Entomology is the branch of science concerned with the study of insects. One might imagine that entomology would have few practical applications in the "world of affairs" but this would be mistaken. The pollination by insects of various agricultural and wild plants is of enormous commercial, biological and ecological importance.
I recently read a fascinating account by M.L. Goff, (The Sciences, July/August 1991) of how useful entomology is in forensic science, particularly in deriving an accurate estimate of the time of death of murder victims.
Recent years have seen a revival of interest in forensic entomology but the field is hardly a new one. Writings from 13th century China tell of a villager murdered one summer in a farming community. The wounds indicated that the murder weapon was a sickle. The magistrate ordered all the villagers to place their sickles on the ground before him. In the summer heat, flies soon began to settle on the sickle which had traces of the victim's blood and its owner confessed to the murder.
In 1894, the French entomologist Pierre Megnin set down the basis for determining post-mortem intervals using insects as a guide. This is the ecological concept of succession. It means that a corpse, left lying on the ground, is invaded in a predictable manner by successive waves of insects. An understanding of this phenomenon allows one to estimate the interval between time of death and discovery of the body.
Studies of decomposing pigs have shown that about 450 insect species visit the corpse. Most insects come to feed on the corpse but some insects just happen to wander on to the corpse and may have no interest in eating it, such as plant-eating insects.
Analysis of the range of insects found on the corpse can also indicate whether a body was moved after the crime, for example if urban-living insects were found on a body discovered in the country.
Goff and his colleagues at the University of Hawaii describe a five-stage sequence of post-mortem decomposition.
The first stage begins when, within 10 minutes of death, blowflies and flesh flies visit the corpse. They deposit eggs or larvae (immature, free-living forms which develop into a different adult form) around body openings, principally the eyes, ears, nose and mouth. Blood and wounds are of little interest to the Hawaiian blowflies but elsewhere wounds are a major attraction for the fly species.
Next comes the bloated stage (it comes after one to two days in Hawaii - it obviously takes longer to arrive in our climate). Gases generated by bacteria in the body cause the abdomen to swell and give the corpse a balloon-like appearance. The putrefaction generates heat and the internal temperature of the body rises. Apart from bacteria, the main organisms associated with the corpse throughout this stage are maggots and adult flies.
The third phase is the decay phase. The skin of the corpse breaks as the maggots feed. The accumulated gases escape and the corpse deflates. The decay phase is marked by a stench and large feeding masses of maggots. The maggots complete their larval development by the end of the decay phase and leave the corpse to develop into pupae (immobile, non-feeding stage between larva and adult) in soil. At the same time, beetles start to arrive. Some feed on the corpse and other types feed on the maggots. At the end of the decay phase, all the flesh has been removed from the corpse and only skin, cartilage and bone remain.
The fourth stage is called post-decay. Flies and maggots are entirely replaced by beetles and these increase in numbers and kinds with time, eating all the remaining skin and cartilage.
The fifth and final stage is when the corpse is reduced to bones and hair. Now insects characteristically associated with carrion are absent.
Isn't it curious that a process such as bodily decomposition, which at first glance would seem difficult or even disgusting to contemplate, becomes non-repellent and even interesting once you understand it? The overall mechanism by which the various organisms break down the body post-mortem is very efficient and orderly (the maggots, who need soft, moist tissue to feed on, do their job before the beetles arrive, who prefer to eat the remaining dry skin and cartilage) and naturally sustains the life cycles of many insects.
Goff described typical cases where entomology figured in murder investigations. The oldest case in the West on record dates from 1850 in France. A husband and wife were charged with the murder of an infant whose mummified remains were found behind the living room mantelpiece. An entomologist examined the mites and empty pupae shells on the remains and concluded that the body had been hidden well before the defendants bought the house. Suspicion moved to the previous occupants.
Although usually applied to investigate circumstances of death, forensic entomology can also be used to look at the living. Goff describes a case where entomological evidence was used to prove child neglect.
A small child was brought to a hospital emergency department suffering from extensive nappy rash and malnutrition. Visual examination revealed maggots in the child's genital and anal areas. Entomological analysis showed that the maggots had been present for four to five days, the minimum period which had elapsed since the child last had its nappy changed.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC