Eye specialisation in waterstriders: an adaptation to life in a flat world

Abstract

The angular distribution of the optical axes of the ommatidia in the eyes of waterstriders (Gerris lacustris) has been investigated using antidromic illumination. 1. Each eye contains about 920 ommatidia arranged in approximately 40 nearly horizontal rows (Figs. 5, 6). 2. There is an acute zone (‘visual streak’) of ±5° around the eye equator with high vertical resolution (Fig. 9). 3. The horizontal interommatidial angle ΔΦoh varies little with altitude and is in the frontal parts of the eye equal to the (horizontal) interrhabdomere angle ΔΦrh (Fig. 10). 4. Above and below the ‘visual streak’ the eye is adapted to the perception of relative height or depth ‘constancy’ (Fig. 13). 5. Reconstruction of the ‘viewing directions’ of the rhabdomeres in the ‘visual streak’ reveals that characteristic groups of rhabdomeres in different ommatidia coincide in their ‘receptive fields’. Optical prerequisites for two types of neural superposition can be found: one of medium neural summation but highest sampling frequency, the other of highest neural summation at the cost of resolution (Fig. 11). 6. The correspondence of the limits of resolution due to diffraction and finite sampling frequency is discussed.