Photoinhibition of arousal in the stick insect Carausius
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The phasmid Carausius assumes a stick posture (thanatosis) during the day but becomes active and feeds at night. Animals placed in darkness during the day start to arouse after a latency of 8–10 minutes, and about half are active after 12 minutes.
Light inhibits arousal. A flux of 4×109 hν sec-1 cm−2 at 483 nm increases the latency to the first arousal to about 12 minutes and the half-time to about 26 minutes.
The action spectrum for the inhibition of arousal peaks at 510–520 nm. Sensitivity falls gently toward the blue, more steeply to longer wavelengths, and is down 2 log units at 620 nm.
Animals with opaque screens on their eyes arouse quickly when placed in dim test lights that inhibit activity in normal controls. Therefore the eyes are the principal receptors.
Animals with eyes surgically removed show residual sensitivity to light. Therefore a higher-threshold, extra-optic receptor is also present.
Normally the insects integrate intensity over large areas of the visual field, for if they are placed on a reflecting substrate, illumination of the posterior half of the body is as effective in inhibiting arousal as light on the anterior end.
KeywordsNormal Control Visual Field Action Spectrum Posterior Half Stick Insect
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