Miniaturisation reduces contrast sensitivity and spatial resolving power in ants

Vision is crucial for animals to find prey, locate conspecifics and navigate within cluttered landscapes. Animals need to discriminate objects against a visually noisy background. However, the ability to detect spatial information is limited by eye size. In insects, as individuals become smaller, the space available for the eyes reduces, which affects the number of ommatidia, the size of the lens and the downstream information-processing capabilities. The evolution of small body size in a lineage, known as miniaturisation, is common in insects. Here, using pattern electroretinography with vertical sinusoidal gratings as stimuli, we studied how miniaturisation affects spatial resolving power and contrast sensitivity in four diurnal ants that live in a similar environment but vary in their body and eye size. We found that ants with fewer and smaller ommatidial facets had lower spatial resolving power and contrast sensitivity. The spatial resolving power was maximum in the largest ant Myrmecia tarsata at 0.60 cycles deg⁻¹ compared with that of the ant with smallest eyes Rhytidoponera inornata at 0.48 cycles deg⁻¹. Maximum contrast sensitivity (minimum contrast threshold) in M. tarsata (2627 facets) was 15.51 (6.4% contrast detection threshold) at 0.1 cycles deg⁻¹, while the smallest ant R. inornata (227 facets) had a maximum contrast sensitivity of 1.34 (74.1% contrast detection threshold) at 0.05 cycles deg⁻¹. Miniaturisation thus dramatically decreases maximum contrast sensitivity and also reduces spatial resolution, which could have implications for visually guided behaviours. This is the first study to physiologically investigate contrast sensitivity in the context of insect allometry.