Fiordland Bat Diaries: Bat-flies

Department of Conservation —  25/02/2020

Local rangers in Fiordland undertake short-tailed bat monitoring in order to protect and learn more about these special, little native critters. But sometimes they are not the only little critter on show…. The New Zealand bat-fly is a wingless fly that lives alongside short-tailed bats and is a creature of nightmares for some.

Bat with a few hitch hikers.
📷: DOC, Louise McLaughlin

Bats are not the only endangered species we run into doing bat work. The New Zealand bat-fly Mystacinobia zelandica is a species of wingless fly that has evolved to live alongside short tailed bats. At some point they decided hitching a ride on the back of a bat was easier than flying and so lost their wings. Unfortunately, this gives them a somewhat spider like appearance which isn’t all that endearing, the first of their un-endearing traits.

Bat-fly having a meal on the go.
📷: DOC, Warren Simpson

Bat-flies are the housekeepers/ waste disposal workers in bat land. They have a symbiotic relationship where they feed on bat guano (bat dung) and keep things clean while the bats provide a warm safe environment with ample food. Females lay their eggs in large communal nurseries in the bottom of the guano pile and tend to the growing larvae.

Bat roosting tree from below.
📷: DOC, Jamie McAulay

However, life isn’t all peachy for bat-flies as there is a constant threat of being left behind at a roost tree when the colony moves on. Every evening when the bats depart, they must make a gamble on whether to stay or leave. Leaving means hitch hiking on a bat for the evening while the bat-flies all night, and staying means feeding at the roost tree. But once the colony moves away from a tree for the final time, any remaining bat-flies will slowly die as their food supply dwindles. This makes those stranded individuals pretty desperate to find a new home and when we climb the tree to take down our monitoring equipment, they swarm out seeking our heat, coming down the climbing rope to greet us.

Masses of bat-flies at a bat roost entrance.
📷: DOC, Colin O’Donnell

For us bat-flies are a part of life while we handle bats. They crawl off their hosts and onto us – on average there are about 5 per bat hanging on (though up to 30 on one individual have been seen). And while we endeavour to re-home as many as we can back onto the bats before release, come the end of the evening there are inevitably flies crawling around on our legs, in our hair, the back of a neck. We find them days later in our beds, packs, and even in a toothbrush…

A stranded juvenile with hitch hikers.
📷: DOC, Bex Jackson

Catch the team next time on the Conservation blog to take a closer look at the mighty trees that these bats roost in.

10 responses to Fiordland Bat Diaries: Bat-flies


    Edit to my other post…I meant symbiont, not saprophyte.


    The article implies that the bat flies only eat bat dung, but other websites about bat flies say they take blood meals, which means they’re parasites, not solely dung-eating saprophytes.

    Kathy Harvey 12/03/2020 at 2:31 pm

    Interesting and entertaining articles – thank you for spreading the info out to many of us who have little to no knowledge of the many NZ Taonga that you take care of

    Greg Jarvis 25/02/2020 at 3:27 pm

    Do the flies have a preference for Batman or Wonder woman?

    Rob Carson-Iles 25/02/2020 at 11:44 am

    Let us not forget that these beauties have been immortalised in song! Thanks to Fatcat & Fishface 🙂


    Thanks for sharing about this relationship in an engaging and fun read:).


    What an amazing series of articles. Well done. At least I think I would prefer the bat flies to those found on wild boars


    You people are totally batty!