Really, is that so?
Most people know that insects have compound eyes with often hundreds and even thousands of hexagonal facets. But what most people do not know is that many insects in addition to the two large compound eyes possess also an additional three much smaller single lens eyes (known as ocelli) on their forehead. These extra eyes are usually arranged in a triangular position on the insect’s forehead with the two lateral ones placed a little higher alongside the single, median ocellus.
Despite the huge amount of research that over the years has been conducted in connection with the compound eyes’ structure and function and has led to a considerable amount of understanding how the visual signals are received, analysed, transmitted to the insect’s brain and the elicit behavioural responses, the role (or roles) of the ocelli are still not fully clear. Although there is some evidence that they help a flying insect to maintain a balanced course and that damage to the ocelli, at least in some insects, interferes with their orientation mechanism, it is puzzling why members of some insect orders possess ocelli and others do not. If the ocelli, as another functional suggestion has it, work in concert with the compound eyes and analogously to a photometer prime the compound eyes by setting up their overall sensitivity to the ambient light level, then one could have perhaps expected to find ocelli in all insect species, but that is clearly not the case. Ocelli are almost always present in those insect orders with aquatic larvae like dragonflies, mayflies, stoneflies and some caddisflies. But they are absent in virtually all 400,000 or so species of beetles and in butterflies, bugs (Hemiptera), lacewings, scorpionflies and true flies (Diptera) some have them and some do not. Ants, wasps, bees, etc, almost always have them, but so do the unrelated winged termite castes.
Structurally these little eyes, where they are present, are rather similar. There is, as could be expected, some variation with regard to the diameters of the ocelli, the curvatures of their corneal lenses, their precise location on the forehead and to what extent hairs on the insect head surrounding them affect their visual field. However, it has convincingly been shown that these eyes are incapable of forming an image on their respective retinas, because the images are always underfocused and would, at best, produce a very blurry representation of the real world. The retinas of the various ocelli in the different insect orders all contain typical insect photoreceptive cells with ultrastructurally similar membrane tubes that house the photopigments in them. The orientation and arrangement of the photoreceptive membranes, however, can vary between species, suggesting that some ocelli may be capable of perceiving linearly polarized light that could help them navigating. Yet again, this would not explain why not all flying insects share this ability and, in fact, why flying beetles do not even possess ocelli at all.
Can they perceive colours? I was perhaps one of the first in the world to test the spectral sensitivity of a dorsal ocellus of a bumble bee electrophysiologically and determined that it had two sensitivity peaks: one in the ultraviolet to light of around 350 nm wavelength and one in the green range of the spectrum around 520 nm wavelength. In terms of their visual field, I found that it covered an approximately 60 degree wide diameter. What I did not examine was the overlap between the visual field of the three ocelli with each other and the compound eyes. This was recently investigated by a group of researchers headed by Emily Baird in Sweden, who were interested why only in bumble bees but not in honey bees the three ocelli are placed in a horizontal row rather than being triangularly positioned and bumble bee males and females had similar eyes while in honey bees they were dissimilar. The researchers found that the occluding hairs around the ocelli played an important role to reduce visual overlaps and that male bumble bees appeared to be foraging more like female bumble bees , while honey bee drones and female honey bees differed much more from each other. The data presented by the Swedish group allowed me to calculate an F-number that shows that the bumble bee’s dorsal ocelli could function under much dimmer light than humans could see in. And yet, as to the precise function of the little insect eyes, well, we still don’t know.
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