Chelicerata

The subphylum (or phylum

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 contaminant 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.

Description

Segmentation and cuticle

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. 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. However in the Acari (mites and ticks) there is no visible division between these sections.

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 while those of horseshoe crabs (Xiphosura) form gills.

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. {| align="right" |- valign="top" | style="margin-left:2px" | of a eurypterid.]] | style="margin-left:2px" | |}

Chelicerae and pedipalps

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. However spiders' have only two sections, and the second forms a fang that folds away behind the first when not in use.

In most chelicerates the pedipalps are relatively small and are used as sensors.

Respiratory 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. 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 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. 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.

Harvestmen are among the minority of living chelicerates that can take solid food, and the group includes predators, vegetarians and scavengers. 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.

Excretion

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. 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. However since chelicerates lose the first segment, which bears antennae in other arthropods, chelicerate brains include only one pair of pre-oral ganglia instead of two. and in scorpions the ganglia of the cephalothorax are fused but the abdomen retains separate pairs of ganglia.

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.

Reproduction

Compsobuthus werneri carrying its young (white)]] Horseshoe crabs, which are aquatic, use external fertilization, in other words the sperm and ova meet outside the parents' bodies. Their trilobite-like larvae look rather like miniature adults as they have full sets of appendages and eyes, but initially they have only two pairs of book-gills and gain three more pairs as they molt. Some spiders care for their young, for example a wolf spider's brood cling to rough bristles on the mother's back,

Evolutionary history

Fossil record

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 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 , 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.

jerami, a trigonotarbid and the oldest known arachnid ]] Until recently the earliest known xiphosuran fossil dated from the Late Llandovery stage of the Silurian , but in 2008 an older specimen was reported from about in the Late Ordovician. Eurypterids have left few good fossils and the earliest confirmed eurypterids appear in the Late Ordovician period a little over .

The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about 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 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.

Relationships with other arthropods

A recent view of chelicerate phylogeny A "traditional" view of chelicerate phylogeny The "traditional" view of the arthropod "family tree" shows chelicerates as less closely related to the other major living groups (crustaceans; hexapods, which includes insects; and myriapods, which includes centipedes and millipedes) than these other groups are to each other. Recent research since 2001, using both molecular phylogenetics (the application of cladistic analysis to biochemistry, especially to organisms' DNA and RNA) and detailed examination of how various arthropods' nervous systems develop in the embryos, suggests that chelicerates are most closely related to myriapods, while hexapods and crustaceans are each other's closest relatives. However these results are derived from analyzing only living arthropods, and including extinct ones such as trilobites causes a swing back to the "traditional" view, placing trilobites as the sister-group of the Tracheata (hexapods plus myriapods) and chelicerates as least closely related to the other groups.

Major sub-groups

It is generally agreed that the Chelicerata contain the classes Arachnida (spiders, scorpions, mites, etc.), Xiphosura (horseshoe crabs) and Eurypterida (sea scorpions, extinct). The extinct Chasmataspida may be a sub-group within Eurypterida. The Pycnogonida (sea spiders) were traditionally classified as chelicerates, but some features suggest they may be representatives of the earliest arthropods from which the well-known groups such as chelicerates evolved.

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.

Diversity

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 Like their ancestors, most living chelicerates are carnivores, mainly on small invertebrates. However many species feed as parasites, vegetarians, scavengers and detritivores. |- ! Scorpiones (scorpions) | 1,400 || Carnivorous |- ! Schizomida (small whipscorpions) | 180 ||   |- ! Amblypygi (whipspiders) | 100 ||   |- ! Uropygi (Thelyphonida – whipscorpions) | 90 || Carnivorous |- ! Palpigradi (micro whipscorpions) | 60 ||   |- ! Ricinulei | 60 ||   |}

Interaction with humans

.]] In the past, Native Americans ate the flesh of horseshoe crabs, and used the tail spines as spear tips and the shells to bail water out of their canoes. More recent attempts to use horseshoe crabs as food for livestock were abandoned when it was found that this gave the meat a bad taste. The blood of horseshoe crabs contains a clotting agent Limulus Amebocyte Lysate which is now used to test that antibiotics and kidney machines are free of dangerous bacteria, and to detect spinal meningitis and some cancers.

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.

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.

Notes

Category:Arthropods