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 stages during their life cycle. Using a blow fly in the family Calliphoridae as an example, the female fly arrives at the body and deposits eggs in body openings associated with the head, anus, and genitals, or in wounds. After hatching, larvae or maggots feed on the decomposing tissues. There are three larval stages, with a molt in between each stage. Once the maggot is fully developed, it ceases to feed and moves away from the remains before pupariation. The puparium is an inactive stage during which the larval tissues are reorganized to produce the adult fly.
The insects encountered on a corpse in any given habitat consist of species unique to that particular habitat and those having a wider distribution. The unique components may be restricted to a particular geographic area or a particular habitat type within a given geographic area. Those taxa having wider distributions are frequently encountered in several different habitat types and are typically highly mobile species. Many of those taxa closely tied to carrion show this wider pattern of distribution. In estimating the postmortem interval, taxa from both components may, under given circumstances, provide essential information on the history of the corpse.
Of those insects having a direct relationship to the corpse, there are four basic relationships, as described below.
Those taxa feeding on the corpse compose this group. This includes many of the Diptera [Calliphoridae (blow flies) and
Sarcophagidae (flesh flies)] and Coleoptera (Silphidae and Dermestidae). These species may be the most significant taxa for use in postmortem interval estimates during the earlier stages of decomposition, defined here as days 1 to 14.
This is the second most significant group of carrion-frequenting taxa and includes Coleoptera (Silphidae, Staphylinidae, and Histeridae), Diptera (Calliphoridae and Stratiomyidae), and hymenopteran parasitoids of larvae and puparia of Diptera. In some instances, dipteran larvae that are necrophages during the early portions of their development turn into predators.
Ants, wasps, and some beetles, which feed on both the corpse and associated arthropods, compose this group. Large populations of these may severely retard the rate of carcass removal by depleting populations of necrophagous species.
This category includes those taxa that 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 that 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, that feed on other acarine groups and nematodes.
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