Fig. 1. The optical apparatus used to photograph the eye shine of butterfly eyes.
The objective lens L1 has a large aperture. A light source is focused by lens
L2 in its back focal plane, where the field diaphragm D1 is positioned. D1 is
in the focal plane of lens L3, which is confocal with L1 because of a
half-mirror placed at 45° with respect to the optical axes of L1 and L3. A
more-or-less parallel beam, depending on the size of D1, enters L1 and is
focused on the deep pseudopupil (DPP) in the centre of the butterfly's eye.
The telescope lens pair L1 and L4 images the DPP in the back focal plane of
L4, where diaphragm D2 is positioned. The image of the corneal eye shine,
projected by lens L5, confocal with L4, is photographed by a photomicroscope.
The dotted lines are the back-focal planes of L1 and L5. Inset: incident light
entering a butterfly ommatidium is focused by the facet lens and crystalline
cone (fc) into the rhabdom (rh) and then propagates to the tapetal reflector
(tr), where it is mirrored back into the rhabdom and out of the eye again,
unless it is absorbed by visual pigments in the rhabdom or by screening
pigments in the medium surrounding the rhabdom. The rhabdom organization of a
pierid butterfly is indicated schematically: the distal part of the rhabdom
consists of the rhabdomeres of photoreceptors R1-R4, the proximal part
consists of the rhabdomeres of photoreceptors R5-R8 and the most basal part
consists only of rhabdomere R9, which is indicated by an asterisk (see
Qiu et al., 2002). The rhabdom
is surrounded by photoreceptor screening pigment that absorbs light from the
propagating light wave.
