthropods have been debated for decades. Although there is considerable agreement about the relationships between most chelicerate sub-groups, the inclusion of the Pycnogonida in this taxon has recently been doubted (see below), and the position of scorpions is still controversial, though they were long considered the most primitive (basal) of the arachnids.
Although the venom of a few spider and scorpion species can be very dangerous to humans, medical researchers are investigating the use of these venoms for the treatment of disorders ranging from cancer to erectile dysfunction. The medical industry also uses the blood of horseshoe crabs as a test for the presence of dangerous bacteria. Genetic engineers have experimented with modifying goats' milk and plants' leaves to produce spider silk. Mites can cause allergies in humans, transmit several diseases to humans and their livestock, and are serious agricultural pests.
The Chelicerata are arthropods as they have: segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins; heads that are composed of several segments that fuse during the development of the embryo; a much reduced coelom; a hemocoel through which the blood circulates, driven by a tube-like heart.:518 Chelicerates' bodies consist of two tagmata, sets of segments that serve similar functions: the foremost one, called the cephalothorax or prosoma, is a complete fusion of the segments that in an insect would form two separate tagmata, the head and thorax; the rear tagma is called the abdomen or opisthosoma.:554–555 However in the Acari (mites and ticks) there is no visible division between these sections.:591–595
The cephalothorax is formed in the embryo by fusion of the acron, which carries the eyes, with segments two to seven, which all have paired appendages, while segment one is lost during the embryo's development. Segment two has a pair of chelicerae, small appendages that often form pincers, segment three has a pair of pedipalps that in most sub-groups perform sensory functions, while the remaining four cephalothorax segments have pairs of legs. In primitive forms the acron has a pair of compound eyes on the sides and four pigment-cup ocelli ("little eyes") in the middle.:554–555 The mouth is between segments two and three.
The abdomen consists of twelve or fewer segments which originally formed two groups, a "preabdomen" or "mesoma" of seven segments and a "postabdomen" or "metasoma" of five, terminating with a telson or spike. The abdominal appendages of modern chelicerates are missing or heavily modified:554–555 – for example in spiders the remaining appendages form spinnerets that extrude silk,:571–584 while those of horseshoe crabs (Xiphosura) form gills.:555–559
Like all arthropods, chelicerates' bodies and appendages are covered with a tough cuticle made mainly of chitin and proteins which are chemically hardened. Since this cannot stretch, the animals have to molt in order to grow, in other words they grow new but still soft cuticles and then cast off the old one and wait for the new one to harden. Until the new cuticle has hardened the animals are defenseless and almost immobilized.:521–525
These appendages vary widely in form and function and the only consistent difference between them is their position: chelicerae arise from segment two, ahead of the mouth, and pedipalps from segment three, behind the mouth.:554–555
The chelicerae ("claw horns") that give the sub-phylum its name normally consist of three sections, and the claw is formed by the third section and a rigid extension of the second.:554–555 However spiders' have only two sections, and the second forms a fang that folds away behind the first when not in use.:571–584 The relative sizes of chelicerae vary widely: those of some eurypterids formed large claws that extended ahead of the body, while scorpions' are tiny pincers that are used in feeding and project only slightly in front of the head.:565–569
In most chelicerates the pedipalps are relatively small and are used as sensors.:554–555 However those of male spiders have bulbous tips that act to syringes to inject sperm into the females' reproductive openings when mating,:571–584 while scorpions' form large claws used for capturing prey.:565–569
Body cavities and circulatory systems
These depend on individual sub-groups' environments. Modern terrestrial chelicerates generally have both book lungs, which deliver oxygen and remove waste gases via the blood, and tracheae, which do the same without using the blood as a transport system.:559–564 The living horseshoe crabs are aquatic and have book gills that lie in a horizontal plane. For a long time it was assumed that the extinct eurypterids had gills, but the fossil evidence was ambiguous. However a fossil of the 45 millimetres (1.8 in) long eurypterid Onychopterella, from the Late Ordovician period, has what appear to be three pairs of vertically-oriented book gills whose internal structure is very similar to that of scorpions' book lungs.
Feeding and digestion
The guts of most modern chelicerates are too narrow to take solid food.:559–564 All scorpions and almost all spiders are predators that "pre-process" food in preoral cavities formed by the chelicerae and the bases of the pedipalps.:565–569 However one predominantly vegetarian spider species is known, and many supplement their diets with nectar and pollen. Many of the Acari (ticks and mites) are blood-sucking parasites, but there are many predatory, vegetarian and scavenger sub-groups. All the Acari have a retractable feeding assembly that consists of the chelicerae, pedipalps and parts of the exoskeleton, and which forms a preoral cavity for pre-processing food.:591–595
Harvestmen are among the minority of living chelicerates that can take solid food, and the group includes predators, vegetarians and scavengers.:588–590 Horseshoe crabs are also capable of processing solid food, and use a distinctive feeding system. Claws at the tips of their legs grab small invertebrates and pass them to a food groove that runs from between the rearmost legs to the mouth, which is on the underside of the head and faces slightly backwards. The bases of the legs form toothed gnathobases that both grind the food and push it towards the mouth.:555–559 This is how the earliest arthropods are thought to have fed.
Horseshoe crabs convert nitrogenous wastes to ammonia and dump it via their gills, and excrete other wastes as feces via the anus. They also have nephridia ("little kidneys"), which extract other wastes for excretion as urine.:555–559 Ammonia is so toxic that it must be diluted rapidly with large quantities of water.:529–530 Most terrestrial chelicerates cannot afford to use so much water and therefore convert nitrogenous wastes to other chemicals which can be excreted as dry matter. Extraction is done by various combinations of nephridia and Malpighian tubules. The tubules filter wastes out of the blood and dump them into the hindgut as solids, a system that has evolved independently in insects and several groups of arachnids.:559–564
As with other arthropods, chelicerates' cuticles would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact spiders and other arthropods have modified their cuticles into elaborate arrays of sensors. Various touch and vibration sensors, mostly bristles called setae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell, often by means of setae.:532–537
Living chelicerates have both compound eyes, mounted on the sides of the head, and pigment-cup ocelli ("little eyes"), mounted in the middle. The eyes of horseshoe crabs can detect movement but not form images.:555–559 At the other extreme, jumping spiders have a very wide field of vision,:571–584 and their main eyes are ten times as acute as those of dragonflies.
Being air-breathing animals, the living arachnids use internal fertilization, which is direct in some species, in other words the males' genitalia make contact with the females'. However in most species fertilization is indirect. Male spiders use their pedipalps as syringes to "inject" sperm into the females' reproductive openings,:571–584 but most arachnids produce spermatophores (packages of sperm) which the females take into their bodies.:559–564 Courtship rituals are common, especially in the most powerful predators, where males risk being eaten before mating. Most arachnids lay eggs, but all scorpions and a few mites keep the eggs inside their bodies until they hatch and offspring rather like miniature adults emerge.:559–564
Levels of parental care for the young range from zero to prolonged. Scorpions carry their young on their backs until the first molt, and in a few semi-social species the young remain with their mother. Some spiders care for their young, for example a wolf spider's brood cling to rough bristles on the mother's back,:571–584 and females of some species respond to the "begging" behavior of their young by giving them their prey, provided it is no longer struggling, or even regurgitate food.
There are large gaps in the chelicerates' fossil record because, like all arthropods, their exoskeletons are organic and hence their fossils are rare except in a few lagerstätten where conditions were exceptionally suited to preserving fairly soft tissues. The Burgess shale animals Sanctacaris and Sidneyia from about 505 million years ago have been classified as chelicerates, the former because of its pattern of tagmosis (how the segments are grouped, especially in the head) and the latter because its appendages resemble those of the Xiphosura (horseshoe crabs). However cladistic analyses that consider wider ranges of characteristics place neither as chelicerates. There is debate about whether Fuxianhuia from earlier in the Cambrian period, about 525 million years ago, was a chelicerate. Another Cambrian fossil, Kodymirus, was originally classified as an aglaspid but may have been a eurypterid and therefore a chelicerate. If any of these was closely related to chelicerates, there is a gap of at least 43 million years in the record between true chelicerates and their nearest not-quite chelicerate relatives.
The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about 420 million years ago in the Silurian period, and had a triangular cephalothorax and segmented abdomen, as well as eight legs and a pair of pedipalps.
Attercopus fimbriunguis, from 386 million years ago in the Devonian period, bears the earliest known silk-producing spigots, and was therefore hailed as a spider, but it lacked spinnerets and hence was not a true spider. Several Carboniferous spiders were members of the Mesothelae, a primitive group now represented only by the Liphistiidae.
The Late Silurian Proscorpius has been classified as a scorpion, but differed significantly from modern scorpions: it appears wholly aquatic since it had gills rather than book lungs or tracheae; its mouth was completely under its head and almost between the first pair of legs, as in the extinct eurypterids and living horseshoe crabs. Fossils of terrestrial scorpions with book lungs have been found in Early Devonian rocks form about 402 million years ago.
Relationships with other arthropods
However the structure of "family tree" relationships within the Chelicerata has been controversial ever since the late 19th century. An attempt in 2002 to combine analysis of RNA features of modern chelicerates and anatomical features of modern and fossil ones produced credible results for many lower-level groups, but its results for the high-level relationships between major sub-groups of chelicerates were unstable, in other words minor changes in the inputs caused significant changes in the outputs of the computer program used (POY). An analysis in 2007 using only anatomical features produced the cladogram on the right, but also noted that many uncertainties remain. 
The position of scorpions is particularly controversial. Some early fossils such as the Late Silurian Proscorpius have been classified by paleontologists as scorpions, but described as wholly aquatic as they had gills rather than book lungs or tracheae. Their mouths are also completely under their heads and almost between the first pair of legs, as in the extinct eurypterids and living horseshoe crabs. This presents a difficult choice: classify Proscorpius and other aquatic fossils as something other than scorpions, despite the similarities; accept that "scorpions" are not monophyletic but consist of separate aquatic and terrestrial groups; or treat scorpions as more closely related to eurypterids and possibly horseshoe crabs than to spiders and other arachnids, so that either scorpions are not arachnids or "arachnids" are not monophyletic.
Although well behind the insects, chelicerates are one of the most diverse groups of animals, with over 77,000 living species that have been described in scientific publications. Some estimates suggest that there may be 130,000 undescribed species of spider and nearly 500,000 undescribed species of mites and ticks. While the earliest chelicerates and the living Pycnogonida (if they are chelicerates) and Xiphosura are marine animals that breathe dissolved oxygen, the vast majority of living species are air-breathers, although a few spider species build "diving bell" webs that enable them to live under water. Like their ancestors, most living chelicerates are carnivores, mainly on small invertebrates. However many species feed as parasites, vegetarians, scavengers and detritivores.:588–595
Cooked tarantula spiders are considered a delicacy in Cambodia, and by the Piaroa Indians of southern Venezuela. Spider venoms may be a less polluting alternative to conventional pesticides as they are deadly to insects but the great majority are harmless to vertebrates. Possible medical uses for spider venoms are being investigated, for the treatment of cardiac arrhythmia, Alzheimer's disease, strokes, and erectile dysfunction. Because spider silk is both light and very strong, attempts are being made to produce it in goats' milk and in the leaves of plants, by means of genetic engineering.  There were about 100 reliably reported deaths from spider bites in the 20th century, compared with 1,500 from jellyfish stings.
Scorpion stings are thought to be a significant danger in less-developed countries, for example they cause about 1,000 deaths per year in Mexico but only one every few years in the USA. Most of these incidents are caused by accidental human "invasions" of scorpion's nests. However medical uses of scorpion venom are being investigated for treatment of brain cancers and bone diseases.
Ticks are parasitic, and some transmit micro-organisms and parasites that can cause diseases in humans, while the saliva of a few species can directly cause tick paralysis if they are not removed within a day or two.:114
A few of the closely-related mites also infest humans, some causing intense itching by their bites and others by burrowing into the skin. Species that normally infest other animals such as rodents may infest humans if their normal hosts are eliminated. Three species of mite are a threat to honey bees and one of these, Varroa destructor, has become the largest single problem faced by beekeepers worldwide. Mites cause several forms of allergic diseases, including hay fever, asthma and eczema, and they aggravate atopic dermatitis. Mites are also significant crop pests, although predatory mites may be useful in controlling some of these.
1. ^ a b c Margulis, Lynn; Schwartz, Karlene (1998), Five Kingdoms, An Illustrated Guide to the Phyla of Life on Earth (third ed.), W.H. Freeman and Company, ISBN 0716730278
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