Cockroaches are insects of the order Blattodea, sometimes called Blattaria, of which about 30 species out of 4,60 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, contrary to popular opinion, extinct cockroach relatives and ‘roachoids’ such as the Carboniferous Archimylacris and the Permian Apthoroblattina were not as large as the biggest modern species.
A cockroach soon after ecdysis
A bush cockroach (Ellipsidion australe)
Cockroaches live in a wide range of environments around the world. Pest species adapt readily to a variety of environments, but prefer warm conditions found within buildings. Many tropical species prefer even warmer environments.
Cockroaches are mainly nocturnal and will run away when exposed to light. A peculiar exception is the Asian cockroach, which is attracted to light.
The spines on the legs were earlier considered to be sensory, but observations of their locomotion on sand and wire meshes have demonstrated they help in locomotion on difficult terrain. The structures have been used as inspiration for robotic legs.
Cockroaches leave chemical trails in their fæces, as well as emitting airborne pheromones for swarming and mating. These chemical trails transmit bacteria onto surfaces. Other cockroaches will follow these trails to discover sources of food and water, and also discover where other cockroaches are hiding. Thus, cockroaches can exhibit emergent behavior, in which group or swarm behavior emerges from a simple set of individual interactions.
Daily rhythms may also be regulated by a complex set of hormonal controls of which only a small subset have been understood. In 2005, the role of one of these proteins, pigment dispersing factor (PDF), was isolated and found to be a key mediator in the circadian rhythms of the cockroach.