The Design of Compound Eyes and the Illumination of Natural Habitats
209
contrast, the ventral part is quite small with facets only half as big. Resolution is not
uniform either, becoming significantly better in the dorsal direction, with a clear
match to the angular distribution of downwelling light (Fig. I OD). The trend becomes
even more extreme in deeper-living species such as Phronima. In these amphipods,
which live below 800 m, each eye is entirely divided, with a huge dorsal eye detached
from a tiny ventral one (Fig. lOE). The dorsal facets are gigantic (146 J.!m wide
compared to 80 J..lm in the ventral eyes), and resolution escalates within a narrow
(15°) dorsal visual field, outside of which resolution is very poor (Fig. !OF).
What do these depth-related changes in eye design mean? Two of the trends are
easy to understand: the enlargement of facets clearly improves sensitivity, and the
increasingly dorsal bias of the eyes reflects the dominance of dorsal illumination.
The third trend is slightly less obvious: with increasing depth dorsal resolution
increases dramatically. In the three species detailed above, interommatidial angle
falls from 4.8° in Thamneus, to 1.1 o in Parapronoe, and to an astonishing 0.25° in
Phronima. At the same time the total dorsal visual field narrows sharply. The
explanation can be found by considering what amphipods need to see. Had the eyes
been designed to view wide extended scenes, the interommatidial angle should have
increased with depth. However, if the role of the dorsal eyes is to spot small silhouetted
objects floating above, then the opposite would be predicted (Land 1989b). Even
though the life histories of amphipods are poorly understood, the design of their
eyes implies that they use them for spotting small prey animals against the dim
downwelling illumination.
Of course, amphipods are not the only animals that have adapted to the aquatic
light environment in this way. Many euphausiid shrimps (krill) have bilobed
superposition eyes that are adapted in exactly the same manner (Fig. 11 ), with deeper
living species having dorsal eyes with more numerous facets and sharper resolution
directed into narrower dorsal visual fields (Land et al. 1979). At this point it is difficult
to avoid mentioning that even in terrestrial habitats the same kinds of adaptations are
found. The males of many mayfly species patrol small forest clearings at dusk in the
hope that females - tiny silhouettes against a narrow and dim patch of sky- might fly
over them. These males typically have divided superposition eyes, with two huge
tube-like dorsal eyes ofhigh resolving power directed into narrow overlapping dorsal
fields of view. In other species the males fly over open fields and females can appear
as silhouettes anywhere within the wide hemisphere of the sky. Their superposition
eyes are not fully divided and have dorsal parts of medium resolution viewing much
broader fields of view (Brannstrom and Nilsson 2000). If nothing else, these wonderful
examples of convergent evolution emphasize the remarkable influence of habitat
illumination on the design of compound eyes.
5 Conclusions
The animal kingdom's most widespread eye design is certainly one of its most
adaptable. In this chapter I have only attempted to describe the way in which
compound eyes have evolved in response to the intensity and direction of natural
209
contrast, the ventral part is quite small with facets only half as big. Resolution is not
uniform either, becoming significantly better in the dorsal direction, with a clear
match to the angular distribution of downwelling light (Fig. I OD). The trend becomes
even more extreme in deeper-living species such as Phronima. In these amphipods,
which live below 800 m, each eye is entirely divided, with a huge dorsal eye detached
from a tiny ventral one (Fig. lOE). The dorsal facets are gigantic (146 J.!m wide
compared to 80 J..lm in the ventral eyes), and resolution escalates within a narrow
(15°) dorsal visual field, outside of which resolution is very poor (Fig. !OF).
What do these depth-related changes in eye design mean? Two of the trends are
easy to understand: the enlargement of facets clearly improves sensitivity, and the
increasingly dorsal bias of the eyes reflects the dominance of dorsal illumination.
The third trend is slightly less obvious: with increasing depth dorsal resolution
increases dramatically. In the three species detailed above, interommatidial angle
falls from 4.8° in Thamneus, to 1.1 o in Parapronoe, and to an astonishing 0.25° in
Phronima. At the same time the total dorsal visual field narrows sharply. The
explanation can be found by considering what amphipods need to see. Had the eyes
been designed to view wide extended scenes, the interommatidial angle should have
increased with depth. However, if the role of the dorsal eyes is to spot small silhouetted
objects floating above, then the opposite would be predicted (Land 1989b). Even
though the life histories of amphipods are poorly understood, the design of their
eyes implies that they use them for spotting small prey animals against the dim
downwelling illumination.
Of course, amphipods are not the only animals that have adapted to the aquatic
light environment in this way. Many euphausiid shrimps (krill) have bilobed
superposition eyes that are adapted in exactly the same manner (Fig. 11 ), with deeper
living species having dorsal eyes with more numerous facets and sharper resolution
directed into narrower dorsal visual fields (Land et al. 1979). At this point it is difficult
to avoid mentioning that even in terrestrial habitats the same kinds of adaptations are
found. The males of many mayfly species patrol small forest clearings at dusk in the
hope that females - tiny silhouettes against a narrow and dim patch of sky- might fly
over them. These males typically have divided superposition eyes, with two huge
tube-like dorsal eyes ofhigh resolving power directed into narrow overlapping dorsal
fields of view. In other species the males fly over open fields and females can appear
as silhouettes anywhere within the wide hemisphere of the sky. Their superposition
eyes are not fully divided and have dorsal parts of medium resolution viewing much
broader fields of view (Brannstrom and Nilsson 2000). If nothing else, these wonderful
examples of convergent evolution emphasize the remarkable influence of habitat
illumination on the design of compound eyes.
5 Conclusions
The animal kingdom's most widespread eye design is certainly one of its most
adaptable. In this chapter I have only attempted to describe the way in which
compound eyes have evolved in response to the intensity and direction of natural