Teleopsis dalmanni

Male flies produce gourd-shaped spermatophores that occupy only a part of the vaginal capacity, as compared to the spermatophores of certain other diopsid flies that occupy the entire vaginal cavity.[6] It has been suggested that the smaller size of the spermatophore is an adaptation to reduce investment in sperm due to a high mating frequency in a short amount of time.[7] Accessory glands are responsible for forming the spermatophore that transports sperm during mating.[4]

The eyestalks of T. dalmanni, as well as other Diopsidae flies, are frequently used to study the fields of sexual selection and mating preference. Eyestalks have been known to range from 4 mm in length up to 17 mm.[8] Eyestalks can even be longer than body length.[9]

Hyperencephaly of the type that resembles the eyestalks in T. dalmanni is seen is other Dipteran organisms, but flies of the family Diopsidae are unique in that both males and females display some degree of eyestalk elongation. In T. dalmanni flies, males have larger eye-spans relative to body length compared to females. This feature of the fly is subject to studies that investigate the genetic background of sexual selection, female mating preferences and reproductive consequences.[5]

Eyestalks play an important role in mating behavior, as studies have shown that females prefer males with larger eye spans relative to body length. It was found that female preference for large eye-spans is evident when the difference between eye-spans of available males is large, not moderate. In cases of low and moderate difference, a female with a large eye-span is more likely to pick the male with larger eye-span.[8] There are multiple theories posited to explain this, including the idea that female preference for male eye-span may have a ceiling, or that female choice is linked genetically to female eye-span.

Sexual selection posits that structures that are perceived as reliable indicators of individual quality are selected for. Teleopsis dalmanni eyestalks are an example of sexual dimorphism that has developed for mating purposes; larger eye-span is selected for as evidence indicates that it is preferred by females.[10] A study also found that female individuals with larger eye-spans had significantly more mature oocytes than those with shorter eye-spans.[10]

Furthermore, eye-span independent of body size or mass has been identified as the key determinant in the outcomes of contests between males.[11] This suggests that longer eyestalks developed in males as a result of mating success pressure, and hence are an example of sexual selection. However, it has been shown that females that mated with males with large eye-spans did not receive short-term fecundity benefits and fertility benefits, and in fact recorded a lower fertility rate than females mating with males with short eye-spans.[2]

Male eye-span is strongly dependent on conditional factors, comparatively more than other non-sexual traits,[11] and eye-span becomes variable under conditions of increased stress. Similar patterns are not seen in comparable nonsexual traits, and are more closely correlated with body size. The hypothesis drawn from these results is that eye-span provides additional information about the condition beyond that provided by body size. Hence eye-span could be an alternative indication to females of the male's greater ability to survive. This relates to the handicap theory of evolution of ornamental male traits.[11]

However, studies cite a lack of evidence to support the idea that the extended eye-stalks are a handicap to male Diopsid flies. While the eye-stalk increases the moment of inertia of male flies, they were not found to suffer a flight performance decrement. Specifically T. dalmanni male flies were found to perform better than females in free-flying turning behavior.[10] If longer eyespans do not pose a handicap to male flies, an alternative explanation may be more successful in explaining the development of this trait.

Teleopsis dalmanni has been used as a study subject in experimental subject in studies on data processing in insect optic centers, as it is easy to record impulse activity in the eyestalk.