Unit studies on the retina of dragonflies

Abstract

Summary The dorsal part of the adult eye contains mainly uniform blue-sensitive receptors with peak near 380 nm in Anax and near 410 nm in Libellula. This is not a simple rhodopsin type of curve. 2. The larval eye and the ventral part of the adult eye contain receptors with a wide range of spectral sensitivity curves, with peaks from 420–520 nm in Anax and 450–550 nm in Libellula. Many of these can be derived from a typical rhodopsin curve when allowance is made for self-absorption in very long receptors.

3. For most adult receptors, rotation of the plane of polarization by 90° from the optimum is equivalent to a decrease of only 60–70 % in intensity.

4. The acceptance angle curve is approximately a Gaussian normal curve with width of 1.0–1.8° at linear half height. A few units have complex fields which may be artefacts.

5. Retinula unit responses in larval compound eyes resemble those of the adult but the larval units are slower in response and usually more sensitive to rotation of the plane of polarization.

6. There are eight retinula cells arranged in two tiers in each ommatidium.

7. In the dark, pigment grains move away from the tip of the cone and the palisade around the rhabdom increases. These effects could increase the sensitivity by a change in the anatomy.

8. In an eye of this type the ERG is an unsatisfactory tool, the angles of acceptance must be measured very carefully, and experiments with a single electrode have only a limited interpretation.