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A specimen of Chaetoclusia sabroskyi (Soos) showing partially infuscated wings and typical clusiid antennae. Click on the image to take a closer look. Image © Owen Lonsdale
The Clusiidae are a family of small, thin, yellow to black flies with the wing usually partially infuscated. The antenna most readily diagnoses the family: the outer and (sometimes) inner distal margins of the pedicel have a triangular extension, and the arista is dorsoapical (separating it from most other acalyptrate families) on an (frequently) orbicular first flagellomere.
As adults are often encountered as solitary figures in old growth deciduous forests, they have been given the common name "druid flies". The term lekking fly might also apply, but this may be misleading as true lekking has only been found in one of the three subfamilies.
Adults are uncommonly collected in the field, but they can be relatively abundant in some microhabitats. Small dung baits and Malaise traps have been the most successful methods of collection, but a number of species have been swept from grass or collected off of foliage, logs and dead patches of wood on standing trees. North American adults appear to prefer mixed and deciduous forests sometimes associated with grass-dominated areas. Tropical species have often been collected along waterways in mossy, humid habitats. Clusiids have been known to feed on nectar, rotting vegetative matter, sap (Soós 1987), and the dung of birds and mammals.
The Clusiidae are one of a handful of acalyptrate families known to engage in lekking behaviour. Males establish dominance in a lekking site by defending territories (devoid of resource) from other males on logs or branches in order to attract females and mate. After mating, clusiid females lay eggs elsewhere, usually under bark or in wood in a state of more advanced decay than the surface used for lekking (Rohácek 1995). The wood likely provides a moist environment with an adequate supply of saprobes for feeding larvae. Selection of clusiid oviposition sites does not appear to be associated with any particular species of tree, but is limited by "humidity, amount of shade, stage of wood decay, [and the] presence of mycelia of certain fungi" (Rohácek 1995). Larvae of Sobarocephala occur in the same environment as the adults, and have been found only within decaying wood and termite colonies (Sóos 1987). Malloch (1918) also found the larvae of S. flaviseta to be "evidently associated with the burrows of coleopterous insects" and to be relatively sluggish and slow moving.
Males of some clusiids, particularly Hendelia Czerny, have enlarged heads or other conspicuous modifications used in mutual assessment on lek sites (see photo below) (McAlpine 1976, Marshall 2000). Sobarocephala latipennis Melander & Argo and several Australian Hendelia have strongly widened heads, and Hendelia kinetrolicros (Caloren & Marshall), Hendelia mirabilis (Frey), and Procerosoma alini (Shatalkin) have spectacular long genal processes that are probably used in male-male agonistic interactions, although these species have never been observed while engaged in such behavior.
Heteromeringia nitida males facing off in a territorial dispute. Image © Steve Marshall