Flies often appear to die in clusters, leading many to wonder if the sight of dead companions somehow triggers their demise. This common observation sparks curiosity, but the reasons behind it are more complex than simple appearance suggests.
Debunking the Myth
Flies do not possess the cognitive capacity to understand death or experience emotions like fear or grief. Therefore, a fly does not die simply from “seeing” a dead fly. Their behaviors are driven by instinctual responses to environmental cues, not an emotional reaction to a conspecific’s demise. The idea that they die from a psychological response is a misconception.
Common Causes of Fly Mortality
Flies face numerous threats throughout their short lives, which typically span about 15 to 30 days for a housefly. Their mortality is influenced by environmental factors, including temperature extremes. For instance, temperatures above 45-50 degrees Celsius (113-122 Fahrenheit) or prolonged freezing conditions can be lethal. Availability of food and water also plays a crucial role; without sustenance, flies may only survive for a few days.
Beyond environmental stressors, flies are susceptible to predators like spiders and birds, and various pathogens. Active flight also contributes to their mortality, being energetically demanding and leading to oxidative damage. Many flies found dead indoors simply succumb to old age or lack of resources after being trapped inside.
The Fungal Connection: A Key Explanation
The primary explanation for observed fly clustering is infection by the entomopathogenic fungus Entomophthora muscae, which targets various fly species. Infection begins when fungal spores, known as conidia, land on a fly’s body and penetrate its exoskeleton. Once inside, the fungus proliferates, consuming the fly’s internal tissues.
As the infection progresses, the fungus manipulates the fly’s behavior, leading to “summit disease.” Infected flies are compelled to seek high perches, such as windows or walls, typically around dusk. Before death, the fly attaches itself firmly to the surface, often spreading its legs and raising its wings. This elevated position is strategic for the fungus, maximizing spore dispersal after the fly dies. The fungus then erupts from the fly’s body, forcibly ejecting new spores that can infect other nearby flies, perpetuating the cycle.
Understanding Fly Aggregation
Live flies are often drawn to areas where other flies have died, an aggregation driven by chemical signals and shared environmental conditions, not empathy. Dead or dying flies, especially those infected with Entomophthora muscae, release specific chemical compounds. These pheromones can attract other flies; for instance, male flies are known to be attracted to dead infected females, sometimes attempting to mate and inadvertently picking up spores.
Odors from decomposition also act as powerful attractants, signaling potential food sources or egg-laying sites. The presence of dead flies, whether from fungal infection or other causes, can thus draw in living flies seeking resources or mates. This clustering, influenced by chemical cues and natural habitat preferences, facilitates the spread of fungus and other pathogens among the fly population.