Arthrorhynchus nycteribiae

Bat flies are ectoparasites that cling to bats and suck their blood. As their name indicates, they are actually flies, but their bodies have been so heavily modified for their parasitic life style that they are barely recognisable as such. Many of them look like spiders with their long crawling legs which allow them to climb all over a bat's furry coat, and some species have even lost their wings. They can be very picky about what species of bat they parasitise, and most bat flies are specialists that are only found on one or two bat species. While they are a pest to bats, these bat flies also have their own ectoparasites to deal with, in the form of a group of fungi, and this post is on a study which examined some of them.

Bat fly Penicillidia conspicua with Arthrorhynchus nycteribiae attached
from Fig. 3. of the paper

These fungi belong to a group call Laboulbeniales, and are more commonly known as the "labouls". The live on the cuticle of their hosts and are not as invasive as other insect-infecting fungi. Labouls are found on a variety of different terrestrial arthropods including mites, millipedes and insects, but most species of labouls are found on beetles - which is to be expected somewhat since most species of terrestrial arthropods are beetles.

Labouls that infect bat flies have been found all over the world, but they in the environment where they do occur, they are relatively rare. In one study, scientists screened over 2500 bat flies and found only 56 laboul-infected flies. In Europe, there are four species of labouls that live on bat flies, all of them belong to the genus Arthrorhynchus. The fungi described in this study came from bat flies which lived on bats in the mountainous region of Hungary and parts of Romania. The samples were collected as a part of a long term bat surveys which took place between 1998 to 2015.

During the course of the survey, researchers caught bats with mist nets which were placed close to roosting sites. The bats that they caught were inspected for bat flies, and then released right after the researchers finished picking off their bat flies. They end up screening 1594 bats and collected a total of 1494 bat flies. Most of the bat flies the researchers collected were free from labouls, and of the eleven bat fly species they came across, only three were hosting labouls from two species - Arthrorhynchus eucampsipodae and Arthrorhynchus nycteribiae. The most commonly infected bat fly was the spider-look-alike bat fly Penicillidia conspicua - about a quarter of all the P. conspicua they found were infected with A. nycteribiae, and they seem to be the preferred host for that fungus.

Regardless of host fly species, the laboul fungi have an overwhelming preference for infecting female flies. This might be due to female flies simply being better hosts for the fungi - they live for longer than male flies (which gives them more opportunity to pick up laboul infections), they grow bigger, and have higher fat reserves (especially during pregnancy - yes, bat flies get pregnant), all of which makes them better hosts for the labouls than male bat flies.

There is still much that we do not known about these ectoparasites of ectoparasites - do all the bat fly labouls have a single common ancestor that initially jumped onto bat flies from some other insect host, then diversified into different species? Or did the different laboul species independently colonised bat flies on their own? Given mixed species roosts are pretty common among bats, how does this affect the transmission and evolution of these fungi on the bat flies? Additional do the labouls affect the interactions between the bat flies and their hosts?

Parasites can themselves become parasitised. Even on the backs of flies that live on the backs of bats, there is an undiscovered world of biological diversity - and we have barely scratched its surface.

Reference:
Haelewaters, D. et al. (2017). Parasites of parasites of bats: Laboulbeniales (Fungi: Ascomycota) on bat flies (Diptera: Nycteribiidae) in central Europe. Parasites & Vectors 10(1): 96.