A scuttle fly, Megaselia sp., was collected on 31 May 2023 on Shelter Key, City of Key Colony Beach, Monroe County, Florida, USA, at a residence, in a carbon dioxide-baited BG Sentinel trap set for routine mosquito surveillance.  A single mite was attached to the dorsal abdomen between segments three and four (Fig. 1).   The mite was mounted on a microscope slide and the fly was point mounted.  The mite was identified as Macrocheles muscaedomesticae (Scopoli) (Halliday 2000).  The fly was determined to be a species in the large and cosmopolitan genus Megaselia Rondani (Peterson 1987).  Both specimens were deposited into the Florida State Collection of Arthropods (Fly: FSCA 00051358; Mite: FSCA 00030264).

The fly superficially appears to be Megaselia scalaris (Loew).  However, the adult of M. scalaris is very difficult to distinguish from two other Megaselia species that occur in the Florida Keys, Megaselia hansonix Disney and Megaselia imitatrix Borgmeier; a fourth, undescribed Megaselia species also is known from the Florida Keys (Hribar et al. 2011).  The likelihood that this fly actually is M. scalaris is supported by the fact that M. scalaris and M. muscaedomesticae can occur together in the same habitats (Marchiori et al. 2000), and that M. hansonix and M. imitatrix are obligately aquatic in the larval stage, whereas larval M. scalaris are primarily terrestrial, although they will occasionally utilize aquatic habitats (Disney 2008, Disney et al. 2009).  Relative abundance of the Megaselia species in the Florida Keys is unknown although Brown and Hartop (2017) collected more M. hansonix than M. scalaris in urban Los Angeles, California, USA.

This is not the first record of an association between Phoridae and Macrochelidae.  Macrocheles disneyi Fain and Greenwood was described from the phorid Diplonevra nitidula (Meigen) (Fain and Greenwood 1991).  Nor is this the first record of Macrocheles muscaedomesticae in the Florida Keys; Hribar (2020) collected this mite species from Drosophilidae on Vaca Key in the City of Marathon.  Macrocheles muscaedomesticae was first reported in Florida as an associate of the stable fly Stomoxys calcitrans (Linnaeus) (Muscidae) (Williams and Rogers 1976).  Macrocheles muscaedomesticae preys on the eggs of flies and can be numerous as to inhibit dispersal of its fly hosts (Axtell 1961, Beresford and Sutcliffe 2009).  Axtell (1964) found that all phoretic mite specimens were adult females, as is this specimen.  Predatory macrochelid mites have been investigated as potential biocontrol against phorid flies in mushroom production (Kumar et al. 2007).  Conversely, phorids are known to parasitize Acari, namely ticks (Olenev 1941, Rocha et al. 1984).