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... are occur. Subtract the first occurrence time with the discovery time, call this time A. Rear the blow flies to adults, let them mate, let them lay eggs on raw beef liver under conditions similar to the crime scene, take the time from opposition to the first occurrence of 1. instar larvae.
Call this time B. By subtracting B-A, one gets C, which is an estimate of the time since opposition to discovery. Similar calculations can be done for other instars as well. If one has good base-line data from before under different temperatures and for different species, one only needs to rear the flies to a stage where they can be identified, and that is the third stage or the adult stage.
One important biological phenomenon that occurs on cadavers are a succession of organisms that thrive on the different parts. E. g. beetles that specialize on bone, will have to wait until bone is exposed.
Predatory rove beetles or parasites that feed on maggots will have to wait until the blow flies arrive and lay their eggs. The succession on cadavers happens in a fairly predictable sequence and can be used in estimating time of death if the body has been lying around for some time. Here is a table over a succession experiment on guinea pigs performed by Bornemizza in 1957. There are several things to note about this table: The first groups to arrive is blow flies, followed shortly by staphylinids.
As putrefaction develops, more groups arrive at the scene, with most groups present just before the body is drying out due to seepage of liquids. After the body is drying out, dermestids, tineids and certain mites will be the dominant animal groups on the body, and blow flies will gradually vanish. Note also how the fauna changes in the soil. This can also be used to estimate time since death. Succession data can be incorporated in a database, and when the forensic entomologist investigates a case, he can use the taxa found on the body as input, and get an estimate of the time of death as output. The hypothetical table above shows the presence (1) and absence (0) of five different taxa (A, B, C, D and E) over ten days.
The tabulated data is usually obtained from decay studies done on pigs or other animals. Let's say the investigator finds taxa C, D and E on the remains. From the table above, we can see that taxa C occurs on the cadaver from day 5 to day 10, and taxa D from day 7 to day 9, and taxa E from day 9 to day 10. By studying the overlap, the entomologist estimate the PMI to be about 9 days. Several insects are specialized in living in very decayed dead bodies. One example is the cheese skipper, Piophila case, where the larvae usually occurs 3 - 6 months after death.
The cheese skipper is a well known pest of cheese and bacon worldwide, and has a cosmopolitan distribution. Adult cheese skippers may occur early after death, but larvae occurs later. The earliest observation on human remains are when the body is two months, and this was under excellent summer conditions. In 1898, Potter examined 150 graves, and found remains of P. case in ten of them.
These graves were from three to ten years old and three to six feet deep. In temperate regions dead bodies often appear in spring, after the snow is gone. The forensic entomologist and the forensic pathologist must then try to determine whether the death occurred during the winter or before the snow set in. If the death occurred before November, it is possible to find dead insects in and on the body. By analyzing the dead insect fauna, and estimating when the insects probably died (this can be found by looking at meteorological records). Another hint is when the different adults stop flying before the winter.
For example: here in Norway, we have had some cases where the bodies have been found in the spring. In one case we found dead third stage blow fly larvae in the back of the mouth. The blow fly larva was of an species that is flying from May to October. It was from this concluded that the eggs probably was laid during October, and since it was relatively few larvae, probably late in October. In another case, we found several live insects on a dead body, and also many dead third stage larvae. The dead larvae was found on the stomach, the arms, the shoulders, and inside the head.
We concluded that the live insects had colonized the dead body in the spring, and that the dead larvae had died during the winter. Based on the widespread occurrence of the larvae, we had to say it was likely that the body was colonized before October, probably in September. If the death occurred in the winter things become difficult in outdoor settings, as very few insects are active in the winter. It is reported that larvae of the winter gnat, Trichocera sp. can develop on carrion in the winter.
By estimating the age of these larvae, if present, it could be possible to estimate the PMI. Poison can be traced in blood, urine, stomach contents, hair and nails. One other important source is maggots occuring on a corpse. After a while it will be impossible to sample stomach contents, urine and blood from the dead body, but it will still be possible to sample from maggots, empty puparium or larval skin cast. The following list of chemicals has to my knowledge been traced in maggots: Many of these chemicals will also influence the life-cycle of the maggot. For example will high dosages of cocain accelerate the development of some sarcophagus.
Malathion, an insecticide, is commonly used in suicide, and is usually taken orally. Presence of malathion in the mouth may lead to a delay in the colonisation of the mouth. Presence of amitriptyline, an antidepressant, can prolong the developmental time with up to 77 hours, at least in one species of Sarcophagidae. Knowledge of drug use in the victim is therefore important not only in finding the death cause, but also in estimating the time of death. The sites of blowfly infestation on the corpse may be important in determining the cause of death, or at least in reconstruction of events prior to death.
For example: if there have been trauma or mutilation of the body prior to death, this may lead to heavy infestation of other body parts than the usual sites when the victim is not mutilated. Under a knife attack, it is usual to guard oneself with arms in front of thorax and head. This may lead to injury on the lower part of the arm. After death, blowfly may oviposit in these wounds.
The usual sites of opposition on dead humans are natural openings. Even here there is preference. Blowflies will most often lay their eggs in the facial region, and more seldom in the genito anal region. If there is a sexual assault prior to death, leading to bleeding in the genito anal region, blowflies will be more likely to oviposit in these regions. Therefore, if there is observed blowfly activity in the genito anal region, one can start to suspect a sexual crime.
This must of course be corroborated with other evidence as well. The interpretation of maggots in the ano genital region becomes very fuzzy after a few (4 - 5) days, as eggs will be oviposit ed in this region during the course of decay in the natural course of decomposition. Bibliography:
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Research essay sample on Time Of Death Dead Bodies
