The Design of Compound Eyes and the Illumination of Natural Habitats
195
compressed to a 97° cone oflight underwater. Within this cone oflight, called Snell's
window, all the features of the terrestrial world above can be found, including the
flat water surface, which is located at the edge of the cone (Walls 1942, p. 378). By
looking upwards along the edge of the cone, a suspended backswimmer is able to
have a periscopic view of the outside water surface and see anything, including prey,
which might be trapped on it. The water surface is an important horizon for the
backswimmer, and the ventral part of the eye possesses a well-developed visual
streak (Fig. 2C) that watches the surface along the edge of Snell's window (Schwind
1980). This matched filtering does not stop at the optics of the eye: in the optic lobe
there are cells which have their visual fields coincident with the visual streak, and
which respond maximally to prey-sized objects on the water surface (Schwind 1978).
But the water surface is not the backswimmer's only horizon. Frontwards, the
backswimmer can also see the environment of the pond and any item of interest that
might be located there. There is a second visual streak which views this direction as
well! (Schwind 1980)
3.1.2 Matched Filters for Detecting Mates and Prey Against the Sky
The need to see other animals, whether they be potential mates, possible food items
or advancing predators, is so important in the lives of animals that it has driven the
evolution of a wide range of visual specializations. In compound eyes this has resulted
in a fantastic variety of acute zones (Land 1989a). In even more extreme cases entire
extra compound eyes have evolved, to create divided eyes, like those of male bibionid
flies which are specialized for the dorsal detection of females (Zeil 1983). We will
encounter divided eyes again when we discuss the superposition eyes of krill.
In many insects the apposition eyes show a strong sexual dimorphism. Female
brachyceran flies, like blowflies and hoverflies, have their eyes spaced well apart,
but in males the eyes are joined at the top of the head. This extra area of eye constitutes
a frontal-dorsal acute zone used by the males to keep sight of females during highspeed pursuits (Land and Eckert 1985). Amusingly, these acute zones are often referred
to as love spots. They are clearly seen in the male hoverfly Volucella pellucens,
which has large love spots located frontally, 20° above the equator (Fig. 3B). The
interommatidial angle (t1cp) here falls to just 0.7° (Warrant and Olsson, unpubl.).
The size of the acute zone (the eye region where, say, t1cp < 1.1 °) occupies 2230
deg 2 of the visual field (shaded area in Fig. 3B). In females there is also an acute
zone, directed exactly frontally (Fig. 3A). As we will see, this is a useful adaptation
for matching the flow field during flight. In females, t1cp only falls to 0.9°, and the
area of the acute zone (L1l/J < 1.1 °) is a mere 23% as large as that of males (51 0 deg 2 :
shaded area in Fig. 3A).
The acute zones of male flies are not restricted to the eye surface. Below the eye
there is an intricate neural pathway that is specific to males. First, the connections of
photoreceptor axons to the lamina are quite different in the acute zone compared to
both the female's eye and the rest of the male's eye (as found in houseflies:
Franceschini et al. 1981 ). Higher up the brain, in the lobula, Strausfeld and colleagues
(Strausfeld 1991; Gilbert and Strausfeld 1991; Gronenberg and Strausfeld 1991)
195
compressed to a 97° cone oflight underwater. Within this cone oflight, called Snell's
window, all the features of the terrestrial world above can be found, including the
flat water surface, which is located at the edge of the cone (Walls 1942, p. 378). By
looking upwards along the edge of the cone, a suspended backswimmer is able to
have a periscopic view of the outside water surface and see anything, including prey,
which might be trapped on it. The water surface is an important horizon for the
backswimmer, and the ventral part of the eye possesses a well-developed visual
streak (Fig. 2C) that watches the surface along the edge of Snell's window (Schwind
1980). This matched filtering does not stop at the optics of the eye: in the optic lobe
there are cells which have their visual fields coincident with the visual streak, and
which respond maximally to prey-sized objects on the water surface (Schwind 1978).
But the water surface is not the backswimmer's only horizon. Frontwards, the
backswimmer can also see the environment of the pond and any item of interest that
might be located there. There is a second visual streak which views this direction as
well! (Schwind 1980)
3.1.2 Matched Filters for Detecting Mates and Prey Against the Sky
The need to see other animals, whether they be potential mates, possible food items
or advancing predators, is so important in the lives of animals that it has driven the
evolution of a wide range of visual specializations. In compound eyes this has resulted
in a fantastic variety of acute zones (Land 1989a). In even more extreme cases entire
extra compound eyes have evolved, to create divided eyes, like those of male bibionid
flies which are specialized for the dorsal detection of females (Zeil 1983). We will
encounter divided eyes again when we discuss the superposition eyes of krill.
In many insects the apposition eyes show a strong sexual dimorphism. Female
brachyceran flies, like blowflies and hoverflies, have their eyes spaced well apart,
but in males the eyes are joined at the top of the head. This extra area of eye constitutes
a frontal-dorsal acute zone used by the males to keep sight of females during highspeed pursuits (Land and Eckert 1985). Amusingly, these acute zones are often referred
to as love spots. They are clearly seen in the male hoverfly Volucella pellucens,
which has large love spots located frontally, 20° above the equator (Fig. 3B). The
interommatidial angle (t1cp) here falls to just 0.7° (Warrant and Olsson, unpubl.).
The size of the acute zone (the eye region where, say, t1cp < 1.1 °) occupies 2230
deg 2 of the visual field (shaded area in Fig. 3B). In females there is also an acute
zone, directed exactly frontally (Fig. 3A). As we will see, this is a useful adaptation
for matching the flow field during flight. In females, t1cp only falls to 0.9°, and the
area of the acute zone (L1l/J < 1.1 °) is a mere 23% as large as that of males (51 0 deg 2 :
shaded area in Fig. 3A).
The acute zones of male flies are not restricted to the eye surface. Below the eye
there is an intricate neural pathway that is specific to males. First, the connections of
photoreceptor axons to the lamina are quite different in the acute zone compared to
both the female's eye and the rest of the male's eye (as found in houseflies:
Franceschini et al. 1981 ). Higher up the brain, in the lobula, Strausfeld and colleagues
(Strausfeld 1991; Gilbert and Strausfeld 1991; Gronenberg and Strausfeld 1991)