Or in other words, like the unholy offspring of a mantis and a medieval mace. Gerarus was the proud owner of an inflated thorax, liberally studded with prominent don't-f***-with-me spines up to a millimetre in length. Wings of different individuals were notably variable in their venation patterns, suggesting relaxed selectional pressure, and this together with the shift in weight that would have resulted from the hypertrophied thorax suggest that Gerarus was probably not a very active flier (Béthoux & Nel, 2003). Instead, it would have clambered on vegetation like a stick insect, relying primarily on its spines to dissuade potential predators. If that wasn't enough, it could escape by jumping and using its wings as passive gliding planes.
The aforementioned variability of Gerarus and the other members of the family Geraridae led to earlier authors describing nearly every specimen as a separate species. Many of these were synonymised by Burnham (1983) in her review of the family, but it is quite possible that the group is still over-split. Initially, gerarids were included in the "Protorthoptera", an unabashedly paraphyletic or polyphyletic grouping of Palaeozoic polyneopteran-grade insects that was believed to be ancestral to such modern groups as cockroaches, crickets and stick insects, and possibly even to all other recent neopteran insects. When some more specific affinity was hypothesised, it was usually to the Orthoptera (crickets and grasshoppers). This hypothesis was challenged by Kukalová-Peck and Brauckmann (1992), who identified an expanded clypeus in Gerarus (the clypeus is the front part of an insect's head). This, together with certain features of the wing venation, lead them to position Gerarus closer to the Paraneoptera, the group including Psocoptera (booklice) and Hemiptera (bugs). Even more notably, they also identified exites on Gerarus' legs.
Kukalová-Peck is best known for her theories on the origin of insect wings. Many fossil arthropods, and modern crustaceans, possess branched legs, and Kukalová-Peck holds that ancestral insects also possessed such legs, with the wings developed from side-branches (exites) that have become dissociated from the legs and moved closer to the top of the thorax. This contrasts with the earlier idea that insect wings were derived from dorsolateral lobes of the thorax itself. Kukalová-Peck's model has certainly got some points in its favour - it avoids the difficulty of a transition from a fixed lateral lobe to a mobile, articulated wing, and genetic studies have shown that similar genes are involved in the development of Drosophila wings as in that of crustacean gills (which are undoubtedly derived from exites). Kukalová-Peck also identified the presence of exites in a number of fossil insects as further support for her model (Kukalová-Peck, 1987).
However, there are a couple of stumbling blocks. Firstly, those fossil insects on which exites have been identified are phylogenetically nested among modern insects with unbranched legs, which would require the convergent loss of exites in a number of independent lineages (not impossible - exite loss seems to be directly connected to adaptation to life on land for arthropods). Secondly, and perhaps more damningly, some of Kukalová-Peck's reconstructions have been accused of (shall we say) a certain excess of imagination. Béthoux & Nel (2003) re-interpreted the wing venation of Gerarus, and found that it did not possess the features cited by Kukalová-Peck & Brauckmann (1992) as indicating paraneopteran relationships. That still left the expanded clypeus and the exites, but those little details were re-interpreted by Béthoux & Briggs (2008) as artefacts seemingly produced by over-enthusiastic preparation. The current indication is that Gerarus is a member of the Panorthoptera, the clade including Orthoptera plus the extinct orders Titanoptera and Caloneurodea. A close relationship between Geraridae and Titanoptera, enormous grasshopper-like insects, was popular for a while, but was rejected by Béthoux (2007)*. The exact affinities of Gerarus still await elucidation.
*Some day I may do a review of Béthoux (2007), a paper which may or may not constitute a glimpse into the fiery depths of hell. Right now, I haven't the strength.
Béthoux, O. 2007. Cladotypic taxonomy applied: titanopterans are orthopterans. Arthropod Systematics and Phylogeny 65 (2): 135-156.
Béthoux, O., & D. E. G. Briggs. 2008. How Gerarus lost its head: stem-group Orthoptera and Paraneoptera revisited. Systematic Entomology 33 (3): 529-547.
Béthoux, O., & A. Nel. 2003. Wing venation morphology and variability of Gerarus fischeri (Brongniart, 1885) sensu Burnham (Panorthoptera; Upper Carboniferous, Commentry, France), with inferences on flight performance. Organisms Diversity & Evolution 3 (3): 173-183.
Burnham, L. 1983. Studies on Upper Carboniferous insects: I. The Geraridae (order Protorthoptera). Psyche 90 (1-2): 1-57.
Kukalová-Peck, J. 1987. New Carboniferous Diplura, Monura, and Thysanura, the hexapod ground plan, and the role of thoracic side lobes in the origin of wings (Insecta). Canadian Journal of Zoology 65: 2327-2345.
Kukalová-Peck, J., & C. Brauckmann. 1992. Most Paleozoic Protorthoptera are ancestral hemipteroids: major wing braces as clues to a new phylogeny of Neoptera (Insecta). Canadian Journal of Zoology 70: 2452–2473.