Self-propulsion via slipping: Frictional swimming in multilegged locomotors.

Locomotion is typically studied either in continuous media where bodies and legs experience forces generated by the flowing medium or on solid substrates dominated by friction. In the former, centralized whole-body coordination is believed to facilitate appropriate slipping through the medium for propulsion. In the latter, slip is often assumed minimal and thus avoided via decentralized control schemes. We find in laboratory experiments that terrestrial locomotion of a meter-scale multisegmented/legged robophysical model resembles undulatory fluid swimming. Experiments varying waves of leg stepping and body bending reveal how these parameters result in effective terrestrial locomotion despite seemingly ineffective isotropic frictional contacts. Dissipation dominates over inertial effects in this macroscopic-scaled regime, resulting in essentially geometric locomotion on land akin to microscopic-scale swimming in fluids. Theoretical analysis demonstrates that the high-dimensional multisegmented/legged dynamics can be simplified to a centralized low-dimensional model, which reveals an effective resistive force theory with an acquired viscous drag anisotropy. We extend our low-dimensional, geometric analysis to illustrate how body undulation can aid performance in non-flat obstacle-rich terrains and also use the scheme to quantitatively model how body undulation affects performance of biological centipede locomotion (the desert centipede Scolopendra polymorpha) moving at relatively high speeds (∼0.5 body lengths/sec). Our results could facilitate control of multilegged robots in complex terradynamic scenarios.

Neuropeptide evolution: Chelicerate neurohormone and neuropeptide genes may reflect one or more whole genome duplications.

Four genomes and two transcriptomes from six Chelicerate species were analyzed for the presence of neuropeptide and neurohormone precursors and their GPCRs. The genome from the spider Stegodyphus mimosarum yielded 87 neuropeptide precursors and 120 neuropeptide GPCRs. Many neuropeptide transcripts were also found in the transcriptomes of three other spiders, Latrodectus hesperus, Parasteatoda tepidariorum and Acanthoscurria geniculata. For the scorpion Mesobuthus martensii the numbers are 79 and 93 respectively. The very small genome of the house dust mite, Dermatophagoides farinae, on the other hand contains a much smaller number of such genes. A few new putative Arthropod neuropeptide genes were discovered. Thus, both spiders and the scorpion have an achatin gene and in spiders there are two different genes encoding myosuppressin-like peptides while spiders also have two genes encoding novel LGamides. Another finding is the presence of trissin in spiders and scorpions, while neuropeptide genes that seem to be orthologs of Lottia LFRYamide and Platynereis CCRFamide were also found. Such genes were also found in various insect species, but seem to be lacking from the Holometabola. The Chelicerate neuropeptide and neuropeptide GPCR genes often have paralogs. As the large majority of these are probably not due to local gene duplications, is plausible that they reflect the effects of one or more ancient whole genome duplications.

Neuropeptide Evolution: Chelicerate Neurohormone and Neuropeptide Genes may reflect one or more whole genome duplications.

Four genomes and two transcriptomes from six Chelicerate species were analyzed for the presence of neuropeptide and neurohormone precursors and their GPCRs. The genome from the spider Stegodyphus mimosarum yielded 87 neuropeptide precursors and 101 neuropeptide GPCRs. High neuropeptide transcripts were also found in the trancriptomes of three other spiders, Latrodectus hesperus, Parasteatoda tepidariorum and Acanthoscurria geniculata. For the scorpion Mesobuthus martensii the numbers are 79 and 74 respectively. The very small genome of the house dust mite, Dermatophagoides farinae, on the other hand contains a much smaller number of such genes. A few new putative Arthropod neuropeptide genes were discovered. Thus, both spiders and the scorpion have an achatin gene and in spiders there are two different genes encoding myosuppressin-like peptides while spiders also have two genes encoding novel LGamides. Another finding is the presence of trissin in spiders and scorpions, while neuropeptide genes that seem to be orthologs of Lottia LFRYamide and Platynereis CCRFamide were also found. Such genes were also found in various insect species, but seem to be lacking from the Holometabola. The Chelicerate neuropeptide and neuropeptide GPCR genes often have paralogs. As the large majority of these are probably not due to local gene duplications, is not impossible that they reflect the effects of one or more ancient whole genome duplications.

Germ cells of the centipede Strigamia maritima are specified early in embryonic development.

We provide the first systematic description of germ cell development with molecular markers in a myriapod, the centipede Strigamia maritima. By examining the expression of Strigamia vasa and nanos orthologues, we find that the primordial germ cells are specified from at least the blastoderm stage. This is a much earlier embryonic stage than previously described for centipedes, or any other member of the Myriapoda. Using these genes as markers, and taking advantage of the developmental synchrony of Strigamia embryos within single clutches, we are able to track the development of the germ cells throughout embryogenesis. We find that the germ cells accumulate at the blastopore; that the cells do not internalize through the hindgut, but rather through the closing blastopore; and that the cells undergo a long-range migration to the embryonic gonad. This is the first evidence for primordial germ cells displaying these behaviours in any myriapod. The myriapods are a phylogenetically important group in the arthropod radiation for which relatively little developmental data is currently available. Our study provides valuable comparative data that complements the growing number of studies in insects, crustaceans and chelicerates, and is important for the correct reconstruction of ancestral states and a fuller understanding of how germ cell development has evolved in different arthropod lineages.

Maneuverable and Efficient Locomotion of a Myriapod Robot with Variable Body-Axis Flexibility via Instability and Bifurcation.

Legged robots have remarkable terrestrial mobility, but are susceptible to falling and leg malfunction during locomotion. The use of a large number of legs, as in centipedes, can overcome these problems, but it makes the body long and leads to many legs being constrained to contact with the ground to support the long body, which impedes maneuverability. A mechanism for maneuverable locomotion using a large number legs is thus desirable. However, controlling a long body with a large number of legs requires huge computational and energy costs. Inspired by agile locomotion in biological systems, this study proposes a control strategy for maneuverable and efficient locomotion of a myriapod robot based on dynamic instability. Specifically, our previous study made the body axis of a 12-legged robot flexible and showed that changing the body-axis flexibility produces pitchfork bifurcation. The bifurcation not only induces the dynamic instability of a straight walk but also a transition to a curved walk, whose curvature is controllable by the body-axis flexibility. This study incorporated a variable stiffness mechanism into the body axis and developed a simple control strategy based on the bifurcation characteristics. With this strategy, maneuverable and autonomous locomotion was achieved, as demonstrated by multiple robot experiments. Our approach does not directly control the movement of the body axis; instead, it controls body-axis flexibility, which significantly reduces computational and energy costs. This study provides a new design principle for maneuverable and efficient locomotion of myriapod robots.

Lithobius (Ezembius) hualongensis sp. nov. and Lithobius (Ezembius) sui sp. nov. (Lithobiomorpha, Lithobiidae), two new species of centipede from northwest China.

Lithobius (Ezembius) hualongensissp. nov. and Lithobius (Ezembius) suisp. nov. (Lithobiomorpha, Lithobiidae) recently discovered from the Qinghai-Tibet Plateau, China are described. Morphologically, the two new species are very similar but can be distinguished by the number of coxosternal teeth: L. (E.) hualongensissp. nov. has 2 + 2 while L. (E.) suisp. nov. has 3 + 3. The two new species resemble L. (E.) multispinipesPei et al., 2016, from the Xinjiang Autonomous Region, but can be readily distinguished by having the Tömösváry's organ slightly larger than the adjoining ocelli rather than smaller, 3 + 3 spurs on female gonopods versus 2 + 2, and the simple terminal claw of female gonopods with a small triangular protuberance on the basal ventral side versus simple, without a small triangular protuberance on the basal ventral side. We also compare the main morphological characters of the two new species with the other Lithobius (Ezembius) species known in Qinghai Province. A key to the Chinese species of Ezembius is presented.

Distributions of Invasive Arthropods across Heterogeneous Urban Landscapes in Southern California: Aridity as a Key Component of Ecological Resistance.

Urban systems often support large numbers of non-native species, but due to the heterogeneity of urban landscapes, species are not evenly distributed. Understanding the drivers of ecological resistance in urban landscapes may help to identify habitats that are most resistant to invasion, and inform efforts to model and conserve native biodiversity. We used pitfall traps to survey non-native ground-dwelling arthropods in three adjacent, low-elevation habitat types in southern California: California sage scrub, non-native grassland, and suburban development. We found that non-native species were fewer and less widely distributed in the sage scrub and grassland habitats. Due to the proximity of our sites, differences in propagule pressure is an unlikely explanation. Instead, we suggest that the absence of water subsidies in the sage scrub and grassland habitats increases those habitats' resistance to arthropod invasions. Comparisons to studies conducted at fragments closer to the coast provide further support for the relationship between aridity and invasibility in southern California. Our findings highlight that inland fragments are important for conserving native arthropod diversity, that models of non-native species distributions in arid and semi-arid urban systems should include aridity measures, and that reducing resource subsidies across the region is critical to mitigating spread of non-natives.

Segmentation, neurogenesis and formation of early axonal pathways in the centipede,Ethmostigmus rubripes (Brandt).

We have examined the embryo of the centipedeEthmostigmus rubripes to determine the degree of evolutionary conservatism in the developmental processes of segmentation, neurogenesis and axon formation between the insects and the myriapods. A conspicuous feature of centipede embryogenesis is the early separation of the left and right sides of the ganglionic primordia by extra-embryonic ectoderm. An antibody to the protein encoded by theDrosophila segmentation geneengrailed binds to cells in the posterior margin of the limb buds in the centipede embryo, in common with insect and crustacean embryos. However, whereas in insects and crustaceans this protein is also expressed in a subset of cells in the neuroectoderm, the anti-engrailed antibody did not bind to cells in the ganglionic primordia of the centipede embryo. Use of the BrdU labelling technique to mark mitotically active cells revealed that neuroblasts, the ubiquitous neuron stem cell type in insects, are not present in the centipede. The earliest central axon pathways in the centipede embryo do not arise from segmentally repeated neurons, as is the case in insects, but rather by the posteriorly directed growth of axons originating from neurons located in the brain. Axonogenesis by segmental neurons begins later in development; the pattern of neurons involved is not obviously homologous to the conservative set of central pioneering neurons found in insects. Our observations point to considerable differences between the insects and the myriapods in mechanisms for neurogenesis and the formation of central axon pathways, suggesting that these developmental processes have not been strongly conserved during arthropod evolution.