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Dubbed the frightful hairy fly when first described by entomologist E. E. Austen (1936), Mormotomyia hirsuta, now more commonly called the terrible hairy fly, is considered the rarest known fly in the world and a member of one of the only two fly families endemic only to tropical Africa (Kirk-Spriggs et al. 2011).  Since its discovery in 1933 many expeditions have searched for it, but only two succeeded in recovering it (Van Emden 1950; Copeland et al. 2011), each time from its type locality: a bat roost wedged into a split boulder on the apex of Ukasi Hill in Eastern Kenya. 

In addition to its extraordinarily limited distribution, this fly is taxonomically unique.  Placed by Austen (1936) into its own family, Mormotomyiidae, its relationship to the rest of the Diptera is controversial (Hennig 1971, Griffiths 1972, Pont 1980, McAlpine 1985, 1989; as cited in Kirk-Spriggs et al. 2011).  A remarkable fly, M. hirsuta has highly reduced, non-functional wings, reduced eyes, reflecting its cave-like habitat, and a body covered with long hair-like setae, hence its name hirsuta.  In 1933 Major H. B. Sharpe, Kenyan district commissioner and the original collector of the type individuals that Austen (1936) went on to describe, related his experience in entering the rock cleft on Ukasi Hill the flies “float[ed] from above like feathers,” in spirals (presumably slowed in their fall by their shaggy bodies). 

The elongated legs of M. hirsuta adults allow them quick, spider-like movement across quartz/felspar/biotite boulder walls and over thick bat guano accumulations.  Youtube videos taken by the 2011 expedition show their fast movement:, feeding and grooming behavior:,, and female guarding behavior:   Although Van Emden (1950) suggested that the adult flies lap up liquid excretions and/or sweat from bat fur, more recent morphological analysis shows a lack of any means for the adult flies to grip onto a potential bat host, indicating a free-living lifestyle rather than a parasitic or symbiotic relationship (Kirk-Spriggs et al. 2011).

With specific adaptations demonstrated to its specialized environment and low likelihood of active dispersal, this little population of M. hirsuta may be the only living relic of this species in the world.  However, while genetic analysis shows evidence of bottleneck constriction, indicating potential population instability, the genetic diversity of this population suggests gene flow from outside sources undiscovered at the time of the genetic analysis.  More recently, in a survey of Kenyan rock faces stained by pink bat urine, Copeland's team announced the finding of three more locations holding these rare flies, one 200 km from the type locality.  As the flies are wingless and have no morphological adaptations for purposefully attaching to vertebrates, scientists tentatively speculate on more haphazard transportation of eggs contained in wet guano and carried on the feet of bats or birds to other appropriate cave roosts (Copeland et al. 2011; MBZ 2014).

Many recognize the imminent threat of extinction of this species.  To preserve this unique fly, the International Centre of Insect Physiology and Ecology, Nairobi and the National Museums of Kenya, Nairobi are working together to declare the Ukasi hill area a protected site.  Copeland et al. (2011) also advocate for recognition of this species on the IUCN Red List.


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© Dana Campbell

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