Active tactile exploration and tactually induced turning in tethered walking stick insects

Identification of available travel paths' size and orientation by antennal mechanosensory system of crickets

Insects use their antennae to explore surroundings and guide locomotion. Recently, we found that crickets modulate escape behavior elicited by airflow stimuli detected by another mechanosensory organ, called cerci, to avoid collision with obstacles detected with their antennae. This suggests the spatial perception ability of crickets through antennal mechano-sensing without visual inputs. However, whether they are able to identify travel paths available for their movements remains unknown. To assess this, we used the same experimental framework with wind stimulation as our previous studies and tested whether crickets could perceive the width or direction of an available escape path in surroundings using their antennae. When detecting a wall in front with gap openings of different widths, the crickets changed the movement direction of their escape responses depending on the gap width. If the gap was wider than their body width, the crickets ran forward, suggesting that they could recognize the space available for passage of their body width. In addition, the crickets adjusted their escape direction toward the wall gap when it was oriented at 30° to the side of their front. These results suggest that the crickets are able to perceive the spatial information of surroundings, such as size and orientation, available for upcoming movements via their antennal mechanosensory system.

Contrast and luminance dependence of target choice and visual orientation in walking stick insects

When presented with static images, animals show robust preferences for particular visual features, and reliably turn towards and approach selected visual landmarks. In target choice paradigms, stick insects tend to approach edges with high image contrast, but also show robust orientation based on luminance alone. To better understand which stimulus features actually govern turning towards static visual targets, this study tests the relative importance of two elementary cues of spatial vision - luminance and contrast. We do so in a large open-field arena, using luminance-modulated, static 360° patterns with and without high-contrast edges. We show that target choice strongly depends on image contrast, though with a bias towards areas of low luminance. Comparison of heading directions during approach with terminal locations at the arena wall suggests an early, coarse orientation based on luminance, with subsequent steering towards high-contrast regions. When walking towards a target with high-contrast edges, the likelihood to turn away towards a Gaussian distractor image increases with decreasing edge contrast of the original target. Subjective equality of the two images occurs for an approximate 2:1 weighing of contrast and luminance, indicating that a stronger contrast-dependent edge-orientation mechanism acts in parallel with a weaker luminance-dependent phototaxis mechanism. Given the significance of stick insects as study organisms for the control of legged locomotion, future research may now test whether the two visual orientation mechanisms lead to distinct turning responses at the level of step pattern or leg movement variables.

Behavioural function and development of body-to-limb proportions and active movement ranges in three stick insect species

Overall body proportions and relative limb length are highly characteristic for most insect taxa. In case of the legs, limb length has mostly been discussed with regard to parameters of locomotor performance and, in particular cases, as an adaptation to environmental factors or to the mating system. Here, we compare three species of stick and leaf insects (Phasmatodea) that differ strongly in the length ratio between antennae and walking legs, with the antennae of Medauroidea extradentata being much shorter than its legs, nearly equal length of antennae and legs in Carausius morosus, and considerably longer antennae than front legs in Aretaon asperrimus. We show that that relative limb length is directly related to the near-range exploration effort, with complementary function of the antennae and front legs irrespective of their length ratio. Assuming that these inter-species differences hold for both sexes and all developmental stages, we further explore how relative limb length differs between sexes and how it changes throughout postembryonic development. We show that the pattern of limb-to-body proportions is species-characteristic despite sexual dimorphism, and find that the change in sexual dimorphism is strongest during the last two moults. Finally, we show that antennal growth rate is consistently higher than that of front legs, but differs categorically between the species investigated. Whereas antennal growth rate is constant in Carausius, the antennae grow exponentially in Medauroidea and with a sudden boost during the last moult in Aretaon.