How many flies have ever existed?

Episode Summary

Executive Summary: BBC CrowdScience investigates how many flies have ever existed, using expert interviews to explore fly biology, diversity, ecology, and the impossibility of exact counting. The episode also answers why houseflies circle indoor landmarks, and concludes that flies are extraordinarily abundant, ancient, and ecologically vital, but their total historical number can only be estimated as astronomically large and uncertain.

Main Topics: Defining what a fly is (Priority: 5/5): The episode clarifies that true flies are Diptera: insects with one pair of wings, suctorial mouthparts, and halteres for flight stabilization. This distinction matters because common names like dragonfly or butterfly can mislead. Fly diversity and abundance today (Priority: 5/5): Experts explain that flies are far more diverse and numerous than most people realize, with huge numbers of species and individuals across habitats from cities to caves to marine systems. Estimating how many flies have ever existed (Priority: 5/5): The program builds a rough historical estimate by combining fly longevity, speciation over 250 million years, and present-day abundance, but repeatedly stresses that the result is highly uncertain. Why houseflies circle landmarks indoors (Priority: 4/5): A listener question about looping flies in apartments is answered with research showing flies often fly in rectangular paths around landmarks and that the behavior is linked to mating competition among males. Flies as ecosystem services (Priority: 5/5): The episode reframes flies as pollinators, decomposers, prey, and predators, emphasizing their importance to nutrient cycling and ecosystem functioning despite their nuisance reputation. Disease vectors and human impact (Priority: 4/5): Mosquitoes are discussed as fly-related disease vectors, including malaria, dengue, chikungunya, and Zika, showing how some flies directly affect human health.

Key Arguments: True flies (Diptera) are defined scientifically by one pair of wings, suctorial mouthparts, and halteres, so not all winged insects are flies. Flies are vastly underappreciated in both diversity and abundance; current species counts likely underestimate real diversity by a large margin. A simple multiplication of present-day abundance by historical generations suggests an enormous cumulative number of flies, but the calculation is too uncertain for a firm answer. Houseflies do not literally fly in circles; they move in short straight segments and sharp turns around landmarks, creating a rectangular pattern. The indoor hovering behavior of male flies is likely a mating strategy, with males competing for position beneath a landmark to intercept females. Flies provide major ecosystem services: pollination, decomposition, and food for other species; mosquitoes also pollinate certain plants. Some flies and mosquitoes are important disease vectors, so their impact on humans is both ecological and medical. Because fly fossils are sparse, especially for soft-bodied stages, the historical record is too incomplete to count extinct fly species precisely.

Data Points: People ever existed (previous episode reference): 117 billion - Used as a comparison point for estimating cumulative fly numbers Flies per human alive today: 17 billion - Erica McAllister’s rough estimate of current fly abundance relative to humans Hoverflies arriving in the UK each spring: 4 billion - Example showing how large insect numbers can be in a single region Fly specimens at Natural History Museum: 36 million - Insect specimens held in the museum’s collection Specimens per drawer: up to 1,000 flies - Museum storage capacity described during the archive visit Named fly species: about 160,000 - Species of flies formally described to date Estimated living fly species: closer to 1 million - David Yeates’ estimate of total extant fly diversity Age of fly origin: about 250 million years - Molecular and fossil evidence for the origin of flies Assumed species lifespan: about 5 million years - Illustrative assumption used to estimate cumulative species turnover Speciation model output: 2^50 = 1 quadrillion, 125 trillion, 899 billion, 900 million, 000,000,000 - Rough estimate for total fly species over 250 million years under a branching model Estimated cumulative individual flies: nearly a billion flies which have ever existed - Approximate result of multiplying rough present abundance by generation turnover; acknowledged as extremely uncertain Malaria deaths annually: over half a million people every year - Illustrates the human health burden carried by mosquito vectors Adult fly movement distance: about 30 cm straight segments, then 20 cm, with 90-degree turns - Pattern described by Jochen Zeil in housefly movement studies Sticky-tape sampling method: strips hung from ceilings in pubs in Australia and Germany - Low-tech method used to identify sex-based height differences in fly flight patterns

Pivotal Quotes: "How many flies have ever existed?" — Jenny: The listener question driving the episode "They are basically the best stabilizers. So they enable flies to be aeronautical masters." — Dr. Erica McAllister: Explaining halteres and fly flight control "I think that Jenny might just have to be content with the idea that there is a crazy, very large number with many zeros in front of it." — Professor David Yeates: Final conclusion that the exact total cannot be known

Implications: Listeners are left with a clear takeaway: flies are unimaginably numerous, scientifically diverse, and ecologically essential, while any exact historical count is beyond current knowledge. The episode also highlights that pests can be key pollinators, decomposers, and disease vectors.