Article: Walking and jumping with Palaeozoic apterygote insects

Abundant arthropod walking and jumping traces, from the Lower Permian Robledo Mountains Formation of southern New Mexico, provide direct evidence of the locomotory techniques of monurans, an extinct group of archaeognathan apterygote insects. The jumping behaviour of monurans is compared with that of the extant machilid archaeognathan Petrobius. The jumping traces are referred to Tonganoxichnus robledoensis, and demonstrate that monurans were capable of forward progression via a linear succession of jumps of several times their body length. Petrobius also employs an unusual, fast, in-phase, jumping gait for normal directed locomotion; however, unlike the T. robledoensis traces, these jumps are only about one body length. In-phase trackways, referred to Stiaria intermedia from the Upper Carboniferous Tonganoxie Sandstone of Kansas, are found in association with Tonganoxichnus traces, indicating that monurans were also capable of such a fast jumping gait. Petrobius employs an escape jump that is more similar in terms of magnitude to those represented by T. robledoensis; however, the escape jump is essentially random in terms of direction and rotation of the body. Out-of-phase trackways from the Robledo Mountains Formation, also referred to Stiaria intermedia, are found preceding or following on from several Tonganoxichnus traces, and demonstrate that monurans also used normal, out-of-phase, walking gaits across open ground. Analysis of these trackways demonstrates that they used a variety of gaits ranging from very slow and stable gait ratios of 1.2 : 8.8 (i.e. the propulsive backstroke phase comprises 88 per cent of the step cycle) following jumps, to fast gait ratios of 3.3 : 6.7 and 5.0 : 5.0 preceding jumps. Petrobius tends not to use such normal walking gaits unless on the undersurface of rocks, preferring to use the unusual, fast, in-phase, jumping gait over open ground. Monurans appear to have been capable of many of the same jumping behaviours as Petrobius, apart from the random escape jump. Archaeognathans are the most primitive group of true insects, and the presence of these similar types of jumping behaviours in monurans and machilids suggests that such behaviours were a primitive method of insect locomotion.