Question

describe four stages of post mortem decomposition, including the role of insect in each

Answer 1

Stages of decomposition

1) Fresh stage

The Fresh Stage begins at the moment of death and continues until bloating of the body becomes evident. There are few distinctive, gross decompositional changes associated with the body during this stage although greenish discoloration of the abdomen, livor, skin cracking, and tache noir may be observed. The insect invasion of the body generally begins with the natural body openings of the head (eyes, nose, mouth and ears), genitals, and wounds present on the body. The first insects to arrive under most circumstances are the Calliphoridae (blow flies) and Sarcophagidae (flesh flies). Female flies will arrive and begin to explore the potential sites for oviposition or larviposition. These flies will often go deep into the openings and either deposit eggs or 1st instar larvae or maggots. Whereas the openings associated with the head are uniformly attractive to flies, the attractiveness of the genital areas may depend on their being exposed or clothed. Wounds inflicted prior to death have been observed to be more attractive to flies for colonization if inflicted prior to death, when blood is flowing, than wounds inflicted postmortem and lacking a blood flow. During this stage, the eggs laid in the body begin to hatch and there is internal feeding activity, although there may be little evidence of this on the surface.

• Insects in Fresh stage

The first insects to arrive at decomposing remains are usually Calliphoridae, commonly referred to as blow flies. These flies have been reported to arrive within minutes of death or exposure, and deposit eggs within 1–3 hours. Adult flies of the families Sarcophagidae (flesh flies) and Muscidae are also common in this first stage of decomposition. First eggs are laid in or near the natural orifices of the head as well as at the site of perimortem wounds.Depending on the rate of decomposition and the development time of particular blowfly species, eggs may hatch and young larvae begin to feed on tissues and liquids while the carcass is still classified in the fresh stage.Adult ants may also be seen at a carcass during the fresh stage. Ants will feed both on the carcass flesh as well as eggs and young larvae of first arriving flies.

Fig.1 Body in Fresh Stage of decomposition.

2) Bloated stage

The principal component of decomposition, putrefaction, begins during the Bloated Stage.The anaerobic bacteria present in the gut and other parts of the body begin to digest thetissues. Their metabolic processes result in the production of gasses that first cause a slightinflation of the abdomen. When this is noted, the Bloated Stage is considered to begin. Asthis progresses, the body may assume a fully inflated, balloon-like appearance. Thecombined processes of putrefaction and the metabolic activities of the maggots begin to cause an increase in the internal temperatures of the body. These temperatures can besignificantly above ambient temperature ([50°C) and the body becomes a distinct habitat,in many ways independent of the surrounding environment. The adult Calliphoridae arestrongly attracted to the body during this stage in decomposition and significant masses ofmaggots are observed associated with the head and other primary invasion sites. Whilethese populations are visible externally, there are larger populations present internally.Internal pressures caused by production of gasses result in the seeping of fluids from thenatural body openings during this stage and the strong smell of ammonia is noted. Thesefluids seep into the substrate beneath the body and this becomes alkaline. The normal soilfauna will leave the area under the body as a result of this change in the pH and theinvasion of a set of organisms more closely associated with decomposition begins.

• Insects in Bloated stage

Blowflies remain present in great numbers during the bloat stage, and blowflies, flesh flies and muscids continue to lay eggs. Insects of the families Piophilidae and Fanniidae arrive during the bloat stage. Ants continue to feed on the eggs and young larvae of flies.The first species of Coleoptera arrive during the bloat stage of decomposition, including members of the families Staphylinidae (rove beetles), Silphidae (carrion beetles) and Cleridae. These beetles are observed feeding on fly eggs and larvae.Beetle species from the families Histeridae may also be collected during this stage, and are often hidden beneath remains.

Fig.2 Body in Bloated stage of decomposition.

3) Decay stage

The start and termination points for the stages of decomposition are largely subjective, butthere is a definite physical event marking the start of the Decay Stage. This is when thecombined activities of the maggot feeding and bacterial putrefaction result in the breakingof the outer layer of the skin and the escape of the gasses from the abdomen. At this point,the body deflates and the Decay Stage is considered to begin. During this stage, strongodors of decomposition are present. The predominant feature of this stage is the presenceof large feeding masses of Diptera larvae. These are present internally, externally and oftenspilling onto the ground beside the body. Some Coleoptera that have been arriving duringearlier stages of decomposition, increase in numbers during the Decay Stage and are oftenquite evident. Some predators, such as the Staphylinidae, are seen during the Bloated Stageand they become more evident now, along with others, such as the Histeridae. In additionto the predators, necrophages are also evident, increasing in numbers as the processcontinues. By the end of this stage, most of the Calliphoridae and Sarcophagidae will havecompleted their development and left the remains to pupariate in the surrounding soil. By the end of the Decay Stage, Diptera larvae will have removed most of the flesh from the body, leaving only skin and cartilage.

• Insects in Decay stage

Feeding larvae of Calliphoridae flies are the dominant insect group at carcasses during the active decay stage. At the beginning of the stage larvae are concentrated in natural orifices, which offer the least resistance to feeding. Towards later stages, when flesh has been removed from the head and orifices, larvae become more concentrated in the thoracic and abdominal cavities.Adult calliphorids and muscids decreased in numbers during this stage, and were not observed to be mating.However, non-Calliphoridae Dipterans are collected from carcasses.The first members of Sepsidae arrive at the carcass during the active decay stage. Members of Coleoptera become the dominant adult insects at the site of remains. In particular, the numbers of staphylinids and histerids increase.

Fig.3 Body in Decay stage of decomposition.

4) Skeletal or Dry stage

This stage is reached when only bones and hair remain. Typically, there are no obviouslycarrion-frequenting taxa seen during this stage. During the earlier portions of the SkeletalStage, there are a number of soil-dwelling taxa, including mites and Collembola, that canbe used in estimating the period of time since death. As time passes, the pH of the soilbegins to return to the original level and there is a gradual return of components of thenormal soil fauna during this stage. There is no definite end point to this stage and theremay be differences in the soil fauna detectable for a period of months or sometimes years,indicating that a body was there at some point in time

• Insects in Dry stage

The greatest number of species are reported to occur in the late decay and dry stages.The dry decay stage is characterized by the movement from previously dominant carrion fauna to new species. Very few adult calliphorids are attracted to the carcass at this stage, and adult piophilids emerge.The dermestid beetles, common in advanced decay, leave the carcass. Non-carrion organisms that commonly arrive at remains in dry decay are centipedes, millipedes, isopods, snails and cockroaches.

Fig.4 Skull in Skeletal or Dry stage of decomposition