Abstract
Scorpions have arguably the most elaborate “tongues” on the planet: two paired ventral combs, called pectines, that are covered in thousands of chemo-tactile peg sensilla and that sweep the ground as the animal walks. Males use their pectines to detect female pheromones during the mating season, but females have pectines too: What additional purpose must the pectines serve? Why are there so many pegs? We take a computational approach to test the hypothesis that scorpions use their pectines to navigate by chemo-textural familiarity in a manner analogous to the visual navigation-by-scene-familiarity hypothesis for hymenopteran insects. We have developed a general model of navigation by familiarity with a local sensor and have chosen a range of plausible parameters for it based on the existing behavioral, physiological, morphological, and neurological understanding of the pectines. Similarly, we constructed virtual environments based on the scorpion’s native sand habitat. Using a novel methodology of highly parallel high-throughput simulations, we comprehensively tested 2160 combinations of sensory and environmental properties in each of 24 different situations, giving a total of 51,840 trials. Our results show that navigation by familiarity with a local pectine-like sensor is feasible. Further, they suggest a subtle interplay between “complexity” and “continuity” in navigation by familiarity and give the surprising result that more complexity — more detail and more information — is not always better for navigational performance.
Author summary Scorpions’ pectines are intricate taste-and-touch sensory appendages that brush the ground as the animal walks. Pectines are involved in detecting pheromones, but their exquisite complexity — a pair of pectines can have around 100,000 sensory neurons — suggests that they do more. One hypothesis is “Navigation by Scene Familiarity,” which explains how bees and ants use their compound eyes to navigate home: the insect visually scans side to side as it moves, compares what it sees to scenes learned along a training path, and moves in the direction that looks most familiar. We propose that the scorpions’ pectines can be used to navigate similarly: instead of looking around, they sweep side to side sensing local chemical and textural information. We crafted simulated scorpions based on current understanding of the pectines and tested their navigational performance in virtual versions of the animals’ sandy habitat. Using a supercomputer, we varied nine environmental, sensory, and situational properties and ran a total of 51,840 trials of simulated navigation. We showed that navigation by familiarity with a local sensor like the pectines is feasible. Surprisingly, we also found that having a more detailed landscape and/or a more sensitive sensor is not always optimal.
