Mid-Infrared Transmission and Reflection Microspectroscopy: Analysis of a Novel Biological Imaging System: The Snake Infrared-Imaging Pit Organ

P it vipers (e.g., rattlesnakes), boas, and pythons possess facial pit organs that are part of unique infrared imaging sensors not found in any other living species. Pit organs allow these snakes to detect thermal contrast in the environment and to form spatial images of their thermal environments within infrared regions of the electromagnetic spectrum (1) (Figure 1). The infrared imaging system augments the snake’s visual system by providing its brain with a spatial image of the thermal environment. Infrared-imaging snakes use the infrared system to effectively detect and target prey even at night and in very dark places, such as animal burrows. Infrared imaging may also allow these snakes to avoid potential predators and to locate suitable sites for thermoregulation (1–4). Snake infrared imaging systems are extraordinarily sensitive, providing greater absolute sensitivity than sophisticated manmade detection devices without supplemental cooling. Therefore, snake infrared imaging systems, and particularly the pit organs, are important natural models for the development of novel biomimetic infrared sensor technologies. Because snakes detect potential thermal targets from some distance, two major windows of atmospheric infrared transmission are likely to be very important to snakes — the middle infrared 3–5 mm and the 8–12 mm wavelength regions. Biological tissues, such as those that make up the bodies of people and animals, contain high concentrations of water, which is highly absorptive in the mid-infrared region of the electromagnetic spectrum. It is therefore important to understand the optical properties of the pit organ epidermis that allows infrared photons to interact with neural receptors located just beneath the epidermal surface. Spectral investigation of the few millimeter size pit organs are facilitated by the use of mid-infrared microspectroscopy tools capable of focusing electromagnetic radiation onto a well-defined small area (Figure 2). Prior research has shown that the epidermis of the pit organ in infrared imaging snakes is two orders of magnitude more reflective of visible light than that of non-pit organ epidermal regions, potentially to protect the infrared imaging sensor from the overwhelming high-energy photon flux from ambient light (5) (Grace, unpublished observations). The absorptive and/or reflective properties of pit organ epidermis may be due in part to unique nanopit arrays — regular arrays of microscopic surface invaginations — located on the epidermis covering the pit organs (5) (Figure 3). While nanopit arrays are found elsewhere on snake epidermal areas not associated with infrared sensing (such as interscale regions, and on the spectacle covering the eye), the architecture of nanopit arrays covering the pit organ epidermis is unique (5) (Safer and Grace, unpublished observations). In a regular renewal process as their bodies grow, snakes periodically shed or slough off the outer layer of their epidermis, including the epidermis overlying the infraredsensing pit organs. In the work reported here, samples of pit organ epidermis were used to investigate the spectral properties of the surface layer containing nanopit arrays, in isolation from the rest of the pit organ. Transmittance and reflectance of mid-infrared radiation were investigated because this region of the electromagnetic spectrum is believed to be important for infrared imaging in snakes (6).