1. The spectral responses of a ‘stored product’ insect, Tenebrio molitor, have been studied electrophysiologically.
2. Although primarily inhabiting a darkened environment the insect possesses a visual mechanism which could enable it to discriminate various wavelengths. The visible range for T. molitor lies between < 350 and 700 mµ; and the efficiency and sensitivity curves both have maxima between 520 and 550 mµ. Latency and amplitude also indicate maximum response in this same region of the spectrum.
3. Under normal conditions of dark adaptation the screening pigment does not affect the spectral sensitivity.
4. Miniature threshold potentials concealed in the normal background noise were detected by the use of the computer of average transients. The two principal components of the ERG, as well as minor components which were occasionally detected, were similarly related to wavelength. This seems to imply that the various ERG components originate in receptors of one single type.