Summary. Cyrtodiopsis whitei (Diopsidae) from
12Malaysia is one of the stalk-eyed flies which show Ifmr a marked sexual dimorphism of eye spans. The particularly long eye stalks of male flies are to be lo regarded as an epigamic feature. Field observations show the males' eye spans to be correlated with the possession of a harem and with that harem's size. The question whether such a harem is 8 acquired by male competition only, or whether female choice plays a part, is solved in female preference tests, offering dummy males of different eye spans for choice. Optical clues are found to be the relevant stimuli.
Some species of stalk-eyed flies (Diopsidae, Diptera) have a sexual dimorphism of eyespan. For example, Cyrtodiopsis whitei males have much longer eyestalks than females of equal body length. Expression of this trait increases with the bearers' size. The slope of the log-log regression line eyespan versus body length is close to two in males, while in females it is roughly one. Behavioural experiments suggest that male eyespan signals quantitatively a male's strength or attractiveness to a competitor or mate. We used 3 pure strains of C. whitei, which were distinguished by their different phosphoglucomutase allele outfit. We compared the reproductive success of males of different sizes and found the number of offspring to be directly proportional to body length. Thus eyespan, rising with the square of body length, provides an exaggerated and highly conclusive signal in the advertisement of fitness.
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