Simple Corneal Eyes In Insects

Insect simple eyes, or ocelli, fall into two main groups: the larval eyes of holometabolous insects and the dorsal ocelli present in most winged adult insects. In both, the curved air/tissue cornea interface is the main refracting surface, although as in vertebrate eyes, a lens of some kind often augments the optical power of the system and aids in the formation of the image.

Larval Ocelli

In insects with a distinct larval stage, the ocelli are the only eyes the larvae possess. They vary greatly in size and complexity. The larvae of flies have no more than a small group of lightsensitive cells on either side of the head. Lepidopteran caterpillars, however, have ocelli with lenses and a structure resembling that of a single ommatidium from a compound

eye. In each ocellus in the Isia, seven receptors contribute to a two-tiered rhabdom containing the photopigment (Fig. 18A). There seems little possibility of spatial resolution within each ocellus, but as it appears that the fields of view of the 12 ocelli do not overlap, they are capable of providing a 12-"pixel" sampling mosaic of the surroundings. These ocelli do, however, resolve color; three spectral types of receptor have been found in butterfly larval ocelli.

The ant lion Euroleon (Neuroptera) also has six ocelli on each side of the head, borne on a small turret (Fig. 18B). Unlike caterpillars, however, each has an extended retina of 40 to 50 receptors, giving interreceptor angles (A9) of 5 to 10°. Although this resolution is not impressive, it is presumably enough to allow the animals to detect their prey, e.g., moving ants, at a distance of about 1 cm. Sawflies (Hymenoptera) have larvae with a single pair of ocelli, each with an in-focus retina covering a hemisphere (Fig. 18C). The rhabdoms in Perga are made up of the contributions from eight receptors (much as in an ordinary compound eye) and are spaced 20 ^m apart, giving an interreceptor angle of 4 to 6°. These larvae are vegetarian, and it seems that the main function of the ocelli is to direct the larvae to their host plants. However, Perga larvae will also track moving objects with their head and defend themselves by spitting regurgitated sap.

The most impressive of all larval ocelli are found in tiger beetles (Cicindela). These have a lifestyle similar to that of ant lions, ambushing insect prey as they pass their burrows (Figs. 18D-18F). There are again six ocelli on each side of the head, but two are much larger than the others. The largest has a diameter of 0.2 mm and a retina containing 6350 receptors. The interreceptor angle is about 1.8°, comparable with or better than the resolution of the compound eyes of most adult insects. This raises the interesting question as to why the insects did not retain eyes like this into adult life.

FIGURE 18 Simple eyes (ocelli) of increasing complexity in larval insects. (A) Lepidopteran, (B) Neuropteran, (C) Hymenopteran. Scale bars, 0.1 mm. (D—F) Large simple eyes of tiger beetle larvae (Cicindela). They are used to spot prey (usually ants), which they ambush from the burrow. (D) Head with six pairs of eyes. (E) Larva in ambush position. (F) Largest ocellus showing corneal lens and retina. Inset: Tangential section of retina. (Reproduced, with permission, from Land and Nilsson, 2002.)

FIGURE 18 Simple eyes (ocelli) of increasing complexity in larval insects. (A) Lepidopteran, (B) Neuropteran, (C) Hymenopteran. Scale bars, 0.1 mm. (D—F) Large simple eyes of tiger beetle larvae (Cicindela). They are used to spot prey (usually ants), which they ambush from the burrow. (D) Head with six pairs of eyes. (E) Larva in ambush position. (F) Largest ocellus showing corneal lens and retina. Inset: Tangential section of retina. (Reproduced, with permission, from Land and Nilsson, 2002.)

FIGURE 19 Dorsal ocelli of adult locust. (A) Positions of frontal and lateral ocelli on head. (B) Section of an ocellus, showing the different layers and the positions of the focus in light- and dark-adapted states. The focus is a long distance behind the receptor layers. (C) Fields of view of the three ocelli straddling the horizon. (Reproduced, with permission, from Land and Nilsson, 2002.)

FIGURE 19 Dorsal ocelli of adult locust. (A) Positions of frontal and lateral ocelli on head. (B) Section of an ocellus, showing the different layers and the positions of the focus in light- and dark-adapted states. The focus is a long distance behind the receptor layers. (C) Fields of view of the three ocelli straddling the horizon. (Reproduced, with permission, from Land and Nilsson, 2002.)

Dorsal Ocelli of Adults

Adult insects that fly typically have three simple eyes on the top of their heads. These dorsal ocelli resemble larval ocelli in possessing a lens and (like sawfly larvae) an extended retina (Fig. 19), but they are not embryologically related to the larval eyes. Some dorsal ocelli have tapeta, and some a mobile iris. They each have a wide field of view of 150° or more and may have as many as 10,000 receptors. So far all this suggests that these are "good" eyes, like those of hunting spiders. However, they are profoundly out of focus, with the retina much too close to the lens. For example, in the blow fly Calliphora the receptors extend from 40 to 100 |lm behind the lens, but the focus is at 120 |lm.

What then are they for? Recent studies mainly support the idea that the ocelli are horizon detectors, involved in enabling an insect to make fast corrections for pitch and roll. The defocus then makes sense; high spatial frequency clutter such as leaves and branches will be removed, allowing the receptors to respond to changes in the overall distribution of light in the sky. The idea that these ocelli contribute to flight equilibrium is supported by the fact that the receptors converge massively onto a relatively few second-order neurons that project directly into the optomotor system.

See Also the Following Articles

Brain and Optic Lobes • Ocelli and Stemmata

Further Reading

Exner, S. (1989). "The Physiology of the Compound Eyes of Insects and Crustaceans." Springer-Verlag, Berlin. [Translated from the 1891 German ed. by R. C. Hardie.] Land, M. F. (1997). Visual acuity in insects. Annu. Rev. Entomol. 42, 147-177.

Land, M. F., and Nilsson, D.-E. (2002). "Animal Eyes." Oxford University Press, London.

Nilsson, D.-E. (1989). Optics and evolution of the compound eye. In "Facets of Vision" (D. G. Stavenga and R. C. Hardie, eds.), pp. 30-73. Springer-Verlag, Berlin. Wehner, R. (1981). Spatial vision in arthropods. In "Handbook of Sensory Physiology VII/6C" (H. Autrum, ed.), pp. 287-616. Springer-Verlag, Berlin.

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