Insect evidence can be used to confirm the PMI established through body temperature, hypostasis and/or rigor. Conversely the insect evidence recovered from a scene may contradict the standard temperature, hypostasis and rigor indications bringing into question the findings. This in turn can lead to the development of new and more accurate hypothesis regarding the circumstances of death and the PMI (Baden & Hennesse, 1989).
As well as the PMI, insect evidence can indicate the season of death. Insect evidence can indicate whether the death occurred in an urban or rural setting. Insect evidence can be used to determine whether a buried body was on the surface for some time after death and then buried. Insect evidence also indicates whether a body has been previously buried.( Anderson 2001)(Goff 2000)(Erzinclioglu, 2000) Aquatic insects can indicate the season and conditions under which the body came to be in the water(Thomas, 1985)Aquatic insects on a body found on land indicate death in a wetter season or movement of the body.
Insects begin to arrive at a corpse in a little as ten minutes after death (Goff, 2000)(Lane, 1992). In buried bodies colonization can be found as much as ten years after death (Erzinclioglu, 2000). Insect colonization can be found on bodies sealed in plastic bags, rugs and cars (Anderson, 2001). Insect colonization occurs on bodies indoors (Anderson, 2001) and those which have been buried (Erzinclioglu, 2000). This means that insect evidence can be used in wide variety of circumstances and over much longer periods of time as opposed to other widely used methods for estimating the PMI.
Stages of Decomposition and Colonization
In A Fly for the Prosecution , M. Goff identifies five distinct stages of decomposition. The stages of decomposition are; “fresh, bloated, decay, post decay and skeletal.”(Goff, 2000, p.43). Each of these stages are accompanied by distinct insect activity. Goff also notes that , “regardless of where the decomposition studies are conducted, I have observed that certain patterns are common to most if not all of them.”.
The Fresh Stage
In the fresh stage the first insects to arrive are the flies (Order Diptera).This is where the literature becomes somewhat confusing. Some researchers refer to the entire colonization as Blowflies (Saferstein, 2004). Others refer to Blowflies and Fleshflies as seperate entities (Erzinclioglu, 2000)(Goff, 2000). Still others refer to Blowflies as being Sarcophigae(Lane 1992) which is incorrect. The most accurate discription is the actual classification. In the family Calliphoridae are the Blowflies also known as Green bottles, Blue bottles and House flies. The fleshflies are in the family Sarcophagidae. Here is a way of distinguishing the classes. Blowflies are metallic in color and may be green, blue or black. Fleshflies are dull in color and may be striped with a red protuberance at the posterior of the abdomen ( Bryd and Castner, 2001). Blowflies lay eggs into any wounds or openings such as eyes, nose, penis, or vagina. Fleshflies deposit live larva onto the body (Erzinclioglu, 2000)(Goff, 2000).
The insects to arrive in the fresh stage are the Green bottle and Blue bottle blowflies. They may arrive anywhere from minutes to several hours after death depending on the environmental conditions. Females lay eggs in any opening of the body. The first ovipostion of eggs may not be immediately noticeable because the eggs are deposited very deeply inside the orifices of the body(Goff, 2000). The eggs of the blowfly are approximately 2mm in length and are white or yellow. Flesh flies may arrive at the same time or several hours after the blowflies (Goff, 2000). As already noted Fleshflies deposit live maggots on the corpse.
In the Fresh stage their may be wasps which prey on the adult flies. Ants may also appear and take eggs and maggots (Erzinclioglu, 2000).
During the fresh stage there are several methods used to estimate the PMI. The investigator may collect the eggs, when that is all that is found, and rear them in the laboratory. In the laboratory the investigator must reproduce the environmental conditions in which the body was found. M. Goff suggests including a sample of the flesh from the body to replicate the food source (Goff, 2000) However, this practices raises moral and ethical issues. Also, this practice could potentially disturb evidence and create false wound patterns. Other researchers suggest beef liver as a good source of food for laboratory rearing of maggots (M.Stoerkay, 2000) (Bryd, 2001)
The eggs are allowed to hatch and proceed through the five stages of development ending in the emergence of adult flies. Some adult flies are collected for identification at this point. Working backwards from the time of collection using the known life cycle of a particular species the investigator can estimate the PMI.
Some literature suggests allowing a second cycle to occur. This enables the investigator to record the exact time in hours of each stage and the total hours for a complete cycle(Bryd, 2001) The Table below shows some development times for some common species of flies.