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Taxonomy

The classification of the Phasmatodea is complex and the relationships between its members are poorly understood. Furthermore, there is much confusion over the ordinal name. Phasmida is preferred by many authors, though it is incorrectly formed; Phasmatodea is correctly formed, and is widely accepted. However, Brock and Marshall argue:

Phasmida is the oldest and simplest name, first used by Leach in 1815 in "Brewster’s Edinburgh Encyclopaedia" volume 9, p. 119, and widely used in major entomological textbooks, dictionaries and many scientific papers and books on phasmids. As there is no compulsion to select the "grammatically correct" name, selection of a long established (and simple) name is reasonable, although the probability of persuading all colleagues to agree on the use of Phasmida is unlikely.

The order Phasmatodea is sometimes considered to be related to other orders, including the Blattodea, Mantodea, Grylloblattodea, Mantophasmatodea and Dermaptera, but the affiliations are uncertain and the grouping (sometimes referred to as "Orthopteroidea") may be paraphyletic (not have a common ancestor) and hence invalid in the traditional circumscription (set of attributes that all members have). Phasmatodea, once considered a suborder of Orthoptera, is now treated as an order of its own. Anatomical features separate them as a monophyletic (descended from a common ancestor) group from the Orthoptera. One is the instance among all species of Phasmatodea of a pair of exocrine glands inside the prothorax used for defense. Another is the presence of a specially formed sclerite (hardened plate), called a vomer, which allows the male to clasp the female during mating.

The order is divided into two, or sometimes three, suborders. The traditional division is into the suborder groups Anareolatae and Areolatae, which are distinguished according to whether the insect has sunken areola, or circular areas, on the underside of the apices of the middle and hind tibiae (Areolate) or not (Anareolate). However the phylogenetic (evolutionary) relationships between the different groups is poorly resolved. The monophyly of Anareolatae has been questioned and the morphology of the eggs may be a better basis for classification. An alternative is to divide the Phasmatodea into three suborders Agathemerodea (1 genus and 8 species), Timematodea (1 genus and 21 species) and Euphasmatodea (or Verophasmatodea) for the remaining taxa. This division is, however, not fully supported by the molecular studies, which recover Agathemerodea as nested within Euphasmatodea rather than being the sister group of the latter group. Recent taxonomic treatements recognise two suborders, with Agathemeridae placed in Pseudophasmatoidea within Euphasmatodea and Agathemerodea treated as nomen dubium

While suggestions have been made that various insects extending back to the Permian epoch represent stem-group phasmatodeans, the earliest unambigious members of the group are the Susumanioidea, which first appeared during the Middle Jurassic, and usually have two large pairs of wings. Modern phasmatodeans first appeared during the Early Cretaceous, with the cuirrently oldest known being Araripephasma from the Early Cretaceous (Aptian) Crato Formation of Brazil, around 113 million years old, which can be confidently assigned to the Euphasmatodea.

The earliest leaf insect (Phylliinae) fossil is Eophyllium messelensis from the 47-million-year-old Eocene of Messel, Germany. In size and cryptic (leaflike) body form, it closely resembles extant species, suggesting that the behavior of the group has changed little since that time.

Over 3,500 species have been described, with many more yet to be described both in museum collections and in the wild.