In the compound eye of Ciulfina (Mantidae) there are large regional differences in interommatidial angle as measured optically from the pseudopupil. Notably there is an acute zone which looks backwards as well as one looking forwards. There are correlated regional differences in the dimensions of the ommatidia. The following anatomical features which influence the optical performance have been measured in different parts of the eye: (a) The facet diameter is greater where the interommatidial

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... le is smaller. This could influence resolving power, but calculation shows that facet size does not exert a dominant effect on the visual fields of the receptors. (b) The rhabdom tip diameter, which theoretically has a strong influence on the size of visual fields, is narrower in eye regions where the interommatidial angle is smaller. (c) The cone length, from which the focal length can be estimated, is greater where the interommatidial angle is smaller. Estimation of the amount of light reaching the rhabdom suggests that different parts of the eye have similar sensitivity to a point source of light, but differ by a factor of at least 10 in sensitivity to an extended source. There is anatomical evidence that in the acute zone the sensitivity has been sacrificed for the sake of resolution. Maps of the theoretical minimum fields of the photoreceptors, plotted in their positions on the eye in angular coordinates, suggest that there are too few ommatidia for the eye as a whole to reconstruct all the visual detail that the individual receptors can resolve. The conclusion from (3) and (4), together with some behavioural evidence, suggests that the eye structure must make possible the resolution of small movements of contrasting edges and of small dark contrasting objects but there is less emphasis on the total reconstruction of fine patterns because the interommatidial angle is greater than the estimate of the acceptance angle.