|Photo of Trichobius johnsonae from Figure 2 of this paper
The public often view bats as repulsive, disease-carrying animals and are subsequently disliked. “Argh! They’re repulsive!” is just one of the many lines I have heard from people walking by while I observed a local colony. But do you know what is even more horrifying than a bat? A bat fly! These ectoparasites belong to two families of flies know as Streblidae and Nycteribiidae. But these hematophagous (blood-feeding) parasites don’t always fly like the name suggests - most species actually have no wings at all, and some look more like spiders than flies.
Disgusting, right? But not to worry, these external parasites have evolved to feed exclusively on bats. The bat flies are quite specific towards their hosts and tend to stay on a particular bat host. They are even picky about where they live on the host, whether on the bat’s fur or hiding within folded wing membranes. Occasionally they can be found in the fur and these individuals possessed comb-like structures (called ctenidia) for attaching to fur. It is assumed that long-legged species move quickly to avoid being scratched by the bat during grooming, whereas the short-legged species hide within the membrane folds to avoid getting licked. Bats use grooming as a behavioural defence against bat flies and other external parasites, and bats with a high number of flies groom more often than those with only a few. For the parasites, action can result in their removal and often their death.
In a study which took place in Belize, Central America, a team of researchers demonstrated just how host-specific the bat flies can be. They examined over thirty two species of bat flies, and in the twenty species for which they were able to collect more than five individuals, they found that eighteen of those species showed strong site preferences. The majority of the bat flies were constrained to a single host-species, and amazingly, bat flies with functional wings (which would allow them to be more mobile) weren't any more or less picky than those without. The study also found that only two species (Trichobius yunkeri and Trichobius dugesioides) weren’t too fussy in respect to host-site preference.
For bat flies that were the dominant species of their respective hosts, six out of those seven species were fur-specific, suggesting that in most cases, bat flies are highly host site-specific. They also discovered an interesting correlation between leg length and host-site preference. Bat flies with longer legs are able to push up over the surface of the fur, and are more likely to be found dwelling in fur. Conversely, short-legged individuals moved much more slowly and were mostly membrane-dwelling.
The team also conducted a study where three bats were restrained and three left unrestrained, with six bat flies placed on each. All unrestrained bats had only one bat fly remaining after five days, whereas all bat flies remained on the restrained bats. This suggests that the elimination of the flies is due to grooming behaviour. This may also be the cause of host-site specificity in bat flies, although further studies are needed Despite their nightmarish appearance, bat flies can still be very fussy eaters, and they have adaptations which allows them to specialise on particular bat species and host-sites.
Hofstede, H., Fenton, M., & Whitaker. J. (2004). Host and host-site specificity of bat flies (Diptera: Streblidae and Nycteribiidae) on neotropical bats (Chiroptera). Canadian Journal of Zoology 82: 616-626.
This post was written by Melissa Chenery