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One of the first things that happens after death is that the temperature in the body starts to drop. Before the temperature in the body core drops, a temperature gradient must be established from the outside to the core. After this gradient has become established the body temperature will drop with a theoretically predictably rate. This fact can be used to estimate time of death. Even if one succeeds in predicting when the temperature of the body core was 37 degrees Celsius, one has to remember that the time it takes to form the temperature gradient will vary from individual to individual, and will vary from almost no time, to over two hours.
After the onset of putrefaction (about two days after death) the body temperature will increase again, due to the metabolic activity of the bacteria and other decomposing organisms. Rigor mortis is a well known phenomenon, and is due to a complex chemical reaction in the body. In the living body muscles can function both aerobic and anaerobic. In the dead body muscle cells can only function anaerobically. When muscle cells work anaerobically the end product is lactic acid. In the living body, lactic acid can be converted back, by means of excessive oxygen uptake after an anaerobic exercise.
In the dead body this can not happen, and the breakdown of glycogen in the muscles leads irreversibly to high levels of lactic acid in the muscles. This leads to a complex reaction where actin and myosin fuses to form a gel. This gel is responsible for the stiffness felt in the body. This stiffness will not be over before decomposition begins.
As rigor mortis is due to a chemical reaction, the reaction time is due to temperature and the initial concentrations of lactic acid. High metabolic activity in the time just before death, for example when running, leads to higher levels of lactic acid, and shorter time for the rigor mortis to develop. Higher environmental temperature also leads to a shorter reaction time. In temperate regions the following rules of thumb can be used in estimating death, but must be used with caution: Temperature of body Stiffness of body Time since death Warm Not stiff Not dead more than three hours Warm Stiff Dead between 3 to 8 hours Cold Stiff Dead between 8 to 36 hours Cold Not stiff Dead in more than 36 hours Rigor mortis should never be the only basis for estimating time of death.
After death, a lot of internal organisms in the intestine will become very active. Escherichia coli and others will start multiplying, and the decomposition begins. First the intestine and the blood will be attacked, and when gas formation and other things leads to rupture of the intestine other organs will be attacked. Organs starts decomposing at different times after death, and may also be used in estimating time of death.
The decomposition of a body can be divided into several stages, even if the duration of each stage will vary a lot: Initial Decay The cadaver appears fresh externally but is decomposing internally due to the activities of bacteria, protozoa and nematodes present in the animal before death Putrefaction The cadaver is swollen by gas produces internally, accompanied by odour of decaying flesh Black putrefaction Flesh of creamy consistence with exposed parts black. Body collapses as gases escapes. Odour of decay very strong Butyric fermentation Cadaver drying out. Some flesh remains at first, and cheesy odour develops. Ventral surface mould from fermentation Dry decay Cadaver almost dry; slow rate of decay In the rest of this document we will focus on the telltale signs that insects can provide in the investigation of suspicious deaths. After the initial decay, and the body begins to smell, different types of insects are attracted to the dead body.
The insects that usually arrives first is the Diptera, in particular the blow flies or Calliphoridae and the flesh flies or Sarcophagidae. The females will lay their eggs on the body, especially around the natural orifices such as the nose, eyes (2), ears (2), anus, penis and vagina. If the body has wounds the eggs are also laid in such. Flesh flies do not lay eggs, but deposits larvae instead. After some short time, depending on species, the egg hatches into a small larvae. This larvae lives on the dead tissue and grows fast.
After a little time the larva molts, and reaches the second larval instar. Then it eats very much, and it molts to its third instar. When the larvae is fully grown it becomes restless and begins to wander. It is now in its pre pupal stage. The pre pupae then molts into a pupae, but keeps the third larval instars skin, which becomes the so-called puparium. Typically it takes between one week and two weeks from the egg to the pupae stage.
The exact time depends on the species and the temperature in the surroundings. A table of life histories to some species of blow flies and flesh flies are available here, and an illustration of the blowfly life cycle is available here. The theory behind estimating time of death, or rather the post mortem interval (PMI for short) with the help of insects are very simple: since insects arrive on the body soon after death, estimating the age of the insects will also lead to an estimation of the time of death. How to estimate age of blowfly eggs, larvae, pupae and adults When blow flies oviposit, their eggs has come very short in their embryonic development. The eggs are approximate 2 mm in length. During the first eight hours or so there is little signs of development.
This changes after that, and one can see the larvae through the chorion of the egg at the end of the egg stage. The egg stage typically lasts a day or so. The blowfly has three instars of larvae. The first instar is approximately 5 mm long after 1. 8 days, the second instar is approximately 10 mm long after 2. 5 days, the third instar is approximately 17 mm long after 4 - 5 days.
Identifying the right instar is the easiest part, and is done relatively easy based on size of larvae, the size of the larva's mouth parts and morphology of the posterior spiracles. The time it takes to reach the different instars depends very much on microclimate, i. e. temperature and humidity. At the end of the third instar the larva becomes restless and starts to move away from the body. The crop will gradually be emptied for blood, and the fat body will gradually obscure the internal features of the larvae.
We say that the larva has become a preppy. The preppy is about 12 mm long, and is seen 8 - 12 days after ovi position. The preppy gradually becomes a pupa, which darkens with age. The pupa which are about 9 mm in length are seen 18 - 24 days after ovi position. The presence of empty pup aria should therefore tell the forensic entomologist that the person in question has been dead in more than approximately 20 days.
Identification can be done based on the remaining mouth parts of the third instar larvae. A more precise way to determine age of larvae and eggs is the use of rearing. For example: the body is found with masses of eggs on it, none have hatched. How long time is it since the eggs was oviposit ed? Note the time of the discovery, note the time when the first 1. instar l...
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Research essay sample on Time It Takes Time Of Death
