How many arthropods have been described

Chelicerata are characterized as having two distinct body regions, a cephlothorax and an abdomen. Chelicerates have six pairs of appendages, the first two pairs being mouthparts and the following four pairs being legs. They do not have antennae. The first pair of mouthparts are the chelicerae sing. They are three-segmented and pincher-like. In spiders, the terminal third segment is often called a fang.

The second pair of mouthparts are the six-segmented pedipalps. They may appear leg-like spiders or claw-like scorpions. The legs of most Chelicerata have seven segments. Compared to insects, whose legs have six segments, there is an extra segment, called the patella, between the third segment the femur and the fourth segment the tibia. Common orders of Arachnida include:.

Order Araneae a-ran-e-uh , the spiders. No antennae. Abdomen joined to cephlothorax by slender pedicel. Four pairs of legs. Mouthparts: One pair chelicerae and one pair of pedipalps. Eyes simple ocelli. Usually eight, sometimes fewer. Poison apparatus opens on the fangs of the chelicerae. Silk apparatus always present at end of abdomen, below anus. This is a large order, having about species in North America.

Although nearly all spiders have venom glands, spiders seldom bite man. Only a few U. In Nebraska, the black widow and the brown recluse are the only seriously venomous spiders. Order Acari ak-a-ri , the mites and ticks. Abdomen broadly joined to cephlothorax no pedicel.

Four pairs of legs in adults; only three pairs at hatching. Ticks and mites occur just about anywhere animal life is found. They may be terrestrial or aquatic, free-living or parasitic.

They may feed on organic debris beneficial scavengers or on living plants or animals. Some of the plant feeders are serious pests of crops. Some of the parasitic forms are pests of animals and man. Some are vectors of diseases. A few of the predatory and parasitic mites are considered beneficial because they feed on insects or other pests. Over 30, species of ticks and mites have been described. Order Scorpiones scor-pi-on-es , the scorpions. No pedicel between cephlothorax and abdomen.

Four pairs of legs on cephlothorax. One pair of chelicerae and one pair of long, pincher-like pedipalpsAbdomen with seven broad segments anteriorly, followed by five narrower segments which end with a large stinger. Scorpions feed mostly on insects and spiders which they catch with their pedipalps. They may or may not sting their prey. Scorpions do not ordinarily attack man, but they will sting if disturbed or cornered. Of the forty-some species that occur in the U. One Arizona species, however, is very venomous, and its sting can be fatal.

Class Chilopoda chi-lo-po-da , the centipedes. Two tagmata head and trunk One pair of antennae with 14 or more segments. Mouthparts: one pair of mandibles and two pairs of maxillae. Appendages on the first trunk segment are clawlike poison jaws or fangs with which centipedes paralyze their prey.

The class name refers to the lip- like appearance of the fused bases of these appendages. Eyes may be present or absent. Centipedes are elongate and flattened.

They are usually found in somewhat protected places, such as in leaf litter, in the soil, under bark, or in rotten logs. One species is commonly found in houses and other buildings. Centipedes are predatory and feed on insects, spiders, and other small animals.

Sea spiders are known as pycnogonids pycno - means closely packed and gonid refers to gonidia, which is a group of asexually reproductive cells. Sea spiders are in the class Pycnogonida. They are not true spiders, or even arachnids. Sea spiders have long legs with relatively small bodies Fig. They are found across the ocean, with over species. Most are small even as small as 1 mm and live in relatively shallow depths.

However some deep and cold water species grow to over 90 cm. The subphylum Myriapoda is represented by land-dwelling arthropods such as centipedes and millipedes. This group contains over 13, species. There are no known marine myriapod species. They have a single pair of antennae and mouthparts roughly similar to those found in chelicerates. The main distinguishing feature of myriapods is the multiple jointed legs that extend from their elongated worm-liked bodies.

The subphylum name comes from Greek root words meaning countless feet. Myriapods have between 10 to legs. The class Insecta is another primarily terrestrial group of arthropods, although insects have been reported from nearly all environments with the exception of deep-sea habitats. Insects are the most diverse members of the subphylum Hexapoda Fig.

Hexapods have a distinct body plan, which includes three large sections: a head, thorax, abdomen with three pairs of thoracic legs and one pair of antennae Fig. Hexapods have three sets of jaws called mandibles, maxillae, and labium Fig. Insects breathe by taking in air through spiracles into trachea tubes. Unlike in the arachnids, these tubes do not terminate at book lungs, but branch into smaller networks of tubes called tracheoles that branch directly into the tissues of the insect for gas exchange Fig.

There is no active pumping of air, but any small movement in the insect body leads to airflow throughout the trachea. Along with their close relatives the centipedes and millipedes, insects have appendages that are unbranched. Their bodies are developed into three distinct segments of head, thorax and abdomen. The wings usually two pairs are contained on the thorax segment along with three pairs of legs.

Insect wings and legs are modified in many ways depending upon the lifestyle of the insect. In most insects, though, the legs include sensory receptors. Other sensory receptors are found on the body, and insects have compound eyes and light sensing ocelli on their heads.

Most marine arthropods belong to the subphylum called Crustacea Fig. Some species of shrimp, called krill, spend their lives as plankton, drifting in the surface waters of the open oceans. Some species of crustaceans, such as crayfish, are common in freshwater lakes and streams; a few—isopods and pill bugs, for example—live on land.

Many crustaceans are used as food, both by larger animals and by humans. While most crustaceans are mobile, one exception is the barnacle. Barnacles are free swimming as larvae, but when they metamorphose, the head reduces and is cemented to the substrate with a strong chemical glue. The barnacle secretes a hard protective shell around itself and filter feeds from inside this shell, using elongated appendages to collect food particles.

Due to their lack of mobility, barnacles are one of the few arthropods that are hermaphroditic. The crustacean body is clearly divided into a head, an abdomen, and a thorax. While the insects have a flexible joint between the head and thorax, crustaceans tend to have the two sections fused together.

Crustaceans have two antennae on the head, followed by mandibles , maxillae , and maxillipeds , all structures for handling food Fig.

The thorax contains the walking and swimming legs. The front pairs of appendages are often modified into claw-bearing appendages. The abdominal section contains appendages called swimmerets Fig.

These appendages produce water currents that pass over the gills for respiration. Female crustaceans typically use swimmerets to hold onto eggs for brooding , or keeping the eggs with them until they are ready to hatch. The abdomen also contains the tail appendages. Lobsters and crayfish have external structures that make it possible to identify their sex.

Certain social insects have large numbers in their nests. An ant nest in Jamaica was calculated to include , individuals. A South American termite nest was found to have 3 million individuals. Locust swarms are said to hold up to one billion individuals.

These great numbers of insect species and individuals were created by a number of factors including their long geological history, the capability of flight, their small size that allows survival in many various habitats, their ability to store sperm for delayed fertilization, and their general adaptive abilities to the environment.

Insects have remarkable fertility and reproductive abilities, which have usually led to the vast numbers of individuals in nature. East African termite queens have been recorded to lay an egg every two seconds, amounting to 43, eggs each day. To appreciate the population potentials of insects the example of the housefly is sometimes used, stating that the descendants of one pair of this insect, provided that they all survived during a five month season, would total quintillion individuals.

Recent figures indicate that there are more than million insects for each human on the planet! A recent article in The New York Times claimed that the world holds pounds of insects for every pound of humans. Erwin, T. Tropical forest canopies: the last biotic frontier. Bulletin of the Entomological Society of America, Volume Janzen, D.