The “vampire fly” is a colloquial term for Philornis downsi, a species of fly known for its unusual feeding habits. Native to parts of South America, this insect exhibits a parasitic lifestyle during a particular stage of its development. Its name derives from this unique characteristic, which involves feeding on the blood of other creatures.
Identifying the Vampire Fly
Philornis downsi, commonly known as the avian vampire fly, is similar in size and appearance to a common house fly, typically displaying a dark coloration. Adult males tend to be larger, averaging around 10.5 mm across, and often have brighter yellow body coloring and eyes that are closer together. Females are generally smaller, measuring about 8.5 mm, and possess darker overall body coloring, along with darker legs.
While the adult flies are free-ranging and non-parasitic, their common name stems from the blood-feeding behavior exhibited by their larval stage, which feeds on the blood and tissue of developing nestling birds.
Life Cycle and Habitat
The life cycle of Philornis downsi involves several distinct stages: egg, larva, pupa, and adult. Adult female flies lay their eggs in bird nests, often on the nest material itself or directly on the nestlings. These eggs typically hatch within one to two days.
The first-instar larvae, upon hatching, often move into the nostril cavities or feather quills of the baby birds. The subsequent second and third-instar larvae primarily reside in the base of the nest material during the day. After feeding on the chicks for approximately 4 to 7 days, the larvae pupate within the nest base. The adult flies then emerge from these pupal cases roughly 10 to 14 days later.
Philornis downsi is native to mainland South America, with records from Trinidad and Brazil. It has also been accidentally introduced to the Galápagos Islands, where it has spread across many of the islands. The fly is found in most habitat types within the Galápagos, ranging from arid lowlands to humid highland zones, and is particularly prevalent in elevated islands with higher rainfall, humid highland forests, and agricultural areas.
Feeding Behavior and Hosts
Adult Philornis downsi flies are primarily vegetarian, feeding on fruits and nectar. The “vampire” aspect of this fly comes from its larval stage, which exhibits a semi-hematophagous, or blood and tissue feeding, parasitic behavior. Female flies lay their eggs in bird nests, with some eggs even found on the nasal openings of nestlings.
Upon hatching, the first-instar larvae can feed internally within the nestlings’ nasal and body cavities. As they develop into second and third-instar larvae, they move to the nest base during the day and emerge at night to feed externally on the blood and tissues of the nestlings. The larvae feed by chewing through the skin and consuming the secreted blood and other fluids.
Philornis downsi larvae parasitize a wide range of bird species, primarily passerine birds, and have been documented in over 150 species. In the Galápagos Islands, they infest nearly every passerine species, including at least 11 of the 14 species of Darwin’s finches. The number of parasites per nestling can be higher in nests with smaller broods, as the feeding pressure is concentrated on fewer individuals. Larger-bodied finch species that build larger nests tend to have more P. downsi larvae.
Ecological Impact and Significance
The avian vampire fly, Philornis downsi, poses a significant threat to bird populations, particularly in the Galápagos Islands where it is an invasive species. Its parasitic larvae cause high nestling mortality, sometimes leading to the death of up to 100% of hatchlings in a nest. This intense parasitism has been a contributing factor to the decline of several endemic and vulnerable bird species, including the mangrove finch (Camarhynchus heliobates) and the medium tree finch (Camarhynchus pauper).
Surviving nestlings can experience reduced growth rates, anemia, and permanent deformations of their beaks and nasal cavities. These deformities can affect later-life behaviors, such as song in some finch species. The possibility of disease transmission by the larvae has been explored; some studies on other Philornis species suggest they can transmit arboviruses to birds, though this role for P. downsi requires further investigation.
Conservation efforts in the Galápagos are focused on managing P. downsi infestations, including localized use of insecticide-laced cotton in nests and research into biological control agents. The ongoing presence of this parasitic fly highlights the complex challenges introduced species present to fragile island ecosystems.