Diopsis sp. (Diptera: Diopsoidea: Diopsidae)
This week’s insect is famous for setting young, budding naturalists’ heads spinning, particularly those whose interests tend to the bizarre (see image). We are speaking about the Diopsidae (Stalk-eyed flies), a small family of flies originally described by the great Linnaeus in 1775, with their greatest diversity in the Afrotropical and Oriental regions. Interestingly, they are unknown from the Neotropical region (South and Central America) normally a hotbed of insect diversity. This distribution suggests that diopsids evolved after the breakup of the supercontinent Gondwana. With the exception of the small superfamily Centrioncinae (containing the genera Centrioncus and Teloglabrus) Diopsidae are among the easiest of fly families to identify in the wild, by virtue of the enormous displacement of the eyes along with the antennae from the rest of the head onto the tip of diverging stalks. In some species the length of the stalks exceeds that of the fly’s body! Species of Centrioncus and Teloglabrus are the exception in that they have “normally” placed eyes (i.e., they are not stalk-eyed), but otherwise resemble their stalk-eyed cousins. Centrioncinae are endemic to the Afrotropical region.
There are approximately 240 described species of Diopsidae placed in 17 genera, and 157 (66%) of these species are recorded from the Afrotropical region. The subfamily Diopsinae is by far the largest within the family and it is within this subfamily that the extinct genus Prospyracephala is placed. Fossilized specimens are known from amber and shale. Extant Diopsidae are associated with moist habitats such as lake shores and riverine forest and woodland. Larvae are mostly saprophagous, feeding on rotting leaves and the like but others favour living plant tissue. Of these, some are borers in grass stems, including rice, in which they can reach serious pest levels. Within the Diopsinae, Diopsis is the largest genus. A third subfamily (Sphyracephalinae) contains the shorter-stalked genera Sphyracephalus and Cladiopsis, the latter an Afrotropical endemic.
Now, what about those eye stalks? Their primary function appears to be in sexual selection, whereby females use stalk length as a proxy for male fitness (i.e. the longer the eye stalks are on a male the better are his chances of finding receptive females). Presumably there is an upper limit to this kind of growth whereby the chances of a predator spotting potential diopsid prey increases in step with size of the diopsid and becomes too great a genetic burden to maintain. Also, diopsids are poor fliers and there is the chance that further increases in stalk length might impair flight dynamics, especially in bushy woodland. Stalk-eyes is a trait that has evolved independently in several other fly families, but only in Diopsidae are the antennae located alongside the eyes at the end of the stalk.
Credits: Dr Robert Copeland