"Roachoids",[1] also known as "Roachids", "Blattoids"[2] or Eoblattodea,[3] are members of the
stem group of
Dictyoptera (the group containing modern
cockroaches,
termites and
praying mantises). They generally resemble cockroaches, but most members, unlike modern dictyopterans, have generally long external
ovipositors, and are thought not to have laid
ootheca like modern dictyopterans.
Systematic position
Cockroaches are popularly thought to be an ancient order of
insects, with their origins in the
Carboniferous.[4] However, since the middle of the 20th century it has been known that the primitive cockroach insects found fossilized in
Palaeozoic strata are the forerunners not only of modern cockroaches and termites but also of
mantises.[5] The origin of these groups from a blattopteran stock are now generally thought to be in the Early
Jurassic; the earliest modern cockroaches appeared during the Early
Cretaceous.[6] Thus the “Palaeozoic cockroaches” are not cockroaches per se, but a
paraphyletic assemblage of primitive relatives.[7] The youngest known roachoids date to the Cretaceous, by which time they were rare compared to modern cockroaches.[8]
Anatomy and habits
The fossils assigned to the "roachoids" are of general cockroach-like build, with a large disc-like
pronotum covering most of the head, long
antennae, legs built for running, flattened body and heavily veined wings with the distinct arched CuP-vein so typical of modern cockroach wings.[9] Like modern cockroaches, the roachids were probably swift litter inhabitants living on a wide range of dead plant and animal matter.
Contrary to modern forms, female roachoids all have a well-developed external
ovipositor. They probably inserted eggs into substrate. The egg pods, called
ootheca, seen in modern dictyopterans is a new shared trait (
synapomorphy) separating them from their primitive ancestors.[10] Some of the roachoid species could reach relatively large sizes compared to most of their modern relatives, like Progonoblattina[11][12] and Necymylacris[13][14] from
Carboniferous reach around 9 centimetres (3+1⁄2 in) in total length, and the largest Opsiomylacris having wings reaching 7.5 centimetres (3 in), close to modern largest cockroach Megaloblatta longipennis.[12]
^Guthrie, D. M. & A. R. Tindal (1968): The Biology of the Cockroach. St. Martin's Press, New York
^Grimaldi, D (1997): A fossil mantis (Insecta: Mantoidea) in Cretaceous amber of New Jersey, with comments on early history of Dictyoptera. American Museum Novitates 3204: 1–11
^Schneider, J. (1983): Die Blattodea (Insecta) des Paleozoicums, Teil II, Morphogenese des Flügelstrukturen und Phylogenie. Freiberger Forchnungshefte, Reie C 391. pp 5-34