The Major Groups of Terrestrial Arthropods

SUBPHYLUM CHELICERATA: Arthropods with two pairs of head appendages used for feeding (chelicerae) and modified for various functions (pedipalps). All terrestrial species are predatory, with exception of the highly diverse mites.

CLASS ARACHNIDA:

Acarina (mites). The Acarina constitute the largest group of arthropods besides insects, with approximately 50,000 species known, and an exceptional diversity of forms and habits. Because of their obscure size, most still need to be described. They include species that feed on fungi and plant tissues, and those that cause plant galls; they are predators and parasites of a great range of arthropods and other animals. Typical acarine parasites of vertebrates include mange (Psoroptidae), chiggers (Trombiculidae), and the blood-feeding ticks (Ixodida).

Araneae (spiders). The Araneae are an ancient group, known first from the Devonian, approximately 400 million years ago. Most spin silk for aerial webs, snag lines, or drag-lines on the ground, to either snare or detect arthropod prey. Some, such as wolf spiders (Lycosi-dae), fishing spiders (Pisauridae), crab spiders (Thomisidae), lynx spiders (Oxyopidae), and jumping spiders (Salticidae) stalk prey and spin little or no silk. Scytodes "spits" a poisonous "glue" to subdue its prey. Some 40,000 species of spiders range in size from barely a millimeter in length to the "bird spider" tarantulas of South America, 20 cm (8 inches) in leg spread. All spiders secrete venom for killing prey, among the most toxic being Lactrodectus (black widows, red backs) and Loxosceles (brown recluses), which occur in warm temperate and tropical regions. Less well known are the funnel-web spider of Australia (Atrax), and probably the most venomous and aggressive of all, the Ctenidae of South America. The bite of one ctenid, Phoneutria fera, secretes a poison that can kill 300 mice. Despite their reputation and even the common phobias about them, spiders are important as among the most notable predators of terrestrial arthropods.

Pseudoscorpionida (pseudoscorpions). Resembling miniature, tail-less versions of true scorpions, generally only 4 mm in length or less, pseudoscorpions are predatory on small organisms such as nematodes and mites. They live in forested habitats among leaf litter and under bark and are much more diverse than scorpions, with approximately 3,000 known

Desert hairy scorpion on gravel (William Dow/Cor-bis)

species. Some disperse via phoresy—attaching to another, more mobile animal, generally a winged insect.

Scorpionida (Scorpions). A well-known group of predatory arachnids that inhabits xeric and tropical environments, scorpions are perhaps 400 million years old and closely related to extinct eurypterids (all of which were marine); the modern fauna of only 1,500 species appears to be a remnant of past diversity. As with spider venoms, toxicity of the sting varies greatly. Species with lethal venom include Centruroides sculpturatus of the American Southwest.

Opilionida (harvestmen, daddy-longlegs). Opilionida are familiar arachnids with eight stiltlike legs, though some tropical species have armored bodies with short legs.

Palpigrada, Ricinuleida, Schizomida, Uropygida, Solpugida, and Amblygygida. These are relatively minor orders of arachnids, with only a few dozen to several hundred species each. The first three orders are small, obscure, and rather generalized in structure. The last three contain some very impressive, swift predators with large, fearsome-looking pedipalps.

SUBPHYLUM CRUSTACEA:

Isopoda and Amphipoda ("pillbugs," beach "fleas"). Most species of these orders are marine; terrestrial species are detritivores. They include the familiar pill bugs, some of which roll into a tight ball when disturbed (for example, Armadillium). Terrestrial amphipods occur among leaves of some wet, tropical forests.

SUBPHYLUM TRACHEATA: This subphylum includes those arthropods that respire via tracheae, and that have uniramous (unbranched) appendages. They are also called Atelocer-ata, in reference to the single pair of antennae.

CLASS DIPLOPODA (millipedes): Most millipedes have thirty or more segments, with most segments bearing two pairs of legs, though newly hatched millipedes have three pairs and gradually add more segments with each molt. Polyxenid millipedes are unusual in having only thirteen pairs of legs in adults and a body covered with tufts of fine scales. Openings to the reproductive organs are near the head in millipedes, not the posterior end. Most species are detritivores or phytophagous; a few are predatory. Many secrete noxious or even toxic substances for defense, and some are warningly colored (aposematic).

CLASS CHILOPODA (centipedes): Centipedes are elongate, flattened forms with fifteen or more pairs of legs, and only one pair per segment. Reproductive openings are at the posterior end of the body. All species are active predators, killing prey with a pair of highly modified first pair of legs, the poison "jaws." Most species are innocuous, but larger ones (up to 25 cm, approximately 12 inches) can inflict bites that are very painful or even deadly to humans.

CLASSES PAUROPODA, SYMPHYLA: These classes consist of small (1-8 mm in length), whitish arthropods that live under rocks and among leaves, with ten to twelve pairs of legs. Unlike millipedes they have only pair of legs per segment, but like millipedes they have the genital openings near the head. All are detritivores.

CLASS HEXAPODA (INSECTA): Insecta are terrestrial arthropods with three pairs of legs. Primitive forms have no wings, though some species of pterygotes (winged insects) have wings that are highly reduced or entirely lost (these generally live in habitats with severe climates, or burrow, or are parasites).

Orders Diplura, Protura, and Collembola (springtails): These three orders comprise approximately 10,000 species of usually small (1-5 mm), obscure hexapods living among stones, logs, and leaf litter. Proturans have lost antennae, with forelegs instead modified for that function. Collembola usually have a furcula that allows them to spring into the air. Both Collembola and Protura have mouthparts recessed into a pocket in the head. All are detritivores. Diplurans have a pair of pincer-like forceps that they use for preying on small arthropods.

Orders Microcoryphia (bristletails) and Thysanura (silverfish). These are flat, scaled, wingless insects that look superficially similar, though the thysanurans are actually more closely related to the winged insects. Some are inquilines in the nests of social insects.

Order Ephemeroptera (mayflies): Mayflies are the most primitive living winged insects, and the only insects in which a molt occurs in the winged stage. Nymphs are detritivores in freshwater. Adults typically emerge en masse ("hatches," to fishermen) to live for a day or less; they have vestigial mouthparts and do not feed, and they form mating swarms that are sometimes of immense size. Dying mayflies from huge swarms have been known to fall to the ground several feet thick. They form an exceptionally important base of aquatic food chains, particularly as food for fish. There are approximately 3,000 species.

Order Odonata (dragonflies, damselflies): This order consists of approximately 5,000 species of predatory insects with large compound eyes; sharp, toothed mandibles; a long, rudderlike abdomen; and strong, maneuvered flight. Dense spines on the legs are used to seine midges and other small insect prey in air, or they pluck insects from plant stems. Very long, thin pseudostigmatine damselflies of the American tropics pluck insect prey from spider webs. Nymphs are aquatic predators.

Order Plecoptera (stoneflies): There are approximately 2,000 species of Plecoptera, with an aquatic nymph stage that is often found clinging to stones in cold, clear streams and lakes. Most nymphs graze on films of algae, though some are predaceous. Adults are flattened and rather generalized in structure. Some males attract females by drumming their abdomen on a branch with a frequency characteristic of the species.

Order Orthoptera (crickets, grasshoppers, katydids): These are primarily herbivorous insects, with enlarged hind femora used for jumping, though some do not jump (mole crickets and analogs) but have molelike forelegs for burrowing. Some, such as a few katydids (Tettigoniidae) and eumastacid grasshoppers, are predatory. Pseudophylline ("false leafed") katydids are exceptional mimics of leaves, complete with leaf veins, splotches, and even chew marks. Orthopter-ans are probably best known for their songs, produced to attract mates. The sounds are produced by stridulation—generally rubbing a "file" structure against a scraper, either on the wings, the legs, or even the mandibles.

Order Phasmida (stick insects): These are all cryptic mimics of the leaves and branches on which they feed; they also protect themselves with noxious secretions. The longest insects are some phasmids that reach well over a foot in length.

Orders Grylloblattodea (rock crawlers), Zoraptera, Dermaptera (earwigs), and Embi-idina (web spinners): The first two orders are the smallest insect orders, with only 20 and 40 species, respectively; there are 1,200 species of Dermaptera and 400 species of Embiidina. Most are very generalized in structure, though earwigs typically have a pair of strong, terminal forceps and short, leathery forewings (hemelytra). Web spinners have bulging, glandular foretarsi that secrete silk for housing their colonies. Dermaptera are worldwide; Zoraptera and Embiidina are primarily tropical; and rock crawlers, which are wingless, are considered a highly relict order that occurs only near or among glaciers in northern North America and Asia. All are detritivorous.

Dictyoptera: Orders Blattodea (cockroaches), Mantodea (mantises), Isoptera (termites). Although biologically very different, termites and mantises appear to have evolved from cockroaches approximately 150 million years ago. All 2,200 species of termites are highly social, living in colonies with specialized castes, including individuals who reproduce, tend the nest and young, or defend the colony. All termites feed on plant cellulose (some on fungi), which they digest via symbiotic protists or bacteria in their gut. All 1,500 species of mantises are cryptic for ambushing prey; those that mimic flowers feed on pollinating insects. Early "roachoids" possessed ovipositors some 100 to 300 million years ago; true roaches (approximately 5,000 species) have lost an ovipositor and usually lay their eggs in a hardened egg sac, the ootheca.

Orders Psocoptera (bark/book "lice"), Phthiraptera (lice), Thysanoptera (thrips): These are all small insects with cryptic habits. Most of the 3,000 species of Psocoptera are winged and live in leaf litter, under bark, or stones, where they are detritivorous. Thrips probably evolved from primitive Psocopterans approximately 250 million years ago. They have distinctive, narrow wings with a fringe of long hairs. Some feed on fungi, others are predatory, and many feed on plants. The Liposcelidae include the common "book lice" (Liposcelis) psocopterans, which are wingless and also found in mammal nests. This family probably gave rise to the true lice, the Phthi-raptera (6,000 species), all of which are wingless ectoparasites of birds and mammals. Some lice species have chewing mouthparts that they use for feeding on keratin, which composes hair, feathers, and skin. Sucking lice have piercing mouthparts and feed on blood. Groups and species of lice are usually very host specific. Human lice are Pediculus humanus (Pediculidae) and Pthirus pubis (Pthiridae), the former of which is more slender. The only other species of Pthirus is one occurring on gorillas.

Hemiptera: The Hemiptera are a group comprising the orders Heteroptera (predatory and plant bugs), Auchenorrhyncha (tree hoppers, plant hoppers), Sternorrhyncha (aphids, whiteflies, scale insects), and the tiny order Coleorrhyncha. All 100,000 species have mouthparts modified into a thin tube for piercing and sucking fluids from plants, other insects, or vertebrate blood. All auchenorrhynchans and sternorrhynchans feed on plant fluids; only some of the so-called higher heteropterans feed on plants, such as the stink bugs (Pentatomi-dae) and the plant and seed bugs (Miridae and Lygaeidae), among others. Most of the primitive Heteroptera are predatory, including water boatmen (Corixidae), backswimmers (Noto-nectidae), and water striders and their relatives (Gerridae and others). Some Belostom-atidae (giant water bugs) can reach 10 cm (nearly 4 inches) in length and feed on small fish and tadpoles. Bedbugs (family Cimicidae) feed on the blood of birds and mammals. The ones that attack humans, Cimex lectularius and C. hemipterus, do not transmit diseases, though the bites are very irritating.

Order Neuropterida (snakeflies, dobsonflies, lacewings, antlions). Most of the 5,000 species composing the Neuropterida are predators as adults and larvae; some are larval ectoparasites of spiders (Mantispidae). The larvae of many have long mandibles with a groove through which seeps poison and enzymes that kill and liquefy the contents of prey. Antlions are larval myrmeleontids that trap ants in conical pits. "Owlfly" larvae (Ascalaphidae) are similar to antlions but are cryptic on bark and in leaf litter and ambush prey with huge, sickle-shaped mandibles.

Order Coleoptera (beetles): The most diverse group of insects or any arthropod, the Coleoptera contain 350,000 known species. They are morphologically conservative, the main feature being the pair of elytra, or hardened forewings. Most of the diversity in the group is due to several huge families, particularly the so-called Phytophaga or the leaf beetles (Chrysomelidae), long horns (Ceramby-cidae), and weevils (Curculionoidea), all of which account for 150,000 species alone. Several predatory families are also very diverse, especially the Carabidae and Staphylinidae, comprising nearly 100,000 species. The order has some of the smallest adult insects (Ptiliidae, to 0.2 mm length) as well as ones with the most body mass (African Goliathus scarabs). The plethora of beetle species bore in wood and stems; feed on leaves, fungi, fruit, and carrion; and are predatory. Only a few families have larvae that are parasitic on other arthropods. Only one species has evolved into an ectoparasite of vertebrates: Platypsylla castoris, on the American beaver.

Orders Mecoptera (hangingflies, scorpi-onflies) and Siphonaptera (fleas): All 2,500 species of fleas are wingless, blood-sucking ectoparasites of birds and mammals. Recent studies indicate their close relationship to the Boreidae, a family of Mecoptera. Boreids, or snow scorpionflies, have rudimentary wings. They are found in cold, mossy habitats in the Northern Hemisphere, where they are often seen walking on the snow in late winter and early spring. Most of the other 500 species of scorpionflies have a more generalized morphology with a long face. Male Panorpidae have large genitalia curled over the abdomen, scorpionlike, hence the common name. They are predaceous and scavengers.

Order Strepsiptera (twisted-winged parasites): This is an extremely unusual order of 500 species with very controversial relationships,

A worker bee builds a wax comb. (Frank Lane Picture Agency/Corbis)

perhaps related to Coleoptera or to Diptera. The eyes and facets are large, like clusters of grapes; males have hind wings with very reduced venation and forewings reduced into small clubs, or halters. Females are wingless and resemble larvae; they live inside the host insect that they parasitize.

Order Diptera (true flies): Most adult flies have one pair of wings, the hind pair reduced to halters (a few species have lost all wings). This is an order of exceptional ecological and morphological diversity, with 100,000 known species and probably more "unknown" species than any other group of insect. Of prime medical importance, the group contains many blood-sucking species: phlebotomine sandflies, mosquitoes (Culicidae), blackflies (Simuliidae), horseflies and deerflies (Taban-

idae), and tsetse (Glossinidae), among others. Many primitive flies (nematocerans) have aquatic larval stages. Some are parasitoids of other insects (for example, Tachinidae); decomposers of carrion (blowflies and fleshflies, Calliphoridae and Sarcophagidae); predators (Asilidae); and pollinators (Bombyliidae, Syr-phidae). The small fruitfly Drosophila melanogaster is used extensively in genetic research and is the best known complex eukaryote.

Order Hymenoptera (wasps, bees, ants): The Hymenoptera are renowned for the great societies constructed by ants (family Formici-dae), which can number millions of individuals for some species of army ants and leafcutter ants. All 12,000 species of ants have advanced sociality (eusociality), meaning that there are specialized castes. In Hymenoptera, the workers, soldiers, foragers, and nurses are all sisters. Eusociality also occurs in some bees (honeybees [Apis, Bombus] and meliponine stingless bees), as well as in vespid wasps (paper wasps/yellow jackets, potter wasps). Sociality in Hymenoptera probably evolved because of the genetic mechanism that determines sex (haplodiploidy), which makes sisters more closely related to each other than to any offspring they could bear themselves. Thus, their genetic fitness is maximized by raising sisters, not their own offspring. Bees (Apoidea), of approximately 20,000 species, are probably the most ecologically important group of insects, since they are the most important group of angiosperm pollinators (see Pollination). Pollen and nectar are collected to provision their larvae. Bees and ants belong to a group of families (the Aculeata) that has an ovipositor modified into a sting for defense. All Hymenoptera have an ovipositor, but in many other forms it is used to lay eggs into host insects within which the larvae develop, consuming the host. These parasitoid wasps are extremely important in regulating the populations of other insects, such as defoliating caterpillars.

Order Trichoptera (caddisflies): This order is closely related to the order Lepidoptera (moths and butterflies), but biologically they are very different. Larval caddisflies live in freshwater, where most construct cases from sand grains, pebbles, or bits of vegetation. Lepidoptera are terrestrial, and the great majority of them have larvae that feed on angiosperms. This order is the largest lineage of plant-feeding organisms (with approximately 110,000 species), and it also appears to be one of the youngest orders. The earliest Lepidoptera appeared approximately 150 million years ago, but they showed little evolution until the angiosperms radiated 100 million years ago. Butterflies (Papilionoidea) are a group of five families of active, day-flying moths that advertise themselves with broad wings having warning or sexual coloration. Lepidoptera are extremely significant ecologically as pollinators and phytophages.

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