Ultrastructure and adaptation in the retina of Aglais urticae (Lepidoptera)

Summary

The apposition eye of Aglais urticae is composed of eucone ommatidia. Serial sectioning of blocks from the laterofrontal and lateroventral eye regions and mapping at different levels revealed that there is no torsion of whole ommatidia along their long axes.

The sensory part of the ommatidium comprises nine retinula cells. The most significant features of the complicated rhabdom structure (diagrammed in Fig. 3) are as follows. The vertically aligned receptor cells V₁ and V₅, which become axonal at the level at which the ninth cell begins, have microvilli arranged in bundles. The microvilli bundles of these cells generally make an angle α between 30 and 55° on one or the other side of the dorsoventral axis in the ommatidium cross section. The two orientations alternate regularly along the length of the rhabdom. The repeated sweeps of these bundles in regular intervals in combination with the curvature of the V-cell microvilli is considered to be a “substitute” for rhabdomere twisting. The four diagonally aligned receptor cells D_2,4,6,8 have rhabdomers that are continuous, though of variable size. These rhabdomeres, like those of the horizontally aligned cells (H₃ and H₇), extend along the entire rhabdom, though there is a small (1–2 μm) interruption in the H-cell rhabdomeres; the latter have the most constant orientation. Pigment granules are most abundant in the D cells, followed by the H cells and finally the V cells. RC₉ lacks pigments.

Light- and dark-adaptation experiments reveal marked horizontal migration of the retinula-cell pigment (pupil reaction) and slight vertical migration. Monochromatic adaptation experiments at wavelengths λ=342, 436, 522, 578, and 626 nm indicate special sensitivity of the D-cells around λ-520 nm. There are indications for sensitivity of V cells in the UV, and possibly of H cells in the blue. The H cells are regarded as suited for the detection of polarized light. The functional significance of these findings is discussed and compared with what is known of other butterfly eyes.