15.6. Forensic entomology
As seen in section 15.3, some flies develop in living flesh, with two waves discernible: primary colonizers that cause initial myiases, with secondary myiases developing in pre-existing wounds. A third wave may follow before death. This ecological succession results from changes in the attractiveness of the substrate to different insects. An analogous succession of insects occurs in a corpse following death (section 9.4), with a somewhat similar course taken whether the corpse is a pig, rabbit, or human. This rather predictable succession in corpses has been used for medico-legal purposes by forensic entomologists as a faunistic method to assess the elapsed time (and even prevailing environmental conditions) since death for human corpses.
The generalized sequence of colonization is as follows. A fresh corpse is rapidly visited by a first wave of Calliphora (blow flies) and Musca (house flies), which oviposit or drop live larvae onto the cadaver. Their subsequent development to mature larvae (which depart the corpse to pupariate away from the larval development site) is temperature-dependent. Given knowledge of the particular species, the larval development times at different temperatures, and the ambient temperature at the corpse, an estimate of the age of a corpse may be made, perhaps accurate to within half a day if fresh, but with diminishing accuracy with increasing exposure.
As the corpse ages, larvae and adults of Dermestes (Coleoptera: Dermestidae) appear, followed by cheese-skipper larvae (Diptera: Piophilidae). As the body becomes drier, it is colonized by a sequence of other dipteran larvae, including those of Drosophilidae (fruit flies) and Eristalis (Diptera: Syrphidae: the rat-tailed maggot, a hover fly). After some months, when the corpse is completely dry, more species of Dermestidae appear and several species of clothes moth (Lepidoptera: Tineidae) scavenge the desiccated remnants.
This simple outline is confounded by a number of factors including:
- geography, with different insect species (though perhaps relatives) present in different regions, especially if considered on a continental scale;
- difficulty in identifying the early stages, of especially blow fly larvae, to species;
- variation in ambient temperatures, with direct sunlight and high temperatures speeding the succession (even leading to rapid mummification), and shelter and cold conditions retarding the process;
- variation in exposure of the corpse, with burial, even partial, slowing the process considerably, and with a very different entomological succession;
- variation in cause and site of death, with death by drowning and subsequent degree of exposure on the shore giving rise to a different necrophagous fauna from those infesting a terrestrial corpse, with differences between freshwater and marine stranding.
Problems with identification of larvae using morphology are being alleviated using DNA-based approaches. Entomological forensic evidence has proved crucial to post-mortem investigations. Forensic entomological evidence has been particularly successful in establishing disparities between the location of a crime scene and the site of discovery of the corpse, and between the time of death (perhaps homicide) and subsequent availability of the corpse for insect colonization.