The Design of Compound Eyes and the Illumination of Natural Habitats
193
ommatidia to focus light onto a single rhabdom. Because of aberrations and optical
misalignments, it is not a trivial task to direct 2000 individual beams oflight s
actually be possible, with a blurry image being the unfortunate result. Even if all the
light is perfectly coincident, it may still be impossible to trap the very wide cone of
incident light rays within the target rhabdom by total internal reflection (Warrant
and Mcintyre 1990a, 1991 ). High sensitivity may come only at the cost of spatial
resolution. Nevertheless, recent work has shown that image quality need not he all
that blurry: many diurnal superposition eyes are actually diffraction limited (Land
1984, Warrant et al. 1999), and aberrations in many others are not as big a problem
as previously thought (Nilsson et al. 2000). In addition, many superposition eyes
have tackled the problem of preserving the image in the retina by isolating their
rhabdoms in reflective tapetal sheaths, ensuring total internal reflection for any angle
of incidence (Land 1984; Warrant and Mcintyre 1991 ).
3 Compound Eyes for Bright Habitats
I mentioned above that the abundance oflight in bright habitats has allowed compound
eyes to develop acute zones, regions where the visual channels are more tightly
packed and sample the world with greater spatial resolution. These acute zones
invariably reveal remarkable matched filters - for habitat structure, motion and the
location of mates and prey - and it is these to which we tum our attention next.
3.1 Apposition Eyes
3.1.1 Matched Filters for Flat Habitats
Matched filtering is clearly seen in the apposition eyes of animals adapted for life in
a flat habitat: a desert ant that runs across a salt pan (Wehner 1987), a water strider
that stands on a pond (Dahmen 1991 ), a backswimmer that looks upwards through
the water surface in search of stranded insects (Schwind 1980), or a fiddler crab that
scans a flat intertidal beach for mates (Zeil et al. 1986). All experience a bright world
dominated by the horizon, and all possess eyes having an elongated horizontal region
of enhanced resolution known as a visual streak (Fig. 2). Because most objects of
interest for these animals occur either at or very near the horizon, visual streaks
concentrate the majority of the eye's sampling stations- and thereby visual capacity
-at the same locations. Take the fiddler crab (Fig. 2A). Its apposition eyes are vertically
elongated, almost cylindrical, and located on long stalks above the carapace. The
shape of the eye means that the radius of curvature in the vertical eye plane is much
greater than in the horizontal eye plane, resulting in the vertical interommatidial
angles (L1¢v) being much smaller (by up to a factor of 4) than the horizontal ones. L1f/Jv
steeply narrows towards the horizon (from both above and below) becoming smallest
(0.3°) just along the eye's equator. The terrain of the fiddler crab's habitat not only
shapes the design of its eye, it also shapes its behaviour. Fiddler crabs categorize
193
ommatidia to focus light onto a single rhabdom. Because of aberrations and optical
misalignments, it is not a trivial task to direct 2000 individual beams oflight s
actually be possible, with a blurry image being the unfortunate result. Even if all the
light is perfectly coincident, it may still be impossible to trap the very wide cone of
incident light rays within the target rhabdom by total internal reflection (Warrant
and Mcintyre 1990a, 1991 ). High sensitivity may come only at the cost of spatial
resolution. Nevertheless, recent work has shown that image quality need not he all
that blurry: many diurnal superposition eyes are actually diffraction limited (Land
1984, Warrant et al. 1999), and aberrations in many others are not as big a problem
as previously thought (Nilsson et al. 2000). In addition, many superposition eyes
have tackled the problem of preserving the image in the retina by isolating their
rhabdoms in reflective tapetal sheaths, ensuring total internal reflection for any angle
of incidence (Land 1984; Warrant and Mcintyre 1991 ).
3 Compound Eyes for Bright Habitats
I mentioned above that the abundance oflight in bright habitats has allowed compound
eyes to develop acute zones, regions where the visual channels are more tightly
packed and sample the world with greater spatial resolution. These acute zones
invariably reveal remarkable matched filters - for habitat structure, motion and the
location of mates and prey - and it is these to which we tum our attention next.
3.1 Apposition Eyes
3.1.1 Matched Filters for Flat Habitats
Matched filtering is clearly seen in the apposition eyes of animals adapted for life in
a flat habitat: a desert ant that runs across a salt pan (Wehner 1987), a water strider
that stands on a pond (Dahmen 1991 ), a backswimmer that looks upwards through
the water surface in search of stranded insects (Schwind 1980), or a fiddler crab that
scans a flat intertidal beach for mates (Zeil et al. 1986). All experience a bright world
dominated by the horizon, and all possess eyes having an elongated horizontal region
of enhanced resolution known as a visual streak (Fig. 2). Because most objects of
interest for these animals occur either at or very near the horizon, visual streaks
concentrate the majority of the eye's sampling stations- and thereby visual capacity
-at the same locations. Take the fiddler crab (Fig. 2A). Its apposition eyes are vertically
elongated, almost cylindrical, and located on long stalks above the carapace. The
shape of the eye means that the radius of curvature in the vertical eye plane is much
greater than in the horizontal eye plane, resulting in the vertical interommatidial
angles (L1¢v) being much smaller (by up to a factor of 4) than the horizontal ones. L1f/Jv
steeply narrows towards the horizon (from both above and below) becoming smallest
(0.3°) just along the eye's equator. The terrain of the fiddler crab's habitat not only
shapes the design of its eye, it also shapes its behaviour. Fiddler crabs categorize