SIGNALING FITNESS - LARGER MALES SIRE MORE OFFSPRING - STUDIES OF THESTALK-EYED FLY CYRTODIOPSIS-WHITEI (DIOPSIDAE, DIPTERA)

Some species of stalk-eyed flies (Diopsidae, Diptera) have a sexual di morphism 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 regr ession line eyespan versus body length is close to two in males, while in females it is roughly one. Behavioural experiments suggest that ma le eyespan signals quantitatively a male's strength or attractiveness to a competitor or mate. We used 3 pure strains of C. whitei, which we re distinguished by their different phosphoglucomutase allele outfit. We compared the reproductive success of males of different sizes and f ound the number of offspring to be directly proportional to body lengt h. Thus eyespan, rising with the square of body length, provides an ex aggerated and highly conclusive signal in the advertisement of fitness .