Blow flies (family Calliphoridae) are typically the first insects to detect and colonize decomposing remains. These metallic-colored flies are widely recognized in forensic science as the primary indicator species for estimating the initial Post-Mortem Interval (PMI). In accessible environments, their presence is generally expected within minutes to a few hours following death, driven by the release of volatile organic compounds from the body. Consequently, the complete absence of any life stage—eggs, larvae, pupae, or adults—at a scene, particularly in a sealed location like a storage building, presents a significant forensic puzzle. This anomaly forces investigators to look for specific physical, chemical, and behavioral factors that prevented colonization.
The Essential Role of Blow Flies in Forensic Science
The significance of blow flies stems from their highly tuned sense of smell, which allows them to quickly locate remains by detecting volatile decomposition compounds (VDCs). Once located, the female fly deposits masses of eggs, often in natural orifices or open wounds, initiating the colonization process. The subsequent life cycle proceeds through three larval stages, a non-feeding wandering stage, and finally the pupa, before emerging as an adult fly. Forensic entomologists use the age of the oldest insect stage collected on the body to calculate the minimum PMI. This calculation relies on known developmental rates, which are species-specific and highly dependent on ambient temperature. The accumulation of thermal energy, often expressed as Accumulated Degree Hours (ADH), is used to model the time required for development between stages. The absence of these insects removes the most reliable biological clock available for determining the time since death.
Physical Barriers and Environmental Exclusion
The physical structure of the storage building itself can serve as a highly effective, passive barrier preventing blow fly colonization. Storage units are often designed with fully sealed doors and no functional windows, creating an airtight environment that physically excludes adult flies. Even small cracks or ventilation gaps may be insufficient if the flies are not actively drawn to the scent.
Temperature extremes within uninsulated storage buildings often exceed the optimal range required for blow fly activity and development. If the temperature dips significantly below 10°C (50°F), adult flies become sluggish and cease searching for oviposition sites. Conversely, excessive heat, perhaps exceeding 40°C (104°F), can be lethal to flies or inhibit their reproductive drive, making the remains unsuitable for egg-laying.
Blow flies exhibit phototaxis, meaning they require light to be active and search for a host. A completely dark storage unit, even if accessible, can dramatically suppress the flies’ ability to locate the remains and deposit eggs. This combination of structural sealing, non-optimal temperatures, and the absence of light creates an environment of environmental exclusion.
Concealment and Chemical Deterrents
Even if a blow fly managed to breach the storage building’s defenses, access to the remains might be blocked by physical concealment measures. Tightly wrapping the body in multiple layers of heavy blankets, thick plastic sheeting, or placing it inside a sealed container creates a physical barrier to oviposition sites. The flies require direct access to lay eggs, and secure wrappings may prevent volatile compounds from diffusing quickly enough to attract them.
Chemical Deterrents
The presence of toxic substances on or within the remains acts as a chemical deterrent, rendering the body unsuitable for colonization. High concentrations of certain drugs, such as cocaine or methamphetamine, within the tissues can be lethal to newly hatched larvae, which females may instinctively detect. Similarly, the deliberate application of household cleaning chemicals or disinfectants to the body surface can kill eggs or repel adult flies before they lay them. Another common chemical reason for absence is the use of insecticides or pesticides in the immediate area. If a surface spray insecticide was used within the unit, the residual chemicals can kill any adult flies attempting to land on the remains or the surrounding floor, preventing colonization.
The Impact of Intentional Human Intervention
The complete lack of blow flies may also be the result of active, intentional disruption by the individuals involved in the crime. The purposeful use of extreme cold, such as freezing or continuous refrigeration, is one of the most effective ways to prevent or stop insect development.
If the body was kept frozen, any eggs or early-stage larvae would be killed, resulting in a body free of entomological evidence upon later discovery. Alternatively, the remains might have been initially colonized at a different, accessible location before being moved to the sealed storage building.
Before or during the movement, any visible eggs or maggots could have been deliberately brushed off or washed away, removing evidence of the initial time of death. Finally, a perpetrator may have applied a caustic cleaning agent or an insecticide directly to the remains after colonization had begun. This active elimination leaves the scene sterile, despite the body being available for a period.