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Eric J. Warrant
see it, which shows that the fly apposition eye is better resolved in bright light than
the hawkmoth superposition eye. By mid-dusk, the fly apposition eye is unable to
collect enough light to see the movements of the finer grating. By the time intensities
have dropped to moonlight levels, it has also lost its ability to see the coarser grating.
At some intensity between mid-dusk and moonlight the fly becomes blind. Even
though the hawkmoth is unable to respond to movements of the fine grating at any
intensity, they maintain a strong response to the coarse grating at all intensities, even
starlight. These physiological results are in good accord with the moth's behaviour:
with the aid of image intensification equipment, I have observed Deilephila flying
and hovering confidently in starlight.
The superior light gathering capacity of superposition eyes- their great advantage
for animals active in dim light - is clearly the reason why the hawkmoth outperforms
the fly at low intensities. Compared to the fly's apposition eye (F-number = 2.6), the
optics of Deilephila's superposition eye (F-number = 0.6), deliver more than 100
times as much light to the retina. At a higher level, spatial and temporal summation
appears to improve sensitivity even more (Warrant and O'Carroll, in prep.).
4.1.2 Apposition Eyes
As already mentioned, apposition eyes are not suited to dim light. Simply put, their
small lenses have trouble collecting enough light for reliable vision. Despite this,
various factors including predation, and competition for a limited food resource,
forced many diurnal animals with apposition eyes to become nocturnal. In the tropics
for instance, some species of bees and wasps, some with very rich visual behaviour,
have become entirely nocturnal, despite their eye design (Warrant, Kelber and W cislo,
in prep.). Other nocturnal insects, such as crane flies and mosquitoes, seem to possess
apposition eyes simply because their ancestors, who were likewise nocturnal, also
possessed them (Nilsson and Ro 1994, Melzer et al. 1997, Land et al. 1999). These
eyes, however, were not ordinary apposition eyes, but had special optical and neural
adaptations for vision in dim light. Some of these adaptations are still present in
nocturnal species even today, and it is to these which we turn our attention now.
A lovely example of how apposition eyes have adapted to dim light can be found
among the mosquitoes (Land et al. 1997, 1999). These insects have conquered all
light intensities, from brightest sunshine (e.g. the nectar-feeding Toxorhynchites
brevipalpis) to starlight (e.g. the blood-sucking Anopheles gambiae). The apposition
eyes of Anopheles have rhabdoms with huge receptive fields (d/f= 37°), and very
wide interommatidial angles (L1if' = 8°). The large corneal lenses, which are almost
hemispherical, have a diameter A = 28 jlm and deliver a tremendously wide (60°)
cone oflight (and thereby a very bright image) to each rhabdom. Such a wide cone
of light is potentially disastrous, because a normal rod-shaped rhabdom, with a
refractive index only slightly higher than that of the surrounding cells, is unable to
internally reflect a cone oflight greater than about 20° (Warrant and Mcintyre 1990a,
1993). Anopheles has overcome this problem with a wonderful adaptation: instead
of having normal rod-shaped rhabdoms, they have conical ones, and these trap up to
nine times as much light (Land et al. 1997: Fig. 9A). In contrast, the day-active
Eric J. Warrant
see it, which shows that the fly apposition eye is better resolved in bright light than
the hawkmoth superposition eye. By mid-dusk, the fly apposition eye is unable to
collect enough light to see the movements of the finer grating. By the time intensities
have dropped to moonlight levels, it has also lost its ability to see the coarser grating.
At some intensity between mid-dusk and moonlight the fly becomes blind. Even
though the hawkmoth is unable to respond to movements of the fine grating at any
intensity, they maintain a strong response to the coarse grating at all intensities, even
starlight. These physiological results are in good accord with the moth's behaviour:
with the aid of image intensification equipment, I have observed Deilephila flying
and hovering confidently in starlight.
The superior light gathering capacity of superposition eyes- their great advantage
for animals active in dim light - is clearly the reason why the hawkmoth outperforms
the fly at low intensities. Compared to the fly's apposition eye (F-number = 2.6), the
optics of Deilephila's superposition eye (F-number = 0.6), deliver more than 100
times as much light to the retina. At a higher level, spatial and temporal summation
appears to improve sensitivity even more (Warrant and O'Carroll, in prep.).
4.1.2 Apposition Eyes
As already mentioned, apposition eyes are not suited to dim light. Simply put, their
small lenses have trouble collecting enough light for reliable vision. Despite this,
various factors including predation, and competition for a limited food resource,
forced many diurnal animals with apposition eyes to become nocturnal. In the tropics
for instance, some species of bees and wasps, some with very rich visual behaviour,
have become entirely nocturnal, despite their eye design (Warrant, Kelber and W cislo,
in prep.). Other nocturnal insects, such as crane flies and mosquitoes, seem to possess
apposition eyes simply because their ancestors, who were likewise nocturnal, also
possessed them (Nilsson and Ro 1994, Melzer et al. 1997, Land et al. 1999). These
eyes, however, were not ordinary apposition eyes, but had special optical and neural
adaptations for vision in dim light. Some of these adaptations are still present in
nocturnal species even today, and it is to these which we turn our attention now.
A lovely example of how apposition eyes have adapted to dim light can be found
among the mosquitoes (Land et al. 1997, 1999). These insects have conquered all
light intensities, from brightest sunshine (e.g. the nectar-feeding Toxorhynchites
brevipalpis) to starlight (e.g. the blood-sucking Anopheles gambiae). The apposition
eyes of Anopheles have rhabdoms with huge receptive fields (d/f= 37°), and very
wide interommatidial angles (L1if' = 8°). The large corneal lenses, which are almost
hemispherical, have a diameter A = 28 jlm and deliver a tremendously wide (60°)
cone oflight (and thereby a very bright image) to each rhabdom. Such a wide cone
of light is potentially disastrous, because a normal rod-shaped rhabdom, with a
refractive index only slightly higher than that of the surrounding cells, is unable to
internally reflect a cone oflight greater than about 20° (Warrant and Mcintyre 1990a,
1993). Anopheles has overcome this problem with a wonderful adaptation: instead
of having normal rod-shaped rhabdoms, they have conical ones, and these trap up to
nine times as much light (Land et al. 1997: Fig. 9A). In contrast, the day-active