Fig. 8. Spatial summation in nocturnal bees. (A) Comparison of the first-order interneurons – L-fibre types L2, L3 and L4 – of the Megalopta genalis female (left) and the worker honeybee Apis mellifera (right). Compared with the worker honeybee, the horizontal branches of L-fibres in the nocturnal halictid bee connect to a much larger number of lamina cartridges, suggesting a possible role in spatial summation. L, lamina; M, medulla. Reconstructions from Golgi-stained frontal sections. Adapted from Greiner et al. (Greiner et al., 2004b) and Ribi (Ribi, 1975). (B,C) Spatial and temporal summation modelled at different light intensities in Megalopta genalis (B) and Apis mellifera (C) for an image velocity (V) of 240° s^–1 [measured from Megalopta genalis during a nocturnal foraging flight (Warrant et al., 2004)]. Light intensities are given for 540 nm, the peak in the bee's spectral sensitivity. Equivalent natural intensities are also shown. The finest spatial detail visible to flying bees (as measured by the maximum detectable spatial frequency, _max) is plotted as a function of light intensity. When bees sum photons optimally in space and time (continuous lines), vision is extended to much lower light intensities (non-zero _max) compared with when summation is absent (broken lines). Note that nocturnal bees can see in dimmer light than honeybees. Grey areas denote the light intensity window within which each species is normally active (although honeybees are also active at intensities higher than those presented on the graph). Adapted from Theobald et al. (Theobald et al., 2006).