The visual mechanisms of Tenebrio molitor: some aspects of the spectral response.

Granaries and food storage depots provide an essentially dark environment offering only occasional low-level illumination which is rarely intense enough to make spectral discrimination possible. Nevertheless insects adapted to such environments have been found to display phototactic responses (Balfour & Carmichael, 1928; Rahman & Sohi, 1939; Perttunen& Lahermaa, 1963; Yinon& Shulov, 1966; Sohi, 1966; Soderstrom, 1968). The low-light habitat of 'stored product' and 'granary' insects seemed to provide an environment which would demand a less well-developed spectrally sensitive visual mechanism than that found for example in field insects. The yellow mealworm beetle Tenebrio molitor was chosen as a suitable 'stored product' insect to test the above hypothesis. The spectral response of 'stored product' and 'granary' insects has been examined behaviourally by Stermer (1959), Sohi (1966) and Yinon & Shulov (1966). In that these insects display a negative phototactic response there seem to be inherent difficulties in quantifying behaviour. Further, owing to the influence of extraneous social factors the behavioural criteria themselves appear to be somewhat equivocal. To circumvent these problems the response of the mediating sense organ (the compound eye) was directly investigated by means of electrophysiological recordings of the insect electroretinogram (ERG).