Biology Of Diptera

One of the major problems facing forensic entomologists is the accurate identification of the larvae collected from the remains. Too frequently, the entomologist must work with dead specimens collected by crime scene investigators and submitted in marginal states of preservation. Even when the local faunas are well known, identification of these specimens is difficult, especially for early instars. Work by Erzinclioglu in England and Liu and Greenberg in the United States has provided identification keys to larvae of forensically important taxa. Given the wide distributions of many of the sarcosaprophagous taxa, these keys have wider application than their regional nature implies.

Seasonal variation in the populations of Calliphoridae have been documented in North America and Europe. There are also successional patterns in Calliphoridae. In northern

Mississippi, for example, Phaenicia caeruleiviridis was the first species to arrive during spring, whereas Cynomyopsis cadaverina was the first during the fall and winter months.

Considerable emphasis has been placed on how temperature influences the duration of the various stadia for different species of flies. Accumulated degree hours (ADH) or accumulated degree days (ADD) can be used to estimate the postmortem interval. Ambient temperature data from weather stations in the vicinity of the corpses are used as an indicator of the temperatures at which larvae developed. These temperatures, however, may not reflect the temperatures at which the larvae actually develop. Based on a Hawaiian study, internal temperatures associated with the maggot masses can be as much as 22°C above ambient. Similar observations have been made for human corpses in the former Soviet Union.

In instances of heavy maggot infestations, it is obvious that there is little, if any, direct relationship between ambient air temperature and the temperatures at which the maggots are developing. Although heat generated by maggot masses influences the rate of larval development, this heat generation may not occur immediately, but requires a period of several days to develop. Such a delay may be because of a lack of an organized maggot mass during the early instars. For corpses found during cool weather when colonizing fly populations are low, ADH or ADD calculations generally are more accurate than in higher temperatures with high fly populations. In addition to the rate of development, temperatures may also serve to limit the species that can use the corpse for development. Only some species of Calliphoridae can tolerate high temperatures inside a maggot mass during development. Thus, maggot-generated heat can influence the rate of maggot development, the nature of the corpse arthropod community, the character of the subcorpse community, and the validity of ADH or ADD calculation-based postmortem interval estimates.

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