Perissommatidae

Definition

A small family of minute nematoceran flies (Diptera) comprising five extant species in the single genus Perissomma, distributed in high-altitude forests of Australia (four species) and Chile (one species). Adults are distinguished by an unusual ocular arrangement suggesting four compound eyes, a slender body less than 2 mm long, and disproportionately large wings that produce weak, fluttering flight. Adults are active in winter and exhibit distinctive sunlit hovering behavior above foliage. Larvae inhabit decaying leaf litter in wet sclerophyll and cool rain forests, with suspected mycophagous or fungus-associated habits in some species.

Etymology

From the type genus Perissomma (Greek perissos 'odd, excessive' + omma 'eye') + -idae (family suffix).

Example

Perissomma macalpinei adults congregate in sunlit foliage clumps and rise in short zigzag flights above the canopy before descending, a behavior that facilitates mate location in the dim winter light of Australian montane forests.

Related Terms

• Diptera
• Nematocera
• Perissomma
• sclerophyll forest
• mycophagy
• compound eye
• montane ecology

Usage Notes

The family is monogeneric and relictual, with a classic Gondwanan distribution pattern linking Australian and Chilean cool-temperate forests. The apparent 'four eyes' represent a divided or duplicated eye structure whose functional significance remains unclear; specialists avoid calling them true supernumerary eyes. The weak flight capacity and winter adult activity are unusual among Diptera and reflect adaptation to stable, cool, humid microclimates. The Chilean species, P. elguetai, represents a significant disjunction that has figured in discussions of southern hemisphere biogeography.