Time Flies-Age Grading of Adult Flies for the Estimation of the Post-Mortem Interval

Abstract

The estimation of the minimum time since death is one of the main applications of forensic entomology. This can be done by calculating the age of the immature stage of necrophagous flies developing on the corpse, which is confined to approximately 2-4 weeks, depending on temperature and species of the first colonizing wave of flies. Adding the age of the adult flies developed on the dead body could extend this time frame up to several weeks when the body is in a building or closed premise. However, the techniques for accurately estimating the age of adult flies are still in their beginning stages or not sufficiently validated. Here we review the current state of the art of analysing the aging of flies by evaluating the ovarian development, the amount of pteridine in the eyes, the degree of wing damage, the modification of their cuticular hydrocarbon patterns, and the increasing number of growth layers in the cuticula. New approaches, including the use of age specific molecular profiles based on the levels of gene and protein expression and the application of near infrared spectroscopy, are introduced, and the forensic relevance of these methods is discussed.

Keywords: NIRS; cuticular hydrocarbons; forensic entomology; gene expression; proteomics; pteridine.

Figure 1

An exemplary PMI_min determination using a fictitious case. The dead body was discovered in an apartment (20 °C ambient temperature), and only larvae, pupae, and empty puparia of one single species, the blow fly Calliphora vicina, were found on the body and its surroundings. In order to determine the PMI_min, the most advanced stage of its development will be of relevance. In the present case, these are empty puparia. Since there is no current method to narrow this down with respect to the time of hatching, the entomologist will assume that the flies hatched immediately before the body was found in order to determine a reliable PMI_min. There is a risk that this may lead to an underestimation of the time period. According to, e.g., Marchenko [11], C. vicina needs about 21 days to complete a complete development cycle at a temperature of 20 °C. These 21 days would be the PMI_min in the present case, based just on the immature specimens (larvae, pupae) and remains (empty puparia). However, there are hundreds of adult C. vicina in the apartment. Thirty to forty specimens are captured, and by using various techniques (see below), their age is estimated to be 15–17 days. Based on the assumption that the animals have developed on the cadaver, this now means that we are giving a PMI_min of 36 days in the entomological report (21 days development of larva and pupa plus 15 days aging of the fly).

Figure 2

Different methods can use/analyse one and the same adult specimen by different techniques.