BackgroundMany legged animals change gaits when increasing speed. In insects, only one gait change has been documented so far, from slow walking to fast running, which is characterised by an alternating tripod. Studies on some fast-running insects suggested a further gait change at higher running speeds. Apart from speed, insect gaits and leg co-ordination have been shown to be influenced by substrate properties, but the detailed effects of speed and substrate on gait changes are still unclear. Here we investigate high-speed locomotion and gait changes of the cockroach Nauphoeta cinerea, on two substrates of different slipperiness.ResultsAnalyses of leg co-ordination and body oscillations for straight and steady escape runs revealed that at high speeds, blaberid cockroaches changed from an alternating tripod to a rather metachronal gait, which to our knowledge, has not been described before for terrestrial arthropods. Despite low duty factors, this new gait is characterised by low vertical amplitudes of the centre of mass (COM), low vertical accelerations and presumably reduced total vertical peak forces. However, lateral amplitudes and accelerations were higher in the faster gait with reduced leg synchronisation than in the tripod gait with distinct leg synchronisation.ConclusionsTemporally distributed leg force application as resulting from metachronal leg coordination at high running speeds may be particularly useful in animals with limited capabilities for elastic energy storage within the legs, as energy efficiency can be increased without the need for elasticity in the legs. It may also facilitate locomotion on slippery surfaces, which usually reduce leg force transmission to the ground. Moreover, increased temporal overlap of the stance phases of the legs likely improves locomotion control, which might result in a higher dynamic stability.Electronic supplementary materialThe online version of this article (10.1186/s12983-017-0232-y) contains supplementary material, which is available to authorized users.

Alvinellid worms are a family of endemic and closely related species from deep-sea hydrothermal vents in the Pacific and Indian Oceans. These annelid worms, sister group to the Ampharetidae, occupy a wide range of ecological niches, some of which include the most thermotolerant marine animals described to date such as the Pompeii worm Alvinella pompejana, and other species living at much lower temperatures such as Paralvinella grasslei or Paralvinella pandorae. The phylogeny of this family has not yet been studied extensively. It is, however, a complex case with conflicting molecular phylogenies, the main question being the monophyletic or polyphyletic character of the genus Paralvinella. We carried out a comprehensive study of the phylogeny of this family using the best molecular data currently available from RNAseq datasets. The study is based on the assembly of several hundred transcripts for 11 of the 14 species described or in description. The results obtained by the most popular phylogenetic inference models (gene concatenation and maximum likelihood, or coalescent-based methods from gene trees) are compared, and an attempt is made to use sequence insertion and deletion information to evaluate and strengthen our choice over the different phylogenies using a newly-developed maximum likelihood method. Although our study does not allow to definitively assert the phylogeny of the Alvinellidae (three species are still missing), we propose to support the initial hypothesis of the monophyly of the Paralvinella proposed by Desbruyeres and Laubier on the basis of the morphology of the species, in which the speciesParalvinella pandorae and Paralvinella unidentata are basal in the genus Paralvinella and grouped together within the subgenus Nautalvinella. Following a clock calibration, the radiation of the Alvinellidae, dated between 55 and 78 Myr, took place very rapidly, resulting in high rates of incomplete lineage sorting between the first ancestors and probable gene transfers between the Alvinella, Nautalvinella and the rest of the Paralvinella lineages.