Arthropods

Arthropods (jointed legs) include the spiders, centipedes, millipedes, insects, and crustaceans. They represent an evolutionary advance that allowed them to form the greatest diversity and number of species of all the phyla, about 1 million species known. The advance was the development of a jointed exoskeleton made of chitin, a nitrogenous poly-saccharide, bound with protein. In crustaceans the exoskeleton also contains calcium salts for added strength. Other innovations with this phylum are increased specialization of the body segments; locomotion via muscles in external appendages; improved sensory capabilities; more efficient respiratory organs, which enables high activity rates and even flight; and even the development of social organization.

Because the rigid exoskeleton places limits on the organism's growth, it must shed its covering periodically in a process called molting. It then secretes a new cuticle, which hardens into a new shell. Each stage in the life of an arthropod between molts is called an instar.

Besides taking on virtually all the kinds of ecological roles there are, arthropods are essential to the survival of many other species, from the many flowering plants that depend on insects to pollinate them, to the many larger animals that depend on them for food. In aquatic ecosystems, for example, microscopic crustaceans form much of the zooplankton, which forms the first animal level of the food chain, and insect larvae are the food source for many fish species.

There are numerous arthropod classes (see Table 8.2). We focus on several of the most familiar and ecologically important. The diversity of this group is so great that we will

TABLE 8.2 Classification of Familiar Arthropods

Phylum Subphylum Class Order

Arthropoda

Trilobites (extinct) Chelicerata

Merostoma [includes horseshoe crabs (Limulus)]

Pycnogonida (sea spiders)

Arachnida

Araneae (spiders) Scorpionida (scorpions) Opiliones (harvestmen, or ''daddy longlegs'') Acari (ticks and mites) Uniramia (two classes not shown)

Chilopoda (centipedes) Diplopoda (millipedes)

Insecta (the terrestrial mandibulates; 16 of 27 orders shown) Anoplura (lice)

Coleoptera (beetles, fireflies, weevils)

Dermaptera (earwig)

Diptera (true flies, mosquitoes)

Ephemeroptera (mayfly)

Isoptera (termites)

Lepidoptera (moths, butterflies)

Hemiptera (true bugs, bedbugs, water striders)

Homoptera (aphids, cicadas)

Hymenoptera (bees, wasps, ants)

Neuroptera (lacewing, dobsonflies)

Odonata (dragonfly)

Orthoptera (roaches, grasshoppers, locusts, crickets, mantids) Plecoptera (stoneflies) Siphonaptera (fleas) Trichoptera (caddis flies) Crustaceans (the aquatic mandibulates; two minor classes not shown) Branchiopods (several other classes not shown) Anostraca (brine shrimp) Cladocera (water fleas, or Daphnia)

Maxillopoda Subclass Ostracoda Subclass Copepoda (e.g., Cyclops) Subclass Cirripedia (includes barnacles) Malacostraca (14 orders, not all shown)

Isopoda (pill bugs or sow bugs) Amphipoda (scuds, sideswimmers) Euphausiacea (krill)

Decapoda (crayfish, lobsters, crabs, ''true'' shrimp)

find numerous familiar species to be in surprising relationships. For example, horseshoe crabs are more closely related to spiders than to crustaceans, and the "daddy longlegs'' is not really a spider. The ''pill bugs'' often found upon overturning a rock in the garden are not insects, but one of the few terrestrial crustaceans.

Arthropods share the segmentation of the annelids; however, the segments are fused into functional parts. For example, the insects have three such parts: the head, thorax, and abdomen. In spiders and crustaceans the head and thorax are further fused into a cephalothorax.

Arachnids include the spiders and several other orders. There are about 35,000 species of spiders. All are venomous predators. However, few are dangerous to humans, not even the tarantula, Rhechostica hentzi. Among the few that are dangerous are the black widow spider, Latrodectus mactans, and the brown recluse, Loxosceles reclusa. These can be recognized by a red ''hourglass'' shape on the bottom of the abdomen and a "violin-shaped" mark on the cephalothorax, respectively. Even most scorpions are not very dangerous, except, again, for certain species in Africa and Mexico. Arachnids have six appendages, but only four are used as legs. The other two are modified into poison fangs. The protinaceous web is spun from an organ on the abdomen, producing silk that is stronger than steel of the same thickness.

Ticks and mites are the most abundant arachids and the most important from an economic and medical viewpoint. They attach to plants and animals, puncturing their surface to suck out fluids or blood. Besides the direct harm this causes, they are the most important insect disease vector after mosquitoes. They spread Lyme disease and Rocky Mountain spotted fever in humans, as well as many cattle diseases.

Crustaceans and insects make up some 80% of all the named species of animals. Whereas insects dominate the land, crustaceans rule the waters. In fact, it may be that the most abundant animals in the world belong to the copepod genus Calanus, a zooplankter.

Crustaceans are distinguished from other arthropods in that they always have two pairs of antennas, and their chitin exoskeleton is hardened with calcium salts. The head and thorax is fused into a cephalothorax, as with spiders. Each of the typically 16 to 20 body segments has a pair of appendages, adapted for various uses, such as grasping, walking, and swimming. They have gills for respiration. An example is the lobster, a decapod (Figure 8.3).

Hold a glass of pond water up to the light and you will see "specks" swimming or darting about. Most likely, you will be seeing one of the planktonic crustaceans: cladocer-ans, ostracods, or copepods (see Figure 15.10). The cladoceran Daphnia spp. are also called water fleas. They feed on algae. Daphnia, along with the brine shrimp, are important in the laboratory for their use in toxicity testing. Ostracods resemble the cladocerans, except that they are enclosed by a two-piece carapace that makes them look like a very small clam. As mentioned above, copepods are extremely common. In the oceans the copepod Calanus and the euphasid krill form the major food source for herring, menhaden, sardines, and some whales and sharks. The copepod Cyclops is very common in freshwater systems.

Barnacles start their lives as planktonic forms resembling copepods. Eventually, most attach to a solid surface and form the familiar shell of calcareous plates that we see on rocks exposed by low tide. When covered with water, they open the slitlike apertures and wave their fanlike legs to filter food particles from the water.

The class Malacostraca is the most diverse. It contains the most familiar crustaceans of all, the decapods, which include crabs, lobsters, and shrimp. As the name implies,

Arthropods Crabs Respiratory System

Walking legs

Figure 8.3 Typical body plan of the lobster, order Decapoda, class Malacostraca, subphylum crustacean. (From Hickman et al., 1997. © The McGraw-Hill Companies, Inc. Used with permission.)

Walking legs

Figure 8.3 Typical body plan of the lobster, order Decapoda, class Malacostraca, subphylum crustacean. (From Hickman et al., 1997. © The McGraw-Hill Companies, Inc. Used with permission.)

decapods have five pairs of walking legs, the first of which may be modified into a claw. The euphasids have only about 90 species but form the important marine food source called krill. Most of them are bioluminescent. The amphipods have a laterally flattened form with immobile compound eyes and two types of legs. For example, one set of legs may be for swimming, the other for jumping. An example is Orchestia, the beach hopper. Isopods are the only crustacean group to include terrestrial species, such as the familiar pill bug. They have a flattened form and compound eyes. Although adapted for land, they are not efficient at conserving water and require moist conditions to survive. Some are parasites of fish or other crustaceans.

Around 1 million species of insects have been named, and this is thought to be only a fraction of the number that exist. There are more species of insects than of all other animals combined. They dominate the land and fresh water, although few marine insects exist. Their success is attributed to the arthropod characteristics, plus their small size and their development of flight. Table 8.2 shows some of the variety in the insect class. In all there are 27 orders. The word "bug" is popularly used to denote any insect, or sometimes any small organism. Environmental engineers often refer to the organisms populating biological wastewater treatment plants, including bacteria and protozoans, as bugs. To an entomologist (a scientist who studies insects), however, a "bug" is an insect of a single order only, the Hemiptera.

An insect is an arthropod with three pairs of legs and (usually) two pairs of wings on the thorax. Some insects have only one pair of wings or none at all. For example, the true flies (order Diptera) have only one pair, and female ants and termites have wings only at certain times. Lice and fleas have no wings. The head of an insect usually has two large compound eyes and one pair of antennas. Most insects eat plants, although parasitic insects are common. Some are predators on other insects or animals. Fleas feed on the blood of mammals. Many wasps lay their eggs in or on spiders, centipedes, or other insects, on which the larvae feed after hatching. Many predatory insects benefit humans by attacking crop pests. Many larvae and beetles eat dead animals.

To accommodate the high oxygen consumption rate necessary for flight, insects have evolved an efficient tracheal system, consisting of tubes to transport air directly to the tissues. Insects, along with arachnids, have evolved an efficient water-conserving excretion mechanism that eliminates nitrogen waste in the form of insoluble uric acid.

Insects are dioecious with internal fertilization. Most female insects lay a large number of eggs. The eggs must be laid in highly specific settings. For example, the monarch butterfly must lay its eggs on milkweed plants, which its caterpillar stage needs to eat. Some insects lay their eggs on water, and their juvenile stages are aquatic.

About 88% of insects go through complete metamorphosis, in which the juvenile form is radically different from the adult insect. The hatchling of an insect that undergoes this process is wormlike and is called a larva, also known as a caterpillar, maggot, grub, and so on, depending on the species. The larva grows through a series of molts and eventually surrounds itself with a cocoon, transforming into a transitional nonfeeding stage called a pupa or chrysalis. When it molts out of this stage, the result is an adult, a form that no longer molts. Other insects undergo gradual metamorphosis, in which the juvenile somewhat resembles the adult and becomes an adult by gradual change. The juvenile forms are called nymphs. Examples of species that have nymphs are grasshoppers, cicadas, mantids, and terrestrial bugs. Both larvae and nymphs pass through several instars before going on to the next stage.

Besides those that attack crops, many insects harm humans directly. Mosquitoes transmit malaria, encephalitis, yellow fever, and filariasis. Fleas transmit plague; the housefly, typhoid; and the louse, typhus fever. Our battle with insects has had signal success—only for us to have found that in some cases, the cure is worse than the disease. Unrestricted use of pesticides starting after World War II had become a crisis by the 1960s, giving rise to the modern environmental movement. Elimination of the more persistent pesticides and more careful use of others has greatly reduced environmental and health risks today. However, alternative approaches are being developed. Parasitic wasps are sometimes cultured to fight crop pests. Viral, bacterial, and fungal insect pathogens are being recruited. For example, the bacteria Bacillus thuringiensis has been found to be effective against lepi-dopterans (moths and caterpillars). A strategy called integrated pest management (IPM) combines plant culturing techniques such as crop rotation and selection of resistant varieties with minimum timed use of pesticides to control insects without the heavier pesticide dose that would otherwise be needed.

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Responses

  • Asmeret
    Why are arthropods thought to be?
    3 years ago
  • ABDULLAH
    What disease is transmited to humanby shrimps crabs lobster water flies copepods?
    2 years ago

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