Positive Phototaxis of Adult Alfalfa Weevils1 to Visible and near-Infrared Radiation

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Abstract
The action spectrum of positive phototaxis in adult alfalfa weevils, Hypera postica (Gyllenhal), was evaluated by placing groups of 50 starved weevils in a closed cylindrical orientation chamber illuminated at one end by a Bausch and Lomb monochromator. Weevils responded well to green light (500–550 nm), but they were most strongly attracted to wavelengths in the far red and near infrared region. The response to red light was almost completely eliminated when the vertex of each weevil's head capsule was covered with an opaque lacquer. Weevils with clear shellac over the top of the head capsule had the same bimodal response to colored light as unpainted weevils, so illumination of the head must either stimulate an extra-ocular photoreceptor or else somehow change the sensitivity of the compound eyes. Three structures which lie just below the cephalic exoskeleton have been identified as possible photoreceptors: the brain, a pair of tegumentary nerves, and an elongate pit at the top of the frons which could mark the site of an internal ocellus.

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