How stalk-eyed flies eye stalk-eyed flies: Observations and measurements of the eyes ofCyrtodiopsis whitei (Diopsidae, Diptera)
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The Malayan stalk-eyed flyCyrtodiopsis whitei, of the family Diopsidae, exhibits sexual dimorphism in that the eyestalk span of the males exceeds the length of the body, whereas that of the females, which have shorter bodies on the average, is smaller than the body length. The ratio of the sexes in a population departs significantly from 1∶1; in samples of newly emerged imagines there are twice as many females as males.
The behaviour ofCyrtodiopsis is very much determined by vision. During the day, temporary territories may be defended by threat behaviour. At dusk the animals gather in small groups on selected threadlike structures, returning to the same site each day. When males of about equal size encounter one another within such a group they may engage in ritualized fights (or occasionally contact fights). Competitors are driven away by the dominant male. Conspecifics are most likely to elicit a threat or flight reaction when they are at a distance of about 50 mm, and reactions to model flies and reflections in a mirror also occur at about this critical distance.
Body length is closely correlated with both eyestalk span and the number of ommatidia in the compound eyes (cf. Results, Sect. 2, Table 1 and Figs. 2, 3, 7 and 8). The largest animals have about 2,600 ommatidia in each eye; the number of optic nerve fibres in the eyestalk is about 6,600.
Each compound eye sees a region of space extending over more than a hemisphere in all directions, so that there is extensive binocular overlap; about 70% of the ommatidia of each eye have binocular partner ommatidia in the opposite eye viewing in the same direction.
The binocular field is most extensive in the frontoventral quadrant, where it reaches over 135 °, and is smallest in the dorsal region.
In large animals the size of the binocular field increases because of the greater number of ommatidia. This increase affects chiefly the frontoventral quadrant. As a result, the blind region immediately ahead of and below the head is independent of body size, extending to a distance of only 3 mm in the median plane.
The divergence angles of adjacent ommatidia are ca. 3 ° in most parts of the eye. In the foveal region this angle is reduced to just over 1 °. The optical axes of the foveal ommatidia point forward in the horizontal plane; the foveal zone is above and adjacent to the region of greatest binocular overlap. In a small part of the lateral monocular region the divergence angles increase to 5 °.
Optomotor experiments show that the head can be moved actively by ca. ±25 ° about the vertical and longitudinal axes, and 20 ° upward and 35 ° downward about the eyestalk axis. Conspecifics are fixated by turning the head and body about the vertical and transverse axes, so as to center the object in the binocular field, either in the horizontal plane or in the adjacent region of greatest binocular overlap. During a ritualized fight the opponents keep their eyestalks parallel and position themselves such that each animal sees the other in a region between the horizontal plane (fovea) and a plane inclined frontoventrad (in the region of greatest binocular overlap), at a distance equal to or smaller than the eyestalk span.
The way conspecifics of various sizes at various distances are imaged in the ommatidial array is calculated for the horizontal plane and a plane tilted downward in front by 20 °. Each combination of size and distance generates a different pattern in the array.
The pattern constellations on the ommatidial array, like the behavioural observations, indicate that the size and distance of a conspecific can be detected over a relatively extensive region, from a few millimeters to a meter away from the viewing animal. The high foveal resolution, the wide separation of the eyes and the large binocular field make this possible.
The eyestalks are probably intraspecific sign stimuli by which a conspecific is recognized and its size (and hence the strength of a potential opponent) is determined.
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