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Eric J. Warrant
superposition eyes. Compared to Anopheles, the comeallenses of Toxorhynchites
are smaller (A= 22 Jlm) and deliver much narrower (<30°) and dimmer cones of
light to rhabdoms of much narrower receptive field (d/f= 3.2°). These features,
together with a much finer interommatidial angle (L1f1= 2.8°), make the light-adapted
diurnal apposition eye of Toxorhynchites an order of magnitude less sensitive than
the dark-adapted nocturnal apposition eye of Anopheles (Land et al. 1999).
The eyes of mosquitoes show that clever optical specializations can tum an
insensitive apposition eye into a decent light collector at night. Although it has not
been shown, it is quite possible that nocturnal mosquitoes collect even more light by
using hard-wired spatial summation. In the eyes of cave beetles, and several other
insects with open rhabdoms like those of Anopheles, the photoreceptor axons do not
simply terminate in the underlying lamina cartridge (as in most apposition eyes).
Instead, they spread laterally within the lamina, coupling up to 40 cartridges (Warrant
and Gislen, in prep.). This type of wiring has the potential to sum signals from large
groups of ommatidia (Fig. 9B) and thereby greatly improve sensitivity in dim light
(albeit at the cost of spatial resolution). This type of wiring, likely to be widespread
within the primitive nocturnal Diptera, is thought to be the ancestral precursor to
neural superposition, the wiring found in higher Diptera (Nilsson and Ro 1994).
The conical rhabdoms of mosquitoes and the hard-wired summation of cave
beetles have most likely never developed among bees and wasps. These animals
have typical diurnal apposition eyes, and their movement into the nocturnal niche is
probably comparatively recent. An interesting example is the honeybee genus Apis,
with four species including the well-known European honeybeeApis mellifera carnica
and its Africanized sister race, Apis mellifera scutellata. The Africanized honeybee,
and the southeast Asian giant honeybee Apis dorsata, will forage during dusk and
dawn, and even throughout the night if a moon half-full or larger is present in the
sky. This is despite the fact that the honeybee apposition eye should in theory be
blind by mid-dusk (Warrant et al. 1996). Behavioural experiments show, however,
that even the strictly day-active European honeybee is capable of seeing course habitat
features, like large pale flowers, at moonlight intensities. This ability can be explained
only if bees optimally sum photons over space and time (Fig. 9C: Warrant et al.
1996). According to theory, an Africanised bee could forage slowly in moonlight if it
possessed a visual exposure time of around 120 ms and summed signals from groups
consisting of no more than seven ommatidia, both requirements being rather modest
and within the capacity of an insect visual system.
4.2 Aquatic Habitats
The deep sea is a very special visual environment. With increasing depth it is not
only the intensity oflight that changes dramatically. Its colour and direction change,
too. Light becomes much bluer in deeper water, finally concentrating to a narrow
wavelength band around 475 nm (Tyler and Smith 1970). In addition to becoming
blue-shifted, almost all of the daylight available for vision comes increasingly from
above. This dim downwelling blue light provides the backdrop against which aquatic
Eric J. Warrant
superposition eyes. Compared to Anopheles, the comeallenses of Toxorhynchites
are smaller (A= 22 Jlm) and deliver much narrower (<30°) and dimmer cones of
light to rhabdoms of much narrower receptive field (d/f= 3.2°). These features,
together with a much finer interommatidial angle (L1f1= 2.8°), make the light-adapted
diurnal apposition eye of Toxorhynchites an order of magnitude less sensitive than
the dark-adapted nocturnal apposition eye of Anopheles (Land et al. 1999).
The eyes of mosquitoes show that clever optical specializations can tum an
insensitive apposition eye into a decent light collector at night. Although it has not
been shown, it is quite possible that nocturnal mosquitoes collect even more light by
using hard-wired spatial summation. In the eyes of cave beetles, and several other
insects with open rhabdoms like those of Anopheles, the photoreceptor axons do not
simply terminate in the underlying lamina cartridge (as in most apposition eyes).
Instead, they spread laterally within the lamina, coupling up to 40 cartridges (Warrant
and Gislen, in prep.). This type of wiring has the potential to sum signals from large
groups of ommatidia (Fig. 9B) and thereby greatly improve sensitivity in dim light
(albeit at the cost of spatial resolution). This type of wiring, likely to be widespread
within the primitive nocturnal Diptera, is thought to be the ancestral precursor to
neural superposition, the wiring found in higher Diptera (Nilsson and Ro 1994).
The conical rhabdoms of mosquitoes and the hard-wired summation of cave
beetles have most likely never developed among bees and wasps. These animals
have typical diurnal apposition eyes, and their movement into the nocturnal niche is
probably comparatively recent. An interesting example is the honeybee genus Apis,
with four species including the well-known European honeybeeApis mellifera carnica
and its Africanized sister race, Apis mellifera scutellata. The Africanized honeybee,
and the southeast Asian giant honeybee Apis dorsata, will forage during dusk and
dawn, and even throughout the night if a moon half-full or larger is present in the
sky. This is despite the fact that the honeybee apposition eye should in theory be
blind by mid-dusk (Warrant et al. 1996). Behavioural experiments show, however,
that even the strictly day-active European honeybee is capable of seeing course habitat
features, like large pale flowers, at moonlight intensities. This ability can be explained
only if bees optimally sum photons over space and time (Fig. 9C: Warrant et al.
1996). According to theory, an Africanised bee could forage slowly in moonlight if it
possessed a visual exposure time of around 120 ms and summed signals from groups
consisting of no more than seven ommatidia, both requirements being rather modest
and within the capacity of an insect visual system.
4.2 Aquatic Habitats
The deep sea is a very special visual environment. With increasing depth it is not
only the intensity oflight that changes dramatically. Its colour and direction change,
too. Light becomes much bluer in deeper water, finally concentrating to a narrow
wavelength band around 475 nm (Tyler and Smith 1970). In addition to becoming
blue-shifted, almost all of the daylight available for vision comes increasingly from
above. This dim downwelling blue light provides the backdrop against which aquatic