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| Fig. 1. Representative variation in external anatomy of the facial pit. (A) Copperhead, Agkistrodon contortrix. (B) Western diamondback rattlesnake, Crotalus atrox. (C) Crotalus basiliscus. (D) Eyelash viper, Bothriechis schlegelii. (E) Viridovipera (Trimeresurus) gumprechti. (F) White-lipped tree viper, Cryptelytrops (Trimeresurus) albolabris. Variation in the relative size of the facial pit opening is conspicuous (A versus E) but the optical significance depends entirely on the relationship to internal anatomy. The arboreal species (D,E,F) show a marked constriction or grooving of the face in front of the pits that appears to increase the sensory field overlap compared with the rounded nose of terrestrial species (A,B), although C. basiliscus (C) shows a similar feature with small grooves anterior to the pits.
Photo credits: A,D,E, G. Westhoff; B, present study; C, H. Krisp; F, C. M. L. Burnett.
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The pitviper facial pit is a pinhole camera-like sensory organ consisting of a flask-shaped cavity divided into two chambers by a suspended membrane. Neurophysiological studies and simplified optical models suggest that facial pits detect thermal radiation and form an image that is combined with visual input in the optic tectum to form a single multispectral image. External pit anatomy varies markedly among taxonomic groups. However, optical function depends on unknown internal anatomy. Therefore, we developed methods for relating anatomy to optical performance. To illustrate, we constructed detailed anatomical models of the internal anatomy of the facial pits of four individuals of four pitviper species using X-ray tomography sections of fresh material. We used these models to define the point spread function, i.e. the distribution of radiation from a point source over the pit membrane, for each species. We then used optical physics, heat transfer physics and computational image processing to define the thermal image formed on the pit membrane for each species. Our computed pit membrane images are consistent with behavioral observations if the sensitivity of membrane receptors equals the most sensitive (ca. 0.001°C) laboratory estimates. Vignetting (variation in optical aperture size with view angle) and differences between body and environmental temperatures can create temperature variation across the membrane that greatly exceeds image temperature contrasts, potentially impairing imaging. Spread functions plotted versus source point azimuth and elevation show distinct patterns that suggest new research directions into the relationships among the optical anatomy, ecology, behavior and sensory neurophysiology of pitvipers.
Key words: pitviper, sensory anatomy, sensory physiology, snake, facial pit, loreal pit, optics, heat transfer, image analysis.
George S. Bakken, Samantha E. Colayori and Taihung Duong. 2012. Analytical methods for the geometric optics of thermal vision illustrated with four species of pitvipers. The Journal of Experimental Biology. 215, 2621-2629. doi:10.1242/jeb.063495
