The disappearance of flies as colder weather arrives highlights the challenge these cold-blooded creatures face each year. As ectotherms, flies cannot internally generate the heat needed to maintain activity when temperatures drop. Their survival hinges on specialized biological and behavioral adaptations that allow them to either escape the cold or endure it in a state of suspended animation. Understanding where flies go and how they manage to reappear each spring reveals their survival mechanisms.
Physiological Adaptation: Entering Diapause
Flies survive winter by entering diapause, which is distinct from simple hibernation. This dormancy is not merely a response to cold temperatures but is triggered by environmental cues like shortening day length in late summer and early autumn. Once initiated, diapause involves a profound suppression of the fly’s metabolism and an arrest of its development, allowing it to conserve energy.
A defining feature of this physiological shift is the production of chemical compounds that function as biological antifreeze. The fly converts stored glycogen and fat into cryoprotectants, such as glycerol, sorbitol, and trehalose, which accumulate in the hemolymph, or insect blood. Glycerol is particularly effective, acting to significantly lower the supercooling point of the fly’s body fluids, which is the temperature at which water spontaneously freezes. This mechanism prevents the formation of lethal ice crystals within the fly’s cells and tissues.
The accumulation of cryoprotectants allows the fly to avoid internal freezing, a strategy known as freeze avoidance, which is common among temperate insects. Cryoprotectants also help to stabilize cell membranes and proteins, protecting them from damage caused by cold and cellular dehydration. This biochemical preparation ensures that the fly’s internal machinery remains intact and ready to resume normal function when warmer conditions signal the end of diapause.
Overwintering Locations and Life Stage Choices
When preparing for diapause, flies must also choose a location that offers insulation and protection. The specific site chosen often depends on the life stage—egg, larva, pupa, or adult—that the species uses to overwinter. Many species do not survive the winter as an adult but instead rely on immature forms to persist until spring.
These immature stages are found buried in the soil, tucked beneath leaf litter, or concealed under the bark of trees, where temperatures are more stable than the air. Pupae, which are encased in a hardened shell called a puparium, are often found several centimeters deep in the soil or in compost and manure piles. These microhabitats provide a consistent buffer against sudden temperature drops and freezing rain.
Adult flies that overwinter will seek out sheltered cavities, often aggregating in large numbers for collective insulation. Natural shelters include hollow logs and rock crevices, but human structures like wall voids, attics, sheds, and abandoned buildings are also commonly utilized. The goal is to find a dark, dry, and temperature-stable environment where their dormant state will not be prematurely interrupted.
Winter Strategies of Common Fly Species
The strategies for surviving winter vary among common fly species. House flies (Musca domestica) do not survive as adults in temperate climates once freezing temperatures arrive. The species overwinters as a pupa or larva, typically in warm, protected masses of decaying organic matter like manure heaps or refuse tips.
These larvae and pupae require a microhabitat that remains above -5°C to survive and a short period above 10°C to resume development. The heat generated by microbial activity within piles of organic waste provides the warmth for the species to persist. Adult house flies that survive the winter find warm pockets within human-heated structures, but most of the population is sustained by the developing immature stages.
Cluster flies (Pollenia rudis) overwinter as adults, often invading homes. As autumn days shorten, these flies seek out the sun-warmed sides of buildings, aggregating before crawling into sheltered spaces like attics, wall voids, and soffits to enter diapause. Their larvae are parasites of earthworms, meaning the adults must find a secluded place away from their outdoor feeding and breeding grounds. If a mid-winter warm spell or the heat from the house breaks their dormancy, the flies emerge from their hiding spots into the living space. They often collect around sunny windows, which is how they earn their common name.