The search behavior of the desert isopod Hemilepistus reaumuri as compared with a systematic search

Abstract

To protect itself from the extreme heat and dryness of its habitat in summer, Hemilepistus reaumuri must spend most of the time in its burrow. However, to forage it has often to make large excursions. As a rule it is able to return from any point to the entrance of the burrow by the shortest way. If a returning isopod misses the entrance by a few millimeters, it must search for it. Not until the second antennae have nearly touched the edge of the burrow entrance does an isopod detect it, by means of chemoreceptors in the terminal antennal segment. Even when the entrance is within reach of the antennae occasionally the animal will not detect it. H. reaumuri searches for the entrance of its burrow in a systematic and successful manner. If the excursion has covered only a short distance, it searches initially in a spiral pattern. If the burrow is not found during this phase the animal extends its range, moving in broad loops and meanders, but returns repeatedly to the starting point of the search. The problem H. reaumuri must solve in finding its burrow is described in quantitative terms. It is assumed that before the search begins the animal does not know the exact position of the entrance but possesses information about the probability density for this position. The difficulties encountered in detecting the burrow entrance are also taken into account. There is a very small probability that an isopod will detect the entrance if it searches only superficially in the region where the entrance actually is, but this probability increases rapidly with the intensity of search. For this reason it is advantageous for H. reaumuri to return after a while to a region it has already searched without success. The theoretical systematic form of search with the highest probability of success is derived under these conditions. The search behavior of H. reaumuri is as effective as this systematic form of search. The isopods also follow the most important rules governing the spatiotemporal properties of this pattern. They concentrate their search within a narrowly circumscribed region around the starting point. In all regions further away search would be unprofitable, for the increase in probability of success achieved by searching in these regions as well is too small in comparison with the additional search time required. Because this region continually expands as long as the search is unsuccessful, the isopods search over progressively larger areas until they have reached their goal. © 1983 Springer-Verlag.