1. Calculating A PMI

2. Introduction Remember, the main reason for studying the presence and life stages of insects on a corpse is ot establish the time since death A method for establishing a PMI is based on an ecological and faunal study of the cadaver Data collection must be detailed and precise if the PMI is to be accurate The basis of the method is to study which insects, or their young, inhabit a dead body and in what sequence they do so

3. Introduction A body that has been dead only one or two days will have infestations primarily of blow flies, such as Cochliomyia macelleria, becaause these insects are attracted to a corpse almost immediately the recognition of each species in all stages and knowledge of the time occupied in each stage allow the entomologist to estimate a time since death Other information could be gleaned from the faunal composition, such as whether the body has been moved

4. Ecological Categories of Insects in the Cadaver Community Four ecological categories exist in the cadaver community The first are the necrophagous species, which feed on the carrion itself, contributing directly to the estimation of PMI Examples of necrophagous insects are diptera, coleoptera, silphidae, and dermestidae

5. Ecological Categories of Insects in the Cadaver Community The second most important group, forensically speaking, is the predatory and parasitic species, such as certain coleoptera, silhidae, and some diptera These insects prey on other insects, including the necrophagous ones, which inhabit the cadaver Some of the species in this group, however, may be necrophagous while immature but become predatory in later instars

6. Ecological Categories of Insects in the Cadaver Community Omnivorous species make up the third category Wasps, ants, and some coleoptera fit into this group because they may eat material from the body, other insects, or whatever food sourcepresents itself

7. Ecological Categories of Insects in the Cadaver Community The last category of insects uses the cadaver simply as an extension of their normal habitat, such as spiders, butterflies, collembola, and others This group can be referred to as the incedental species

8. A More Complicated Study Remember when we talked about using insects to calculate PMI A more complete, although more complicated, method of MI estimation involves the study of the succession of insect species on and within a body The forensic entomologist employs a model that is based on information about the ecological and environmental events between the time of death and the appearance of a particular insect species The simplest model would be one in which the forensic entomologist estimates the age of a larva and the time between death and the insect landing on the body

9. A More Complicated Study The activities of the insects, especially fly larvae, accelerate the putrefaction and disintegration of the body The number of waves of insects in the succession on a body has been interpreted to be between two and eight Such a model provides both a minimum and maximum PMI and yields much more accurate estimates

10. A More Complicated Study Many environmental factors – such as whether the body is on the surface, buried, in water; the temperature; weather; humidity; amount of light/shade; season; and even manner of death – influence the number, type, appearance, and life cycles of necrophagous insects Regardless of the complexity, the forensic entomologist must choose a model of insect development or succession, sometimes drawn from published experimental data

11. Experimentation Sometimes experimentation is required The experimental conditions should be as close to those at the crime scene as possible; this logically means that a forensic entomologist should be collecting data on decomposition in his or her ecological zone year round The closer the experimental data are to the crime scene conditions, the lower the margin of error will be in the PMI estimate The subjects for these decomposition studies are typically small pigs (under 50 pounds), which have been shown to be appropriate stand-ins for humans despite their smaller size

12. Experimentation One of the most influential factors in estimating MI is temperature Temperature has a direct effect on the metabolism and development of insects This is true not only of ambient (air) temperature but also the amount of sun or shade to which a body is exposed The larvae of necrophagous flies (maggots) are essentially “eating machines,” and they have a metabolism and feeding rate that is much higher than other immature insect forms

13. Experimentation When maggots are living, feeding, and moving all in approximately the same area, the temperature can soar by many degrees: This is termed the maggot mass effect The temperature at the center of a maggot mass can be 100°C while the ambient temperature is in the 30°F range, and this could obviously bias a forensic entomologist’s PMI estimation

14. Preserved Species The forensic entomologist studies insect samples that were killed and preserved at the time of collection as well as those kept alive or rearing The time when the preserved samples were collected is the starting point for the PMI, and it is from here forward that the entomologist makes his or her calculations for the maximal time since death

15. Computers Computers are now being used to create very complex but highly realistic models that provide forensic entomologists with improved models for PMI estimates As humans and computers become more adept at handling large amounts of complex data, the estimates of PMI based on entomological information will become more realistic and accurate