Between 3500 and 4000 species of cockroaches have been identified, with one relatively simple classification scheme dividing this group into five families as follows:
• Cryptocercidae is the most primitive family and consists of one genus with fewer than 10 species. These cockroaches live as isolated family groups in decaying logs and occur in the United States, Korea, China, and Russia. They are large, reddish brown insects that are wingless at adults.
• Blattidae is a diverse family with many genera and hundreds of species. Those classified as Periplaneta and Blatta are widely distributed, while other genera are more regional. They are large insects that tend to live outdoors. Several species are referred to locally as palmetto bugs.
• Blattellidae is also a diverse family with many genera and around 1000 species. These cockroaches are widely distributed in the world but are concentrated in the tropics and subtropics. Blattellids are mostly small outdoor cockroaches, including those called wood cockroaches. The genus Blattella contains the German cockroach.
• Blaberidae is the largest family of cockroaches, with dozens of genera and more than 2000 species. These insects are widely distributed outdoors in tropical and subtropical regions. Some members of the genus Blaberus are extremely large, reaching more than 80 mm in length. These are the most highly evolved cockroaches, having developed the ability to incubate their eggs internally and, in some species, to nourish them.
• Polyphagidae is a small family with only a few described genera and 100 to 200 species. Females of most species are wingless. These cockroaches are widely distributed in harsh environments, such as deserts and other arid climates. Some members of the genus Arenivaga have evolved structures that can absorb moisture from humid air.
Other, more complex, classification schemes for cockroaches also exist, indicating that the subject is not entirely settled. In addition, insect collections from formerly unexplored locations often include many undescribed cockroach species. Thus, it is likely that the total number of extant species is much higher than the figures just given. Indeed, the vast majority of cockroaches live in the tropical regions of the world, many of which have not been adequately assessed to establish the diversity of insect life, including cockroaches, occurring there.
Cockroaches, being hemimetabolous insects, have egg, nymph, and adult stages and grow through a series of molts. They vary greatly in size, ranging from a few millimeters to over 100 mm in length. Many cockroaches are dark brown, but some are black or tan, and others show a surprising amount of color variation and cuticular color patterns. Most species have four wings as adults, and some are capable of rapid, sustained flight; others are wingless or have wings that are variously reduced in size. The majority of species are either nocturnal (or are hidden from view because of where they live), but some are diurnal. Cockroaches occupy diverse habitats, such as among or under dead or decaying leaves, under stones or rubbish, under the bark of trees, under drift materials near beaches, on flowers, leaves, grass, or brush, in the canopy of tall trees, in caves or burrows, in the nests of ants, wasps, or termites, in semiaquatic environments, and burrowing in wood. Thus, the common view of cockroaches as pests is not representative of the group as a whole.
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