Arthropods' body plan of repeated segments, each with a pair of appendages, is so versatile that they have been compared to Swiss Army knives, and has enabled them to become the most species-rich members of all ecological guilds in most environments; they have over a million described species, making up more than 80% of all described living species. In fact they are one of only two groups that are very successful in dry environments – the other is amniotes. They range in size from microscopic plankton up to forms a few metres long.
Although arthropods are coelomates, their main internal cavity is a hemocoel, which accommodates their internal organs and through which their blood circulates – they have open circulatory systems. Like their exteriors, arthropods' internal organs are generally built of repeated segments. Their nervous system are "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of these segments' ganglia and encircle the esophagus.
Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information; but spiders' main eyes are ocelli that can form images and, in a few cases, can swivel to track prey. They also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles.
Arthropods' methods of reproduction and development are similarly diverse. All terrestrial species use internal fertilization, but often by indirect transfer of the sperm via an appendage of the ground, rather by direct injection. Aquatic species use either internal or external fertilization. Almost all lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mothers. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from zero to prolonged, as in scorpions.
The versatility of arthropods' modular body plan has made it difficult for zoologists and paleontologists to classify them and work out their evolutionary ancestry, which dates back to the Cambrian period. From the late 1950s to late 1970s it was widely thought that arthropods were polyphyletic, in other words there was no single arthropod ancestor. Now, however, they are generally regarded as monophyletic. Traditionally it was thought that arthropods' closest evolutionary relatives were annelid worms, as both groups have segmented bodies. It is now generally thought that arthropods belong to the superphylum Ecdysozoa ("animals that molt"), while annelids belong to another superphylum, Lophotrochozoa. The relationships between various arthropod groups are still actively debated.
Although arthropods contribute to human food suply both directly as food and more importantly as pollinators of crops, they also spread some of the most severe diseases and do considerable damage to livestock and crops.
The embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods probably consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways; The three-part appearance of many insects' bodies and the two-part appearance of spiders' is a result of this grouping; in fact there are no external signs of segmentation in mites. Arthropods also have two body elements which are not part of this serially repeated pattern of segments, an acron at the front, ahead of the mouth, and a telson at the rear, behind the anus. The eyes are mounted on the acron.
}} The original structure of arthropod appendages was probably biramous, with the upper branch acting as a gill while the lower branch was used for walking. In some segments of all known arthropods the appendages are modified, for example to form gills, mouth-parts, antennae for collecting information, or claws for grasping – arthropods are "like Swiss Army knives, each equipped with a unique set of specialized tools." Appendages often vanish from some regions of the body, and it is particularly common for abdominal appendages to disappear or be highly modified.
The most conspicuous specialization of segments is in the head. The four major groups of arthropods - Chelicerata (includes spiders and scorpions), Crustacea (shrimps, lobsters, crabs, etc.), Tracheata (arthropods that breathe via channels into their bodies; includes insects and myriapods), and the extinct trilobites – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways. In addition some extinct arthropods, such as Marrella, belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages. Working out the evolutionary stages by which all these different combinations could have appeared is so difficult that it has long been known as "the Arthropod head problem". In 1960 R.E. Snodgrass even hoped it would not be solved, as trying to work out solutions was so much fun.
The exoskeletons of most aquatic crustaceans are biomineralized with calcium carbonate extracted from the water. Some terrestrial crustaceans have developed means of storing the mineral, to deal with the unavailability of dissolved calcium carbonate. Biomineralization generally affects the exocuticle and the outer part of the endocuticle. Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals are that it provides tougher defensive armor, and that it allows animals to grow larger and stronger by providing more rigid skeletons; and in either case a mineral-organic composite exoskeleton is cheaper to build than an all-organic one of comparable strength.
The cuticle can have setae (bristles) growing from special cells in the epidermis. Setae are as varied in form and function as appendages, for example: they are often used as sensors to detect air or water currents, or contact with objects; aquatic arthropods use feather-like setae to increase the surface area of swimming appendages and to filter food particles out of water; aquatic insects, which are air-breathers, use thick felt-like coats of setae to trap air, extending the time they can spend under water; heavy, rigid setae serve as defensive spines.
Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, some still use hydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors - for example all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.
In the initial phase of molting , the animal stops feeding and its epidermis releases molting fluid, a mixture of enzymes that digests the endocuticle and thus detaches the old cuticle. This phase begins when the epidermis has secreted a new epicuticle to protect it from the enzymes, and the epidermis secretes the new exocuticle while the old cuticle is detaching. When this stage is complete, the animal makes its body swell by taking in a large quantity of water or air, and this makes the old cuticle split along predefined weaknesses where the old exocuticle was thinnest. It commonly takes several minutes for the animal to struggle out of the old cuticle. At this point the new one is wrinkled and so soft that the animal cannot support itself and finds it very difficult to move, and the new endocuticle has not yet formed. The animal continues to pump itself up to stretch the new cuticle as much as possible, then hardens the new exocuticle and eliminates the excess air or water. By the end of this phase the new endocuticle has formed. Many arthropods then eat the discarded cuticle to reclaim its materials.
Because arthropods are unprotected and nearly immobilized until the new cuticle has hardened, they are in danger both of being trapped in the old cuticle and of being attacked by predators. Molting may be responsible for 80 to 90% of all arthropod deaths.
Arthropods have open circulatory systems - although most have a few short, open-ended arteries. In chelicerates and crustaceans the blood carries oxygen to the tissues, while hexapods use a separate system of tracheae. Many crustaceans but few chelicerates and tracheates use respiratory pigments to assist oxygen transport. The most common respiratory pigment in arthropods is copper-based hemocyanin: this is used by many crustaceans and a few centipedes. A few crustaceans and insects use iron-based hemoglobin, the respiratory pigment used by vertebrates. As with other invertebrates and unlike among vertebrates, the respiratory pigments of those arthropods that have them are generally dissolved in the blood and rarely enclosed in corpuscles.
The heart is typically a muscular tube that runs just under the back and for most of the length of the hemocoel. It contracts in ripples that run from rear to front, pushing blood forwards. Elastic ligaments or small muscles connect the heart to the body wall and expand sections that are not being squeezed by the heart muscle. Along the heart runs a series of paired ostia, non-return valves that allow blood to enter the heart but prevent it from leaving before it reaches the front.
Arthropods have a wide variety of respiratory systems. Small species often do not have any, since their high ratio of surface area to volume enables simple diffusion through the body surface to supply enough oxygen. Crustacea usually have gills that are modified appendages. Many archnids have book lungs. Tracheae, systems of branching tunnels that run from the openings in the body walls, deliver oxygen directly to individual cells in many insects, myriapods and arachnids.
Living arthropods have paired main nerve cords running along their bodies below the gut, and in each segent the cords form a pair of ganglia from which sensory and motor nerves run to other parts of the segment. Although the pairs of ganglia in each segment often appear physically fused, they are connected by commissures (relatively large bundles of nerves), which give arthropod nervous systems a characteristic "ladder-like" appearance. The brain is in the head, encircling and mainly above the esophagus. It consists of the fused ganglia of the acron and one or two of the foremost segments that form the head - a total of three pairs of ganglia in most arthropods, but only two in chelicerates, which do not have antennae or the ganglion connected to them. The ganglia of other head segments are often close to the brain and function as part of it. In insects these other head ganglia combine into a pair of subesophageal ganglia, under and behind the esophagus. Spiders take this process a step further, as all the segmental ganglia are incorporated into the subesophageal ganglia, which occupy most of the space in the cephalothorax (front "super-segment").
There are two different types of arthropod excretory system. In aquatic arthropods the end-product of biochemical reactions that metabolise nitrogen is ammonia, which is so toxic that it needs to be diluted as much as possible with water. The ammonia is then eliminated via any permeable membrane, mainly through the gills. All crustaceans use this system, and its high consumption of water may be responsible for crustaceans' relative lack of success as land animals. Various groups of terrestrial arthropods have independently developed a different system: the end-product of nitrogen metabolism is uric acid, which can be excreted as dry material; Malphigian tubules filter the uric acid and other nitrogenous waste out of the blood in the homecoel, and dump these materials into the hindgut, from which they are expelled as feces. Most aquatic arthropods and some terrestrial ones also have organs called nephridia ("little kidneys"), which extract other wastes for excretion as urine.
Most arthropods have sophisticated visual systems that include one or more usually both of compound eyes and pigment-cup ocelli ("little eyes"). In most cases ocelli are only capable of detecting the direction from which light is coming, using the shadow cast by the walls of the cup. However the main eyes of spiders are pigment-cup ocelli that are capable of forming images, and those of jumping spiders can rotate to track prey.
Compound eyes consist of fifteen to several thousand independent ommatidia, columns that are usually hexagonal in cross-section. Each ommatidium is an independent sensor, with its own light-sensitive cells and often with its own lens and cornea. Compound eyes have a wide field of view, and can detect fast movement and, in some cases, the polarization of light. On the other hand the relatively large size of ommatidia makes the images rather coarse, and compound eyes are shorter-sighted than those of birds and mammals – although this is not a severe disadvantage, as objects and events within are most important to most arthropods. Several arthropods have color vision, and that of some insects has been studied in detail - for example bees' ommatidia contain receptors for both green and ultra-violet.
Most arthropods lack balance and acceleration sensors and rely on their eyes to tell them which way is up. The self-righting behavior of cockroaches is triggered when pressure sensors on the underside of the feet report no pressure. However many malacostracan crustaceans have statocysts, which provide the same sort of information as the balance and motion sensors of the vertebrate inner ear.
Arthropods' proprioceptors, sensors that report the force exerted by muscles and the degree of bending in the body and joints, are well understood. On the other hand little is known about what other internal sensors Arthropods may have.
A few arthropods, such as barnacles, are hermaphroditic, in other words can function as both sexes. However individuals of most species remain of one sex all their lives. Aquatic arthropods may breed by external fertilization, as for example frogs do, or by internal fertilization, where the ova remain in the female's body and the sperm must must somehow be inserted. All known terrestrial arthropods use internal fertilization, as unprotected sperm and ova would not survive long in these environments. In a few cases the sperm transfer is direct from the male's penis to the female's oviduct, but it is more often indirect. Some crustaceans and spiders use modified appendages to transfer the sperm to the female. On the other hand many male terrestrial arthropods produce spermatophores, water-proof packets of sperm, and then the females take these into their bodies. A few such species rely on females to find spermatophores that have already been deposited on the ground, but in most cases males only deposit spermatophores when complex courtship rituals look likely be successful.
Most arthropods lay eggs, but scorpions are viviparous, in other words produce live young after the eggs have hatched inside the mother. Scorpions are also notable for their prolonged maternal care.
Newly-born arthropods have similarly diverse forms. Insects alone cover the range of extremes: some hatch as apparently miniature adults (direct development), although in some cases, such as silverfish, the hatchlings do not feed and may be helpless until after their first molt; many insects hatch as grubs or caterpillars which do not have segmented limbs or hardened cuticles, and metamorphose into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body; while dragonfly larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws. Crustaceans commonly hatch as tiny nauplius larvae that have only three segments and pairs of appendages.
From the late 1950s to the late 1970s Sidnie Manon and others argued that arthropods were polyphyletic, in other words they do not share a common ancestor that was itself an arthropod. Instead they proposed that three separate groups of "arthropods" evolved separately from worm-like ancestors: the chelicerates, including spiders and scorpions; the crustaceans; and the uniramia, consisting of onychophorans, myriapods and hexapods – these arguments usually bypassed trilobites, whose evolutionary relationships were unclear. Proponents of polyphly argued that: the similarities between these groups are the results of convergent evolution, as natural consequences of having rigid, segmented exoskeletons; the three groups use different chemical means of hardening the cuticle; there were significant differences in the construction of their compound eyes; it is hard to see how such different configurations of segments and appendages in the head could have evolved from the same ancestor; crustaceans have biramous limbs with separate gill and leg branches, while the other two groups have uniramous limbs in which the single branch serves as a leg.However further analysis and discoveries in the 1990s led to acceptance that arthropods are monophyletic, in other words they share a common ancestor that was itself an arthropod. For example Graham Budd's analyses of Kerygmachela in 1993 and of Opabinia in 1996 convinced him that these animals were similar to onychophorans and to various Early Cambrian "lobopods", and he presented an "evolutionary family tree" that showed these as "aunts" and "cousins" of all arthropods. These changes made the scope of the term "arthropod" unclear, and Claus Nielsen proposed that the wider group should be labelled "Panarthropoda" ("all the arthropods") while the animals with jointed limbs and hardened cuticles should be called "Euarthropoda" ("true arthropods"). Traditionally the Annelida have been considered the closest relatives of the Panarthropoda, since both groups have segmented bodies; the combination of these groups was labelled Articulata. There had been competing proposals that arthropods were closely related to other groups such as nematodes, priapulids and tardigrades, but these remained minority views because it was difficult to specify in detail the relationships between these groups. However in the 1990s molecular phylogenetics analyses that compared sequences of RNA and DNA produced a coherent scheme showing arthropods as members of a superphylum labelled Ecdysozoa ("animals that molt"), which contained nematodes, priapulids and tardigrades but excluded annelids. This was backed up by studies of these animals' anatomy and development, which showed that many of the features that supported the Articulata hypothesis showed significant differences between annelids and the earliest Panarthropods in their details, and some were hardly present all in arthropods. This hypothesis groups annelids with molluscs and brachiopods. If the Ecdysozoa hypothesis is correct, then segmentation of arthropods and annelids either has evolved convergently or has been inherited from a much older ancestor, and has been subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa.
Aside from these major groups, there are also a number of fossil forms, mostly from the Early Cambrian, which are difficult to place, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them. Marrella was the first one to be recognized as significantly different from the well-known groups.
The phylogeny of the arthropods has been an area of considerable interest and dispute. The validity of many of the arthropod groups suggested by earlier authors is being questioned by recent studies; these include Mandibulata, Uniramia and Atelocerata. The most recent studies tend to suggest a paraphyletic Crustacea with different hexapod groups nested within it. The remaining clade of Myriapoda and Chelicerata is referred to as Paradoxopoda or Myriochelata. Since the International Code of Zoological Nomenclature recognises no priority above the rank of family, many of the higher-level groups can be referred to by a variety of different names.
|Disease||Insect||Cases per year||Deaths per year|
|Malaria||Anopheles mosquito||267 M||1 to 2 M|
|Yellow fever||Aedes mosquito||4,432||1,177|
|Filariasis||Culex mosquito||250 M||unknown|