▪ Abstract The acuity of compound eyes is determined by interommatidial angles, optical quality, and rhabdom dimensions. It is also affected by light levels and speed of movement. In insects, interommatidial angles vary from tens of degrees in Apterygota, to as little as 0.24° in dragonflies. Resolution better than this is not attainable in compound eyes of realistic size. The smaller the interommatidial angle the greater the distance at which objects—prey, predators, or foliage—can be resolved. Insects with different lifestyles have contrasting patterns of interommatidial angle distribution, related to forward flight, capture on the wing, and predation on horizontal surfaces.

Visual acuity in insects.

http://link.springer.com/content/pdf/10.1007/978-3-642-66907-1_4.pdf

Optics and Vision in Invertebrates

Since the earth formed more than 5 billion years ago, sunlight has been the most potent selective force to control the evolution of living organisms. Consequences of this solar selection are most evident in eyes, the premier sensory outposts of the brain. Because organisms use light to see, eyes have evolved into many shapes, sizes, and designs; within these structures, highly conserved protein molecules for catching photons and bending light rays have also evolved. Although eyes themselves demonstrate many different solutions to the problem of obtaining an image--solutions reached relatively late in evolution--some of the molecules important for sight are, in fact, the same as in the earliest times. This suggests that once suitable biochemical solutions are found, they are retained, even though their \"packaging\" varies greatly. In this review, we concentrate on the diversity of eye types and their optical evolution, but first we consider briefly evolution at the more fundamental levels of molecules and cells.

The evolution of eyes.

Invertebrates are the class par excellence where one finds all manner of variations in the structure and arrangement of photoreceptors and their associated light-gathering and processing structures. We are compelled to search for a physiologic explanation of these unique forms. This article offers a highly personalized view of the physics suited to the task.

Physics of Vision in Compound Eyes

Acuity, the fineness with which sensory systems perceive and parse information, limits the information that organisms can extract from stimuli. Here, we focus on visual acuity (the ability to perceive static spatial detail) to discuss relationships between acuity and signal form and evolution. Research suggests that acuity varies by orders of magnitude across species, and that most animals have much lower acuity than humans. Thus, hypotheses regarding the function of spatial patterns must account for the acuity of relevant viewers. New data quantifying acuity in a range of taxa allow us to examine correlations between acuity and ecology, elucidate the selective forces that receiver acuity places on signal evolution, and examine how signals might appear to viewers with different acuities.

Visual Acuity and the Evolution of Signals

The changes in the directions of view of marked larval ommatidia were observed after the emergence of the adult. Those ommatidia that had been present during the first larval instar had the most posterior directions of view in the adult visual field while the newest ommatidia that had not been functional for vision in the aquatic larva contributed to the anterior and dorsal foveas of the aerial adults. The changes in interommatidial angles at emergence are discussed. Contrary to the general trend for interommatidial angles between retained larval ommatidia to decrease at emergence, the interommatidial angles in the larval fovea of aeshnid visual predators increase at emergence. The modifications in an odonate compound eye at emergence are like an exaggeration of the modifications that occur at the moult from one larval instar to the next, except that the newest ommatidia do not have any compromises in their design for use in the aquatic vision of the larvae. This is in contrast to the ommatidia retained from the earliest larval instars which have to have the most compromises in their design so that they can be adapted for the visual requirements of every larval instar, as well as the adult. This is discussed in relation to the trend among advanced species of odonates to replace the larval ommatidia by an entirely new set of adult ommatidia.

Development of the compound eyes of dragonflies (odonata). IV. Development of the adult compound eyes

Chironomidae were collected in floating emergence traps on 27.5 m deep Findley Lake in the coniferous forest of the Cascade Mountains, USA, from 1972 to 1975. There was considerable yearly variation in the date of thaw, the total number of Chironomidae that emerged and the relative abundance of each species. In 1972, 1973 and 1975 when there were early thaws, Tanytarsus and Procladius were the most common Chironomidae. Many of the common species started to emerge before the lake had completely thawed. In 1973 when most of the lake thawed June 7 and the surface water reached a maximum temperature of 19.25 °C in July, the Tanytarsus chinyensis group, Stictochironomus and Chironomus started to emerge from deep water when there was still ice along shore. Procladius nr gretis and Orthocladius started to emerge two days after the last ice had melted. In 1974 when most of the lake did not thaw until July 30–31, the surface water reached a maximum temperature of only 12.5 °C in August. Only a fifth as many Chironomidae emerged as in 1973. Most of these were Procladius and Orthocladius which had their maximum emergence when most of the lake was still frozen. The emergence of each species was reduced except Orthocladius. 
Procladius contributed the greatest biomass that emerged each year, but the second greatest biomass was contributed by different genera each year.

Emergence of chironomidae from Findley Lake in the coniferous forest of the cascade mountains after early and late thaws

From Findley Lake and two ponds in the Cascade Mountains (U.S.A.) Chironomidae started to emerge as soon as the ice thawed in 1972 and 1973. The 1051 Chironomidae that emerged per m2 from Findley Lake included 307 individuals ofTanytarsus and 358 ofProcladius. Tanytarsus outnumberedChironomus at all depths from 0 to 19.3 m. The 1506 Chironomidae that emerged per m2 from the 4.5 m deep pond included 997 individuals ofTanytarsus. The 1490 Chironomidae that emerged per m2 from the 2 m deep pond included 1363 Tanytarsus. The emerging biomass was 216 and 186 mg dry weight per m2 in Findley Lake and the 4.5 m deep pond, respectively, and only 78 mg in the 2 m deep pond. Most species had their maximum emergence where there was organic detritus from the surrounding forest.Orthocladius emerged from sand bottom at 2.7 to 5.2 m depth.Stictochironomus emerged from mud and sand at 8.4 m depth.Chironomus emerged from soft mud at 15 to 27.5 m depth.

Emergence of Chironomidae from Findley Lake and two ponds in the Cascade Mountains, U.S.A.

The distribution of ommatidial diameters and interommatidial angles, as determined by measuring the angles between the optic axes of adjacent ommatidia, are mapped across the surface of the compound eyes of a variety of species selected for different adult behaviors, developmental histories, and taxonomic positions. The size of the visual fields, prey capture foveas, foveas composed of large dorsal ommatidia, and other specializations in the numbers of ommatidia that view various directions in the visual field are discussed in relation to adult behavior. Advanced species have less resemblance between their larval and adult eyes than primitive species. In contrast to their larvae, adults increase the monocular resolution of each eye at the expense of binocular vision. Most species have foveas which view in approximately the anterior direction, instead of in a region of binocular overlap, and many species have foveal bands which view along the horizon. Some advanced perching species, which approach their prey and other odonates from below, have an additional vertical foveal band that views along a vertical plane from the anterior direction to a more dorsal direction. The most unusual foveal band is seen in active flying species. The large dorsal ommatidia of the migratory Anax junius, which cover approximately one third of the eye surface, view a narrow region of the visual field that extends along a plane from the most lateral direction of one eye to a dorsal direction, and continues without interruption to the most lateral direction of the other eye.

Development of the compound eyes of dragonflies (odonata). III. Adult compound eyes

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Development of the compound eyes of dragonflies (odonata). IV. Development of the adult compound eyes

Emergence of chironomidae from Findley Lake in the coniferous forest of the cascade mountains after early and late thaws

Emergence of Chironomidae from Findley Lake and two ponds in the Cascade Mountains, U.S.A.

Development of the compound eyes of dragonflies (odonata). III. Adult compound eyes