The small compound eyes of water fleas are usually regarded as unsophisticated, with low visual acuity. Indeed this is true for the most common water flea, Daphnia (Young & Downing, 1976). However, another species, Polyphemus pediculus, is a visually guided predator (Ischtereyt, 1933). The cyclopic eye of Polyphemus is not larger than that of Daphnia but it certainly appears more elaborated. Looking at the pseudopupil, it is obvious that the interommatidial angles are different in different regions of the eye. To reveal the extent of these regional differences, the interommatidial angles were recorded. This was done by mounting an animal in a goniometer and observing the position of the pseudopupil while the goniometer was turned (Horridge, 1977 a) (Fig. 1A). The recorded co-ordinates can be characterized as points on a spherical surface. Transforming this spherical surface to a flat map inevitably...

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