Visual control of cursorial prey pursuit by tiger beetles (Cicindelidae)
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Ó Springer-Verlag 1997
ORIGINAL PAPER
C. Gilbert
Visual control of cursorial prey pursuit by tiger beetles (Cicindelidae)
Accepted: 7 April 1997
Abstract Target detection poses problems for moving animals, such as tiger beetles, that track targets visually. The pursuer's movements degrade target image contrast and induce reaerent image movement that confounds continuous detection of prey. In nature, beetles pursue prey discontinuously with several iterations of stop-andgo running. The beetle's dynamics were analyzed by ®lming pursuits of prey or experimenter-controlled dummies. Durations of stops are inversely related to prey visual angular velocity; as the prey image moves between neighboring ommatidial ®elds, the beetle relocalizes it and renews running. During subsequent runs, translation and rotation depend upon prey visual angular velocity and position, respectively, seen during the previous stop. The beetle runs, then stops, while prey continues moving. After two to three iterations of stopand-go the beetle catches its prey, suggesting open-loop control of running. Computational model simulations produce good qualitative spatio-temporal ®t with actual pursuits. However, when pursuing prey dummies, beetles track continuously and quickly follow changes in target position, suggesting closed-loop control using a position-sensitive servo mechanism. Dierences between these types of pursuit control system are discussed with respect to limitations in signal detection, particularly spatio-temporal contrast, that may force beetles to use an open-loop system. Key words Tracking á Open loop á Signal detection Predatory behavior á Insect Abbreviations he the angular position of prey relative to the beetle's body axis á h_ e the relative angular velocity
C. Gilbert Department of Entomology, Cornell University, Ithaca, NY 14853, USA, Tel.: +1-607/255-3985, Fax: +1-607/255-0939, e-mail: [email protected]
of prey and the beetle's body axis á / the beetle's angular orientation á /_ the beetle's angular velocity
Introduction Visually-guided pursuit of prey presents mobile predators with the problem of maximizing their speed of approach while reliably detecting a prey image which is degraded by the relative angular velocity of the moving observer and the prey (Horridge 1977; Wehner 1981). Some predators use stealth to reduce their separation from the prey after initial detection and only ``break cover'' for a relatively short run or pounce (Curio 1976) that is characterized by high linear velocity, but low angular velocity. Thus, the image of the prey is not much degraded during the slow approach phase and the fast ®nal phase is often made under open-loop control without visual guidance (Copeland and Carlson 1979). Among insects, such pursuit tactics have been documented for praying mantids (Holling 1966), assassin bugs (Haridass et al. 1988), jumping spiders (Drees 1952), and others (Bauer 1981). On the other hand, many predators, including tiger beetles, are not particularly stealthy and ``break cover'' upon initially
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