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

Abstract

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.

Self‐Organized Stick Insect‐Like Locomotion under Decentralized Adaptive Neural Control: From Biological Investigation to Robot Simulation

“…[ 35 ] Strongly elongated front legs are often found in species with comparably short antennae since they take over the function of the antennae. [ 36 ] We did not observe extensive tactile exploration of the nearer environment in our experiments. However, the stick insects used herein were not subjected to changes in the near‐range environment, with no inclines, [ 44–46,49 ] obstacles, [ 41–43 ] or restricted walking paths.…”

“…Front legs can be used for tactile exploration in addition to walking and body weight support. [ 36 ] In the species used in our experiments, the sensing function of the antennae is strongly reduced compared to other species. [ 35,36 ] This enables the legs of the insects to receive immediate feedback on the structure of the walking path, without feedback from the antennae and to immediately adapt to the entire walking control.…”

“…[ 36 ] In the species used in our experiments, the sensing function of the antennae is strongly reduced compared to other species. [ 35,36 ] This enables the legs of the insects to receive immediate feedback on the structure of the walking path, without feedback from the antennae and to immediately adapt to the entire walking control. [ 55,56 ] However, both the trajectories of the legs and dynamics of the swing and stance phases may be subject to a more complex control compared to equally elongated legs to account for the heterogeneous leg lengths.…”

“…[ 35 ] The active motion ranges of antennae and legs correlate with each other and the reduction of antennal length is compensated by front leg elongation and the increasing exploration effort of the legs. [ 36 ] The secondary functionalization of legs for sensing the environment is shown to have further implications, with the tactile exploration of the environment and walking performed simultaneously. Stick insects adapt their leg dynamics and single‐step coordination of the six legs to various influences at once.…”

“…Stick insects adapt their leg dynamics and single-step coordination of the six legs to various influences at once. [14,16,37] Besides adjusting the step coordination for tasks like gap crossing, [17,[38][39][40] overcoming obstacles, [41][42][43] dealing with substrate inclination, [44][45][46] and other perturbations, [47,48] it is noteworthy, that the dynamics of the stepping patterns differ according to the body morphometrics, [35,36] adjusting for the loss of legs. [49] Overall, the control of leg used in stick insects is shown to be highly adaptive with some effort spent on understanding the adaptive leg-joint coordination of single steps (i.e., intralimb coordination).…”