The application of insect evidences or damage for purposes of civil or criminal law. Forensic entomology includes three subdisciplines: stored product entomology, structural entomology, and medicolegal entomology. Stored product entomology deals with damage to stored products, such as food materials or clothing, caused by insect activities. Structural entomology deals with damage to buildings and other structures by pest species such as termites or carpenter ants. Both these areas typically involve civil actions when there is a need to assess monetary damages resulting from insect activity. Medicolegal entomology deals with insects as evidence in criminal events, most frequently homicides. This is the area most commonly recognized by entomologists, law enforcement, and the public as forensic entomology. The application of entomological evidence to criminal investigations is not new, with the first recorded use coming from twelfth-century China. Beginning in the early 1980s, there has been a resurgence of interest in the field, and new applications of insect evidence are being exploited. See also: Criminalistics
Insects can serve as evidence in criminal investigations in several different ways. They can provide data for estimating the time since death (postmortem interval), detecting possible movement of the body following death and circumstances of the crime scene, and assessing antemortem versus postmortem wounds on the body. In addition, insect evidence can provide alternative specimens for toxicological analyses in highly decomposed bodies, yield DNA material linking a suspect to a victim for ectoparasitic taxa, and help assess periods of abuse or neglect of children and the elderly. Of these, the most frequent application is the estimation of the postmortem interval. See also: Death
A dead body provides a changing and ephemeral food source for a wide variety of organisms, ranging from bacteria to vertebrate scavengers. Among these organisms, insects are major factors because they arrive at decomposing remains in a predictable pattern and develop in known patterns. Insects arrive shortly following death, often within minutes. The first arrivals are most frequently flies in the families Calliphoridae (blowflies and bottle flies) and Sarcophagidae (flesh flies). The female flies arrive and begin to investigate the natural body openings of the head, anus, and genitals. Wounds present on the body provide another area for activity. The female flies take a meal of blood or other exudates from the body and then deposit their eggs or, in the case of the Sarcophagidae, larvae into these body openings. This action starts a biological clock which is stopped and interpreted by the forensic entomologist when the body is discovered and the insects are collected. The use of insects to estimate the postmortem interval requires an understanding of the insect's life cycle, the relationship of the insect to the remains, and the relationship of the remains to the habitat in which they are discovered.
Insects pass through a number of distinct life-cycle stages. Using the blowfly in the family Calliphoridae as an example, the female fly will arrive at the body and deposit eggs (illus. a) in the natural body openings or in wounds. These eggs will hatch into the larvae (maggots) which will feed on the decomposing tissues. There are three distinct larval stages (illus. b), called instars, with a molt between each stage. Once the maggot has reached complete development, it will cease feeding and move away from the remains to enter the pupal stage (illus. c), an inactive stage during which the larval tissues are reorganized to produce the adult fly (illus. d).
The insects encountered on a corpse in any given habitat will consist of some species unique to that particular habitat and some species having a wider distribution. The former group may be restricted to a particular geographic area or a particular habitat type within a given geographic area. For example, in Hawaii there are taxa that are restricted to a rainforest habitat, while others are specific to a more arid tropical habitat. Taxa having wider distributions are frequently encountered in several different habitat types and are typically highly mobile species, such as those taxa that are found both in rainforests and on arid beaches. Many of those taxa closely tied to carrion show this wider pattern of distribution. In estimating the postmortem interval, taxa from both groups may, under given circumstances, provide essential information concerning the history of the corpse. See also: Insecta
Of those insect taxa having a direct relationship to the corpse, there are four generally recognized relationships:
1. Necrophagous species actually feed on the corpse. Included are many of the Diptera (Calliphoridae and Sarcophagidae) and Coleoptera (families Silphidae and Dermestidae). Species in this group may be the most significant isolatable taxa for use in postmortem interval estimates during the earlier stages of decomposition, defined here as days 1–14. See also: Diptera
2. Parasites and predators of necrophagous species constitute the second most significant group of carrion-frequenting taxa. Many of the Coleoptera (Silphidae, Staphylinidae, and Histeridae), Diptera (Calliphoridae and Stratiomyidae), and Hymenoptera parasites of Diptera larvae and puparia are included. In some instances, Diptera larvae, which are necrophages during the early portions of their development, become predators during their later larval development. See also: Coleoptera
3. Omnivorous species include taxa such as ants, wasps, and some beetles, which feed on both the corpse and associated arthropods. Large populations of these may severely retard the rate of carcass removal by depleting populations of necrophagous species. See also: Arthropoda
4. Adventive species include those taxa which use the corpse as an extension of their own natural habitat, as in the case of the Collembola, spiders, and centipedes. Acari in the families Acaridae, Lardoglyphidae, and Winterschmidtiidae, which feed on molds and fungi growing on the corpse, may be included in this category. Of less certain association are the various Gamasida and Actinedida, including the Macrochelidae, Parasitidae, Parholaspidae, Cheyletidae, and Raphignathidae, which feed on other acarine groups and nematodes. In these investigations, what is being estimated is actually a period of insect activity rather than an actual time since death.
During the early stages of decomposition, the estimate of the postmortem interval is most frequently based on the development rates of individual species of Diptera, most frequently the flies in the family Calliphoridae. The most mature specimens collected from the body are preserved and identified. Given the particular species and stage of development represented, the entomologist can determine the period of time required to reach that stage. As the insects are dependent on environmental conditions, particularly temperature, for their rate of development, this must be factored into the estimate. Data obtained from detailed laboratory studies of life cycles conducted under controlled conditions can be correlated with conditions at a crime scene using weather data from National Oceanic and Atmospheric Administration (NOAA) stations and the concept of accumulated degree hours or accumulated degree days, a technique originally developed to predict pest outbreaks in agriculture. This calculated time period will represent the minimum postmortem interval. It is important to note that the time period is the period of insect activity and not the actual postmortem interval. Under most conditions, these will be quite similar, but there may be factors that delay the onset of insect activity, such as wrapping or concealment of the body.
After the first 2–3 weeks, depending on the location and environmental conditions, those flies in the initial wave of invaders will have completed their development and departed the body. At this point, the estimation of the postmortem interval is based on the ecological succession of insects onto the body. Those insects initially colonizing the body change the body by their activities. These changes make the body attractive to another group of insects, which feed and change the nature of the body, thus making it attractive to yet another group of insects. This process continues until the resource of the body has been completely exhausted. By making complete collections of the insects and other organisms present on a decomposing body and comparing these taxa with results of detailed decomposition studies conducted in similar habitats and areas, periods of time during which the death most probably occurred can be determined. Care must be taken in applications of results from decomposition studies as even geographically proximate localities may support different arthropod populations. These differences in species composition and relative abundance may serve to alter the successional picture. Generally speaking, the closer to the time of death, the more accurate the estimated postmortem interval will be. The estimate begins in terms of hours and proceeds to days, months, and finally seasons of the year.
Entomological evidence is not present in all cases and, when present, may not ultimately prove to be of major significance. When insect evidence is present and properly interpreted, it may prove to be a powerful tool for the solution of the case.