Bat flies are parasitic insects that live almost exclusively on bats, feeding on their blood and spending most or all of their lives clinging to their host’s fur or skin. They belong to two families within the order Diptera (true flies), making them genuine flies despite the fact that many species have lost their wings entirely. Found on bats across every continent where bats occur, bat flies are among the most host-dependent parasites in the insect world, with some species so specialized that they can survive on only a single bat species. Their biology is strange even by parasitology standards: they give live birth, feed their larvae internally with a milk-like secretion, and serve as vectors for bat-specific diseases while sometimes falling prey to their own parasites.
Two Families, One Lifestyle
Bat flies split into two distinct families: the Nycteribiidae and the Streblidae. Both belong to the superfamily Hippoboscoidea, which also includes tsetse flies and louse flies (parasites of birds and other mammals). Molecular phylogenetic work confirms that the bat flies as a group form a lineage within this superfamily, though the internal relationships are messier than the tidy two-family split suggests. Streblidae, in particular, appear to be paraphyletic, meaning some streblid lineages are more closely related to nycteribiids than to other streblids.1PubMed. The phylogeny and evolution of host choice in the Hippoboscoidea (Diptera) as reconstructed using four molecular markers
In practical terms, the two families tend to divide the world’s bats between them. Nycteribiids are most diverse on Old World bats, while streblids dominate in the New World tropics. Their body plans differ in ways that reflect how tightly they’ve committed to life on a bat. Nycteribiids are almost universally wingless, with spider-like bodies, flattened heads that fold back against the thorax, and powerful clinging legs. They look less like a fly and more like a tick with unusually long limbs. Streblids are more variable: some retain functional wings and can fly short distances between hosts or from a roosting surface to a bat, while others have reduced or absent wings and are fully committed to crawling through fur.
How They Reproduce
Bat flies have one of the most unusual reproductive strategies in the insect world. Rather than laying eggs that hatch into free-living larvae, a female bat fly retains a single larva inside her uterus and nourishes it through internal glands that produce a nutrient-rich secretion, often compared to mammalian milk. The larva passes through all three larval stages inside the mother, essentially being “pregnant” with a nearly full-grown offspring. When the larva is finally deposited, it pupates almost immediately, skipping the typical free-living larval phase that most flies go through.
This strategy, called adenotrophic viviparity, is shared with tsetse flies and louse flies. But bat flies add another layer. Researchers have found that the milk gland tubules of bat flies harbor endosymbiotic bacteria, and these bacteria are present in the milk gland secretion itself. That means the mother is not just feeding her larva but inoculating it with the bacterial symbionts it will need to survive as an adult.2The ISME Journal. Reductive genome evolution, host–symbiont co-speciation and uterine transmission of endosymbiotic bacteria in bat flies These endosymbionts likely help the fly synthesize B vitamins and other nutrients missing from a blood-only diet, much as similar bacteria do in tsetse flies. The transmission route through milk ensures every generation inherits its microbial partners.
Finding a Host After Pupation
Once a female bat fly deposits her mature larva, the pupa sits on a surface inside a cave, tree hollow, or whatever structure the bat colony uses as a roost. But the pupae don’t just end up beneath the roosting cluster. A study of streblid bat flies in a Mexican cave found that the overwhelming majority of pupae were deposited not in the main roosting areas but in the flyway passages that bats use to enter and exit the cave, sometimes at a considerable distance from the roost itself. Pupal density correlated with the average flight height of bats passing through those corridors.3PubMed. Pupal deposition and ecology of bat flies (Diptera: Streblidae): Trichobius sp. (caecus group) in a Mexican cave habitat
This pattern suggests something about how newly emerged adult bat flies find their hosts. Rather than hatching directly beneath a cluster of roosting bats, the flies emerge in transit zones where bats regularly pass by, and then actively move onto passing or roosting bats. For winged streblids, this could involve short flights; for wingless nycteribiids, it likely means climbing onto a bat from a wall or ceiling surface. Either way, the adult fly faces a narrow window in which to locate and board a host before it starves. Blood is the only food source, and the clock starts ticking the moment the fly emerges from its pupal case.
What Determines Which Bats Carry Which Flies
Bat flies have a reputation for being extremely host-specific, and many species are indeed found on only one or a few bat species. But the picture is more complicated than a simple one-to-one pairing. Research across Central Panama found that the modular structure of bat-fly interaction networks (meaning which groups of fly species cluster on which groups of bat species) was driven not by how closely related the bats were to each other but by whether the bats shared roosting structures.4Insect Conservation and Diversity. Bat–bat fly interactions in Central Panama: host traits relate to modularity in a highly specialised network In other words, bats that sleep in the same tree hollow or cave chamber tend to share fly species, regardless of their evolutionary relationships.
A study in Singapore reinforced this idea from a different angle. Bat fly species there appeared highly host-specific, but the researchers attributed this to the limited availability of suitable hosts in Singapore’s relatively depauperate bat fauna rather than to strict coevolutionary constraints. In nearby countries with higher bat diversity, the same fly species sometimes used multiple bat hosts.5International Journal for Parasitology: Parasites and Wildlife. Ecology of bat flies in Singapore: A study on the diversity, infestation bias and host specificity (Diptera: Nycteribiidae) Host specificity, then, can be a product of ecology and opportunity as much as deep evolutionary commitment.
Work on neotropical bats further clarified what makes some bat species more heavily parasitized than others. Roosting habits and body mass were the strongest predictors, together accounting for roughly a fifth to two-fifths of the variation in parasitism. Bats that use permanent, enclosed roosts carry heavier fly loads than those roosting in open or ephemeral sites. Broader measures of a bat species’ geographic range or abundance, by contrast, didn’t predict fly loads, suggesting that what happens at the roost matters more than how common the bat is across the landscape.6SpringerLink / Parasitology Research. Parasitism by bat flies (Diptera: Streblidae) on neotropical bats: effects of host body size, distribution, and abundance
The Coevolution Question
Given how tightly bat flies depend on their hosts, you might expect their family trees to mirror each other neatly: when a bat lineage splits, its flies split too, and the two genealogies march in lockstep through time. This pattern, called phylogenetic congruence, is the hallmark of strict coevolution. But a broad analysis of bat-fly relationships found that the evidence for this lockstep is surprisingly weak. Bat flies showed variable degrees of host specialization, sometimes parasitized bat species that were not close relatives at all, and exhibited low phylogenetic congruence with their hosts overall. The coevolutionary signal was small and not statistically significant as a predictor of how specialized any given fly species would be.7bioRxiv. Phylogenetic and Ecological Trends in Specialization: Disentangling the Drivers of Ectoparasite Host Specificity
This doesn’t mean coevolution plays no role. But it suggests that ecological factors, like shared roosting sites, body size compatibility, and local host availability, can reshuffle host associations faster than the slow grind of co-speciation. Bat flies are obligate parasites, yes, but they are not necessarily locked to a single host lineage for millions of years. Host-switching, even across distantly related bat species, appears to be part of their evolutionary playbook.
Disease Vectors on a Bat
Bat flies don’t just take blood; they can move pathogens between bats in the process. The best-studied case involves bacteria in the genus Bartonella, which cause persistent blood infections in many bat species. A screening study in Malaysia found Bartonella DNA in about two-thirds of bat fly pools tested, with both nycteribiid and streblid flies carrying the bacteria.8Heliyon. Molecular prevalence of Bartonella spp. in bat flies in east coast Malaysia That high molecular prevalence hints at how efficiently the flies can shuttle these bacteria around a colony.
An experimental study using a captive colony of fruit bats in Ghana offered more direct evidence. Researchers manipulated bat fly numbers and observed corresponding changes in the Bartonella community within the bats: when flies declined, so did the bacterial diversity, and when flies were restocked, the bacterial community shifted again.9PubMed Central. Manipulating vector transmission reveals local processes in Bartonella communities of bats This kind of experimental manipulation is rare in wildlife parasitology and provided strong circumstantial evidence that bat flies are genuine vectors, not just passive carriers.
Bat flies also transmit malaria-like parasites. The genus Polychromophilus, a haemosporidian parasite related to human malaria parasites, infects bats across multiple continents and uses nycteribiid bat flies as its vectors.10Infection, Genetics and Evolution. Europe-wide distribution and bat-host specific lineages in the malarial parasite Polychromophilus murinus revealed through genetic screening of bat flies In Gabon, researchers detected Polychromophilus melanipherus in multiple bat fly species collected from cave-dwelling bats, with two nycteribiid species appearing to be the primary vectors.11PubMed Central. Bat flies (Diptera: Nycteribiidae and Streblidae) infesting cave-dwelling bats in Gabon: diversity, dynamics and potential role in Polychromophilus melanipherus transmission In Brazil, the first molecular detection of Polychromophilus in a vector came from nycteribiid flies of the genus Basilia, collected from small Myotis bats, confirming what had been suspected since the parasite was first described morphologically in bat fly salivary glands decades earlier.12PubMed Central. Polychromophilus spp. (Haemosporida: Plasmodiidae): First Molecular Detection in Bat Flies From Brazilian Bats
Whether bat flies pose any risk to humans is a question that comes up occasionally. The short answer is that they are not known to bite people, and the pathogens they carry are bat-specific. Bat-associated Bartonella species are genetically distinct from the ones that cause cat scratch disease or trench fever in humans. Researchers handling bats sometimes find a bat fly crawling on their hand, but the fly has no interest in feeding on a non-bat host and typically dies quickly without one.
Parasites of the Parasites
In one of biology’s more satisfying recursive arrangements, bat flies have their own parasites. Tiny fungi in the order Laboulbeniales grow on the exoskeletons of bat flies, forming small stalked structures called thalli that absorb nutrients from the fly’s cuticle. These fungi are obligate hyperparasites: they live only on bat flies, which live only on bats, creating a three-tiered chain of dependence.
In a large neotropical dataset spanning over 1,700 bats and thousands of bat flies, nine out of 39 sampled fly species carried Laboulbeniales infections. The most commonly encountered fungal species was Gloeandromyces streblae, found on 33 individual flies across three fly species.13PubMed Central. Laboulbeniales hyperparasites (Fungi, Ascomycota) of bat flies: Independent origins and host associations In European bat fly populations, the infection rate tends to be low overall but varies dramatically by fly species. One central European study found Laboulbeniales on about 3% of screened bat flies, but the rate on one species, Penicillidia conspicua, reached 25%. The fungi also showed a preference for female bat flies, which the researchers attributed to females’ longer lifespan and larger body size during pregnancy, both factors that give the fungus more time and more surface area to colonize.14PubMed Central. Parasites of parasites of bats: Laboulbeniales (Fungi: Ascomycota) on bat flies (Diptera: Nycteribiidae) in central Europe
A broader European screening of bat flies from the bent-winged bat Miniopterus schreibersii found a 9% overall prevalence of Laboulbeniales infection, with two fungal species present across three bat fly species.15PubMed Central. Laboulbeniales infection of bat flies (Diptera: Nycteribiidae) from Miniopterus schreibersii across Europe Whether these fungi actually harm the flies they colonize or are more like tolerable freeloaders remains unclear. Heavy infections could plausibly reduce a fly’s mobility or ability to feed, but direct fitness costs have been difficult to measure.
How Bats Fight Back
Bats are not passive hosts. Grooming is the primary defense, and bats that carry heavier fly loads scratch themselves more frequently. A study of neotropical bats found that while individual-level grooming rates didn’t differ between bats with and without flies, species that consistently harbored high fly densities had higher scratching rates overall.16Journal of Zoology. Relationships between roost preferences, ectoparasite density, and grooming behaviour of neotropical bats This suggests that grooming intensity is partly a species-level adaptation to chronic parasite pressure rather than something each individual ramps up in response to feeling a fly crawling on it.
Some bats go beyond simple scratching and actively consume the parasites they remove. Researchers studying Madagascar’s Rousettus madagascariensis, a fruit bat, estimated that individual adults consume roughly 37 bat flies per day through grooming. The authors noted that this could provide a meaningful nutritional supplement, particularly for a frugivorous species whose diet is otherwise low in protein. Gravid female bat flies, swollen with developing larvae, would be especially calorie-dense.17PubMed Central. Rates of hematophagous ectoparasite consumption during grooming by an endemic Madagascar fruit bat It’s an appealing irony: the blood-feeding parasite ends up as a protein snack for the host it was exploiting.
Seasonal Swings and Host Sex
Bat fly populations are not static. They fluctuate seasonally, and the patterns are consistent enough across studies to point to some general rules. In temperate regions, fly prevalence and intensity tend to be lower in spring and higher in autumn. A large European study found spring prevalence around 42% compared to roughly 49% in autumn, with mean intensity also climbing from about two flies per infested bat in spring to about two and a half in autumn.18Scientific Reports. Season and host-community composition inside roosts may affect host-specificity of bat flies Male bats showed the most dramatic seasonal shift, going from about 32% prevalence in spring to 48% in autumn. Female bats stayed more consistently parasitized across seasons.
In subtropical and tropical regions, the wet season generally brings higher fly abundance and diversity. A study in Amazonian savannahs found greater bat fly species richness and abundance during the wet season, while the dry season produced networks that were more specialized and modular, meaning flies stuck more tightly to particular bat hosts.19PubMed. Seasonal variation and host sex affect bat-bat fly interaction networks in the Amazonian savannahs A parallel study on bent-winged bats found that parasite abundance peaked during the bats’ breeding period, with lactating females carrying the heaviest loads. These females also had the highest body condition, suggesting that the energetic demands of reproduction might suppress immune defenses or that the warmth and blood flow associated with lactation make them more attractive to flies.20PubMed Central. Effects of host state and body condition on parasite infestation of bent-wing bats
Host sex matters beyond just the breeding season. Across multiple studies, female bats tend to carry more flies than males. One contributing factor is that females in many bat species form large maternity colonies during the summer, packing together in warm, humid roosts that create ideal conditions for fly reproduction and transmission. Males, which often roost alone or in smaller groups during the same period, are simply less accessible to flies looking for new hosts.
When Bats Disappear, So Do Their Flies
Because bat flies cannot survive without bats, the decline or extinction of a bat species means the loss of every organism that depends exclusively on it. A network analysis of bat-fly interactions in Brazil examined 106 fly species and 182 bat species and found that roughly a third of the bats had any documented fly associations at all, but among those that did, 33% of the bats and 64% of the flies were linked to only one partner species. Four fly species were identified as being at immediate risk of coextinction because their sole bat hosts are threatened or near-threatened. In some cases, the extinction of a single bat species would take five fly species with it.21Animal Conservation. Hidden Losses: Assessing the Risk of Coextinction Among Ectoparasitic Flies and Their Bat Host Species in Brazil
This kind of dependent extinction is almost certainly underestimated in global biodiversity tallies. Conservation assessments focus on the species people notice, and parasites are invisible until someone specifically looks for them. A bat species listed as endangered carries with it a community of flies, endosymbiotic bacteria inside those flies, and hyperparasitic fungi on those flies. The loss of one charismatic host potentially collapses an entire micro-ecosystem that most people never knew existed. For researchers who study bat flies, the conservation message is less about saving the parasites for their own sake and more about recognizing that biodiversity is deeper and more interconnected than headline species counts suggest.