Cockroaches are insects of the order Blattaria or Blattodea, of which about 30 species out of 4,500 total are associated with human habitats. About four species are well known as pests.
Among the best-known pest species are the American cockroach, Periplaneta americana, which is about 30 mm (1.2 in) long; the German cockroach, Blattella germanica, about 15 mm (0.59 in) long; the Asian cockroach, Blattella asahinai, also about 15 mm (0.59 in) in length; and the Oriental cockroach, Blatta orientalis, about 25 mm (0.98 in). Tropical cockroaches are often much bigger, and extinct cockroach relatives and 'roachoids' such as the Carboniferous Archimylacris and the Permian Apthoroblattina were not as large as the biggest modern species.
Cockroaches are generally rather large insects. Most species are about the size of a thumbnail, but several species are bigger. The world's heaviest cockroach is the Australian giant burrowing cockroach, which can reach 9 cm (3.5 in) in length and weigh more than 30 g (1.1 oz). Comparable in size is the Central American giant cockroach Blaberus giganteus, which grows to a similar length but is not as heavy.
Cockroaches have broad, flattened bodies and relatively small heads. They are generalized insects, with few special adaptations, and may be among the most primitive living neopteran insects. The mouthparts are on the underside of the head and include generalised chewing mandibles. They have large compound eyes, two ocelli, and long, flexible, antennae.