Louse flies are members of the family Hippoboscidae. They are dorso-ventrally flattened blood-sucking muscoid flies, the adults of which live as external parasites of birds and mammals. They fossick amongst the hairs or feathers and the flattened body plan aids in this activity. Some have complex life histories involving "adenoptrophic viviparity". This means that eggs are nourished and hatch within the female and living larvae are deposited rather than eggs. This happens in a number of unrelated flies, the most awful example of which is an Australian brown blowfly.
I have often seen hippoboscids scuttling about on the wings and bodies of Brush Turkeys and wondered how to photograph them. To my surpsrise one appeared on the light sheet. A variety of birds and mammals play host to these flies and wallabies have some special ones. I have no idea the host of this fly, but I have myself been mistaken for a host by hippoboscids. They alight and when they discover their mistake are quick retreat.Note the needle-like mouthparts. They use this beak to penetrate the tissues of the host and withdraw blood.