Forensic entomology applies the study of insects and other arthropods to legal investigations. This field provides valuable insights into circumstances surrounding a death by examining insect communities found on or near remains. While insect presence provides crucial evidence, their absence at a scene, such as inside a storage building, is also a notable event. Understanding why these insects, especially blow flies, might be missing is as important as analyzing their presence.
The Forensic Significance of Blow Flies
Blow flies are frequently among the first organisms to arrive at decaying remains, often within minutes of death. They are highly attracted to the volatile organic compounds released during decomposition. Adult female blow flies lay their eggs on the remains, typically in natural orifices or wounds. These eggs hatch into larvae, commonly known as maggots, which feed on the decaying tissue.
The life cycle of blow flies, which progresses through egg, larval (three instars), pupal, and adult stages, is central to forensic investigations. By studying the developmental stage of the blow fly larvae and pupae, forensic entomologists can estimate the minimum post-mortem interval (PMI), which is the time elapsed since insects first colonized the body. This estimation is possible because each species has a characteristic temperature-dependent growth rate; therefore, analyzing insect development in conjunction with environmental data like temperature allows for a more precise determination of the time since death.
Essential Conditions for Blow Fly Presence
Blow flies require specific environmental and biological conditions to effectively locate, colonize, and develop on remains. A primary requirement is direct access to the decomposing material. Adult blow flies must be able to reach the remains to lay their eggs, as their larvae rely on this decaying matter for sustenance. Without such access, colonization cannot occur.
Temperature plays a significant role in blow fly activity and development, as these insects are cold-blooded and their metabolic rates are directly influenced by ambient temperatures. Optimal developmental temperatures for many blow fly species typically range between approximately 18°C and 30°C. Development can be significantly retarded or even cease at temperatures outside these ranges, with higher mortality rates observed at temperature extremes. Additionally, blow flies are more active in bright light conditions and high humidity can enhance their activity.
How Storage Environments Deter Blow Flies
Storage buildings often present an environment that significantly deters blow flies, preventing their presence at a crime scene. Sealed structures, a common feature of storage units, act as a physical barrier, effectively limiting or preventing adult blow flies from accessing the interior where remains might be located. Without direct access, colonization cannot begin.
Temperature fluctuations within storage units can also fall outside the optimal range for blow fly development. Non-climate-controlled units can experience extreme temperatures, potentially soaring to 49-54°C (120-130°F) in summer or dropping below freezing in winter. Even climate-controlled units, typically maintained between 13-27°C (55-80°F), might still present conditions that inhibit consistent blow fly activity or development if temperatures are at the lower or higher ends of this range for extended periods. Furthermore, the lack of natural light and limited air circulation common in enclosed storage spaces create an unsuitable habitat for these insects, which prefer bright conditions and rely on air currents to detect the odors of decomposition. The presence of certain chemicals or cleaning agents used within a contained storage space could also potentially act as deterrents, further contributing to the absence of blow flies.
Investigative Implications of Absence
The absence of blow flies at a crime scene within a storage building does not halt a forensic investigation; rather, it provides valuable information about the scene’s conditions. While their absence complicates the estimation of the post-mortem interval (PMI) using blow fly development, it indicates factors like restricted access or extreme environmental conditions. Forensic entomologists then broaden their search for alternative insect indicators that might thrive in such contained or atypical environments.
Beetles, for instance, are often found in later stages of decomposition or in drier conditions where blow flies may be less prevalent. Some species of beetles, like dermestids or clerids, are known to colonize remains even when they are dry. Mites, although tiny, can also provide crucial information, particularly those adapted to specific microenvironments. If insect evidence is minimal or absent, investigators will rely more heavily on non-entomological methods, such as studying decomposition rates based on factors like temperature, humidity, and the presence of other biological indicators, to estimate the time since death. The very lack of expected insect activity helps to reconstruct the events surrounding the death and the conditions of the scene.