Automatic spectrosensitometry of photoreceptors inLethrus (Coleoptera, Scarabaeidae)

Summary

• 1. The colour substitution spectrosensitometer is a stimulator with alternating test light and reference light so that a stimulus of any wavelength or polarization plane can be adjusted in intensity to give the same response as does the reference beam (Fig. 1). Spectral sensitivity (SS) and polarization sensitivity (PS) of the eye of the lamellicorn beetle,Lethrus apterus Laxm. which is known to be capable of accurate astro-orientation were measured with this device.

• 2. The “SS-curves” of 13 dorsal eyes were recorded using an electroretinogram (ERG) as the criterion of light sensitivity. The curves proved to fall into two groups, those with maximum near 355 nm, termed UV-type (9 eyes), and those with maximum near 525 nm, termed LW-type (4 eyes). Each type showed a good fit to Dartnall's absorption spectrum of a thin layer of visual pigment (Figs. 2, 3, 4).

• 3. Of 38 single photoreceptors recorded 24 were found to be the LW-type and 14 the UV-type (Figs. 5, 6, 7).

• 4. The peak spectral sensitivity of a receptor lies within the range 345–365 nm, or 465–540 nm in the UV- and LW-group respectively (Fig. 8).

• 5. Among 11 UV-receptors, 7 were narrower than the theoretical absorption spectrum calculated from Dartnall's nomogram for rhodopsins. The same appeared to be true for 10 LW-receptors of the 20 measured (Fig. 9).

• 6. Both LW- and UV-receptors often displayed secondary sensitivity peaks but these did not exceed half the height of the main peak in most cases (Figs. 10, 11).

• 7. Polarization sensitivity of both types was unexpectedly low. Of 26 photoreceptors tested 17 showed PS≦2 whereas only one LW-unit had PS=4.5 and one UV-unit had PS=7.8 (Figs. 12, 13). The latter was presumably a specialized UV-sensitive polarization plane detector similar to those in the honey bee eye.

• 8. In 7 double-peaked photoreceptors there is an angle between the two E-vector planes corresponding to maximal PS's at the two spectral peaks (Fig. 14). This angle never equaled 90°; this is evidence against double-cell recordings because the microvilli of different cells are at 90° to each other.