CALCULATING A(PMI)
The main reason for studying the presence and life stages of insects on a corpse is to establish the time since death. A method for establishing a PMI is based on an ecological and faunal study of the cadaver, demonstrated in Figure 1. 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. A body that has been dead only 1 or 2 days will have infestations primarily of blowflies, such as Cochliomyia macellaria, because 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 a time since death to be estimated. Other information could be gleaned from the faunal composition, such as whether the body has been moved. 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. The second most important group, forensically speaking, is the predatory and parasitic species, such as certain coleoptera, silphidae, 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. 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 source presents itself. 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 incidental species. The faunal succession on carrion is linked to the natural changes that take place in a body following death. After death, the body temperature falls to that of its ambient environment. Cellular breakdown begins after several hours, which results in the release of gases such as ammonia (NH3), hydrogen sulfide (H2S), carbon dioxide (CO2), and nitrogen (N2). Putrefaction follows due to the activity of microbes, especially those from the body’s own intestinal flora. These chemical and micro biological consequences of death are the earliest, and tend to be the most accurate, indicators of time since death. Their accuracy and utility diminish as time moves forward, however, and other information, namely ecological, must be used.
FIGURE 1 Data collected by entomologists and other biologists, as well as weather data, all contribute to the estimation of a postmortem interval, the time since death occurred.
At a minimum, the estimated age of an immature insect can provide a PMI, but this estimate does not provide a maximum limit because the amount of time between death and egg/larval deposition is unknown. Necrophagous insects appear almost immediately as the cellular breakdown begins; some species of flies are so sensitive to the chemistry of death that they appear within minutes of the cessation of life. The level of larval development can provide an estimate that is accurate from less than 1 day to just over 1 month, depending on conditions and the species reared. A more complete, although more complicated, method of PMI 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. 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 2 and 8. Such a model provides both a minimum and maximum PMI and yields much more accurate estimates. Many environmental factors, such as whether the body is on the surface, buried, or 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. For an example, see “In More Detail: PMI in Hawaii.”
If no data are available that take into account the parameters that the forensic entomologist faces, then 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 eco logical zone(s) 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. In the second part of a study funded by the National Institute of Justice, Haskell et al. (2002) showed that both pigs and humans attract a large majority of the same arthropod species, but also the most common and moderately common species. This was true regardless of the pigs’ size, but small pigs are easier to physically handle than large ones, practically speaking. Importantly, their work also indicates that cross-comparison of “pig studies” from differing geographical zones may be a viable research interest. One of the most influential factors in estimating PMI 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. When a group of maggots is 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 °F while the ambient temperature is in the 30 °F range, and this could obviously bias a forensic entomologist’s PMI estimation. The forensic entomologist studies insect samples that were killed and preserved at the time of collection as well as those kept alive for rearing. The time when the
FIGURE 2 150 200 250 300 Because the ending point of the simulation is known, but the starting point is not, it is necessary to run the model at all possible starting points using historical environmental data to generate possible solutions from each hour in the past. Each model run assumes a time of death at the point where the model starts. Computers logically lend themselves to calculations involving a large amount of data, such as a PMI. This example shows the data collected, graphed, and interpreted using specific mathematical models for the particular geographical area in question. Other models would need to be employed if the victim were found in another area or at another time of year.
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. Because every death scene has unique circumstances and environs, no one algorithm best calculates all PMI estimates. As shown in Figure 2, 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.
FIGURE 3 Faulkner Haskell Goff Hall Chart of the various timelines in the Westerfield trial. During the trial, an anthropologist, two pathologists, and four entomologists offered estimates of PMI for Danielle Van Dam; the lighter colored areas indicate times conceded to on the stand. Methodology and expertise played a role in the range of variation of their estimates.
