Q. Forensic Investigation: PMI using entomological techniques. Introduction In this lecture we learned that for a forensic entomologist, the most important part of life cycle studies concerns the rate of development of the different stages. Identification of the species represents step one of the analysis; step two involves gauging the stage of the life cycle the most advanced specimen is in (in terms of growth). We are going to focus on step two to try to understand the basics of time estimation, setting the scene for later lectures where we examine estimation of post mortem interval (PMI) in greater detail. The assignment is divided into two parts – you are expected to complete both parts. Part 1 examines the number of flies at a carcass, extrapolating from that to calculate how many maggots could be expected. Part 2 looks at the effects of temperature on maggot development. Part 1. It is very useful to understand the impact that insects make on a corpse over a relatively short time frame. We know, of course, the rate that this happens is temperature dependent. We are going to use the number of adults visiting a carcass to try to estimate the potential number of maggots that may develop under favourable conditions. Q1. Examine photograph 1: SixPiglets. Count how many adult flies you observe on the piglet corpses and adjacent vegetation. To do this you could either print the image and mark the flies off with a pencil, or you could use media software to annotate the flies as you count. If you use the latter method, it may help to zoom in one step first. Show your calculation and highlight your final answer.
Photograph 1: Q2. We could assume that half the flies are males, hoping to mate with the females arriving at the locus. Divide your number in half. Show your calculation and highlight your final answer. Q3. We also know that the average female blowfly can lay approximately 3000 eggs in a lifetime. Assume she lays half of these on this carcass. Multiply your answer for step 2 by the number of eggs laid on this occasion. This is the potential number of maggots expected in a body; of course not all females will oviposit on this occasion and most are unlikely to lay a full (or even 50%) complement of her eggs. Show your calculation and highlight your final answer. The main point I wish to demonstrate from this exercise is the vast population of flies that results from just one body (or in this case group of bodies) – imagine the situation where multiple bodies are located. The amazing part is that on the whole, we don’t see enormous populations of flies out in the countryside (unless living in certain places like the outback of Australia), nevertheless they are there.
Secondary to that main aim, is the two-fold problem of estimating numbers of insects at a locus. If we try to do this in vivo it quickly becomes clear that what looks like a simple task is in fact an impossible situation, because the flies move around too much. Taking a snapshot looks like a good alternative, but we then encounter many problems. Photographic resolution may not be detailed enough to do more than ‘guestimate’ at some parts of the photograph (clusters for instance and partially concealed or out of focus specimens). Sometimes in case work the need for a forensic entomologist is a secondary consideration after the start of the investigation and perhaps even after the photograph was taken, so we are sometimes presented with photographs such as these. The only pragmatic solution, if indeed it really is important to know how many flies are present, is to capture every single one and count them. I have done this under experimental conditions using the body of a road-kill stoat (Mustela erminea) weighing about 210g, from which a total of just over 300 adult flies emerged. Extrapolating this to a grown adult human produces an incredible 88,571 flies, using the world average body mass of 62kg. I am not suggesting that such extrapolations are valid, nor that all the maggots in a body survive, but the numbers speak for themselves. Part 2. Background A complicated forensic case from Canada involved a severed head found on 28 June and a life insurance claim by the husband of the deceased lady. The victim was reported missing 11 days prior (17 June) by the husband, who also submitted an insurance claim on the basis of her disappearance, but on 20 June was told by the insurance agent that missing was not proof of deceased, so the claim was invalid. There was no reason at this stage of the investigation to link the two, but suspicions were raised. Larvae of Calliphora vomitoria (Linnaeus, 1758) just developing from third instar (L3) to the post feeding stage were found only on the neck, and not in the orifices of the remains, suggesting that the body had been secreted away where flies could not find it and then the head was severed post mortem, after which it was left exposed on the vegetation where it was found. This being the case it is possible to place an estimated time of severing using the development of the maggots – or at least a time since blow fly eggs hatched after being lain on the severed flesh. The local meteorological mean ambient temperature was 18.4°C over the 11 days preceding the discovery of the body part, which was in a shaded locality. For the practical purposes of this exercise, use this mean temperature as the developmental temperature of the maggots, but remember that this is an imprecise measure of their real developmental temperature. Exercise Q1. From the table below, decide which row expresses the most appropriate temperature scale to use in this case (show your calculation and highlight your final answer). Development Rate Chart (hours) for Calliphora vomitoria (Linnaeus, 1758) Temp °C Egg Larva 1 Larva 2 Larva 3 L1-L3 Post-feeding Larva Pupa 12.5 38-64.8″ 55.2 60.0 278.4 393.6 156 717.6 23.0 21.6 25.2 19.2 128.4 172.8 86.4 247.2 29.0 16.8 10.8 18 88.8 117.6 74.4 –
– represent missing experimental data ” represent ranges from different experiments Note: the calculations are using SI units for temperature, i.e. degrees Celsius. Q2. Now sum the length (in hours) of all appropriate immature stages, up to and including the last completed stage of the oldest specimen, using the temperature closest to the crime scene mean as the basis for your estimate (Show your calculation and highlight your final answer). Hint be careful which stages you choose, as this will affect your end result. Q3. On the basis of your calculation, convert to days and count backward in time and estimate when the first eggs were laid.
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