Arthropods

Arthropods are invertebrates such as insects, spiders and other arachnids, and crustaceans that comprise the phylum Arthropoda. The phylum Arthropoda includes three major classes—the Insecta, Arachnida, and Crustacea.

Arthropods are characterized by their external skeleton (exoskeleton), which is made mostly of chitin. Chitin is a complex, rigid carbohydrate usually covered by a waxy, waterproof cuticle. This integument is important in reducing water loss in terrestrial habitats, in providing protection, and in providing a rigid skeleton against which muscles can work. The exoskeleton is segmented, which allows for easy movement of the body, and there are numerous paired, segmented appendages, such as legs, antennae, and external mouth parts. Periodically, the entire rigid exoskeleton is shed; the temporarily soft animal swells in size; and its new, larger exoskeleton hardens.

Most arthropods have compound eyes, each with numerous lenses capable of forming complex, composite images. Arthropods have various mechanisms for the exchange of respiratory gases which, depending on the group, include gills, chambered structures known as book lungs, tracheal tubes, and various moist areas of the body surface.

Most arthropods exhibit sexual dimorphism; males look distinctly different from the females, at least in the appearance of their external genitalia. Arthropods have internal fertilization, and they lay eggs. Arthropods have a complex life cycle, generally involving eggs, a juvenile larval stage, and the adult form, with complex metamorphoses occurring during the transitions between these stages. In some insects, there is an additional stage between the larva and the adult, known as a pupa.

Arthropods are extremely diverse in species richness. Almost 875,000 living species of arthropods have been recognized and named, comprising more than 80% of all identified animal species. Many more species likely remain to be discovered. The scientific consensus is that these yet-to-be-discovered species are small beetles and other inconspicuous arthropods, and most of these occur in old-growth tropical rainforests, a biome that has not yet been well explored and studied by taxonomists and ecologists.

Species of arthropods utilize an enormous variety of Earth’s habitats. Most species of crustaceans are aquatic, although a few, such as woodlice and land crabs, occur in moist habitats on land. The spiders, mites, scorpions, and other arachnids are almost entirely terrestrial animals, as are the extremely diverse insects.

Some species of arthropods are very important to humans. A few species are important as vectors in the transmission of microbial diseases, such as malaria, yellow fever, encephalitis, plague, Chagas disease, and Lyme disease. Some arthropods are venomous, for example, scorpions, some spiders, and bees and wasps, and can hurt or kill people by single, or multiple, stinging. Some arthropods are a highly nutritious source of food for people, as is the case with lobsters, crayfish, shrimp, many species of crabs, and some insects.

However, the most critical importance of arthropods relates to the extremely diverse and beneficial ecological functions that they carry out. Arthropods play an important role in nutrient cycling and other aspects of ecological food webs. Earth’s ecosystems would be in a great deal of trouble if there was any substantial decline in the myriad species of arthropods in the biosphere.