There are about 2,500 species of stick-insect described worldwide. Stick-insects are hemimetabolous (they do not have a pupal stage in their development). Most species are native to the tropics. Many stick-insects are extremely long and thin, mimicking sticks, though others mimic leaves with large broad forewings and flat appendages on the sides of the abdomen and thighs. Many species are wingless, especially females. Stick insects are herbivorous, some specialising in individual food plants. They are nocturnal, sitting motionless on branches during the day. Some species can change their colour depending on lighting, temperature and moisture.
Several stick-insect species are parthenogenetic, meaning they reproduce asexually – females lay unfertilised eggs which develop into new females. Males and sexual reproduction do occur in these species, though very rarely.
Three stick-insect species form colonies in the wild in Britain, the Prickly Stick-insect Acanthoxyla geisovii, the Smooth Stick-insect Clitarchus hookeri, and the Unarmed Stick-insect Acanthoxyla inermis. All are native to New Zealand and almost certainly arrived here on imported plants, either as nymphs or adults or, more likely, eggs.
The Phasmida Species File is a taxonomic database of the world's Phasmida.
The Phasmid Study Group is a worldwide organisation dedicated to the study and captive rearing of stick and leaf-insects (aka phasmids).
Wikipedia article on Phasmida