Columbines

Can nocturnal bees really see so well with such tiny eyes? Remarkably yes. But there is no question that their visual systems are operating near the limit and that only the coarser and slower features of the world can be seen. Light intensity plays a deciding role in whether a particular bee species is able to forage or not — many species of bees that are capable of visual foraging in the early dusk may be forced back to the nest just a short time later before light levels have become unacceptably dim. Those able to forage at the very dimmest light levels (such as Megalopta genalis (Figure 1) and the giant Indian carpenter bee Xylocopa sp.) typically have visual specialisations that permit nocturnal foraging. Apart from giant ocelli, these bees have very large ommatidial facet lenses and massive photoreceptors, adaptations that maximise light capture and improve vision. Moreover, it is very likely that nocturnal bees (and indeed many other nocturnal insects) are also able to enhance vision at a higher level in the visual system by neurally summing photons in space and time. This strategy, which improves visual sensitivity at the expense of spatial and temporal resolution, requires the presence of specialised circuits in the optic lobes of the brain. Even though we have considerable evidence for the presence of such circuits, their existence remains to be established with certainty. But the remarkable visual abilities of nocturnal bees — which allow landmark-based homing at light levels where humans are practically blind — are beyond question.