Many biological and chemical changes begin to take place in the body immediately after death and progress in a fairly orderly manner until the body disintegrates. Estimates have been based on a series of changes to the body, including Livor mortis, Algor mortis, Rigor mortis, and similar occurrences. However, modification of the decomposition process can significantly alter the estimate of the time of death. These changes will be described and their relative significance discussed.
This paper documents the factors affecting the rate of decomposition and invertebrate colonization of exposed human remains.
STAGES OF DECOMPOSITION
Decomposition is a continual process of gradual decay beginning at the moment of death and ending when the body is reduced to a dried skeleton, this can take from weeks to years, depending on the environment.
The human body, like other animal remains will pass through a series of stages that offer evidence to allow post-mortem estimate to be made. The stages of decomposition can be divided into five stages as proposed by Catts and Haskell (1990); these include: fresh, bloat, active decay, advanced decay, and dry remains.
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These different stages of decomposition reflect the physical conditions of the corpse and the insects most often found during those periods in the decay process, and can also help in determining how long a body has been dead.
The fresh stage shows little or no signs of decomposition, but external colour changes begin to appear. Internal changes are occurring as a result of bacterial and protozoal activity. (Gary R. Mullen, Gary Mullen, Lance Durden; 2009).
These changes include rigor mortis, liver mortis and algor mortis which happen in the first few hours following death.
Rigor mortis is the stiffening of muscles and limbs; it becomes noticeable about 3-4 hours after death and advances until approximately 12 hours, when it is generally complete.
This process takes time to fully complete because cell death and the build up of waste products do not occur immediately. Once the process has completely subsided, the body will be limp and flexible.
This process is the most useful in determining time since death, as the changes in duration is relatively slight, and is also easy to recognize.
The second process that is sometimes used to determine post mortem interval is livor mortis. (Kaatsch et al., 1994)
Spitz and Fisher (1980) describe livor mortis as a purple discoloration of the skin of the dependent parts of the dead body that begins immediately after death. It is caused by the settling of the blood into the capillaries of the skin as they become dilated after circulation ceases. Livor mortis is usually well developed within four hours and reaches a maximum between eight and twelve hours and at about this time, it is said to become "fixed."
The third process, algor mortis, refers to the normal cooling of the body following death. According to Oxenham (2008) the rate of cooling can be used to estimate post mortem interval but it may be ineffective in hot climates.
Each of these processes can be used to estimate a time since death that is generally accurate within a few hours after death; however, these techniques will cease to be at thirty six hours post mortem, making a need to develop a timeline for the processes later in the decomposition sequence.
From the moment of death flies are attracted to bodies, and without the defence of a living human, blowflies are able to lay eggs around wounds if any and natural cadaveric openings such as the mouth, nose, etc. These eggs then hatch and move into the body; usually within 24 hours. This stage usually ends with the first evidence of the bloated stage.
After these preliminary changes have occurred, a body moves into the bloated stage of decomposition.
The bloated stage results from the accumulation of gases associated with anaerobic metabolism. After death, the body's defences cease which causes the bacteria that are normally inside the intestines of a living person to break the body down by feeding off the tissues. Eventually these bacteria break out into the body cavity and begin to feed on the other organs. The body's digestive enzymes then leak out and spread through the body; helping to break down more organs and tissues.
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At the same time, enzymes inside individual cells leak out and digest themselves and chemicals such as the stomach acids from the dead cells and tissues.
As a result, various gases such as hydrogen sulphide, sulphure dioxide, carbon dioxide, methane, and mercaptans are released, and it is these gases that create pressure within the body. This pressure slightly inflates the abdomen and scrotum, and later the whole body inflates; forcing fluids to escape from natural cadaveric openings and flow into the soil.
As the fluids from decomposition filter down into the soil, the area underneath the body becomes attractive to a series of organisms more specific to the decomposition process (Goff 2001), and as a result the rate of decay increases.
It has been suggested that the greatest number of eggs and maggots are deposited during the early to middle portion of the bloated stage. (Goff 2001)
The next stage to take place is the active decay stage, this stage begins when the gas escapes and the bloated body collapses. Active decomposition results from autolysis of tissues following the release of enzymes from cells and the action of bacteria and fungi growing on the remains. The skin begins to liquefy and the exposed parts of the body appear dark in colour and the odour of decay is very strong.
By this stage, numerous generations of maggots are present on the dead body and some have become fully grown. The majority of the maggots then migrate away from the body and burry themselves in the soil in order to pupate. At this point, the body enters the advanced putrefaction stage; this is the period when butyric fermentation occurs in anaerobic pockets of the body, to which a different group of insects are attracted. These include the silphid beetles Nicrophorus humutar the histerids Hister cadavaverinus and Sparinus rotundatus, and the muscid fly Hydrotaea capensis as illustrated in Figure1.
In the advanced stage of decay all that remains of the body are some flesh, skin, cartilage and bones. The biggest indicator of this stage is an increase in the presence of beetles and a reduction of in the dominance of the flies (Diptera) on the body. (Gennard 2007)
The final stage of the decomposition process involves the slow decay of the remaining dried tissues, hair, teeth, and bones over the course of months or years. No obvious group of insects are associated with this stage, although beetle of the family Nitidulidae can sometimes be found and Tineid moths remain on the body as long as traces of hair remain; which is dependent on the amount of hair available (Figure 1). Once a body has reached this stage, determination of time since death can be difficult, because bone preservation will depend more on the environmental circumstances than natural decay. Various factors can affect how well bone material remains intact.
The above stages of decomposition often follow one after the other but no clear distinction can be made of when one ends and the other starts as they are dependent on factors such as insect abundance and activity, geographical location, temperature, body size, etc, those factors will be discussed in the following section.
Fig1: Insect species most commonly associated with different stages of decomposition (Mullen et al 2009)
Stage of decomposition
Common genera and species
Calliphoridae (blow flies)
Phaenicia sericata, Phormia regina,
Cochliomyia macellaria, Calliphora vecina, C.vomitoria
Calliphoridae (as above) plus: Sarcophagidae (Flesh flies) Muscidae (House, latrine & dump flies)
Musca domestica, fannia scolaris, Hydrotaea aenescens
Staphylinidae (rove beetles)
Calliphoridae, Sarcophagidae & Muscidae (as above)
Staphylinidae (as above) plus: Silphidae (carrion beetles)
Necrophorus spp., oiceoptoma spp.
Staphylinidae & Silphidae (as above) plus: Histeridae (Hister beetles)
Sepsidae (Black scavenger flies)
Sphaeroceridae (Small Dung flies)
Scathophagidae (Dung flies)
Stratiomydidae (Soldier flies)
Phoridae (Scuttle flies)
Hister spp. Sapriaus spp.
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Piophilidae (Skipper flies)
Cleridae (Checkered beetles)
Nitidulidae (Sap beetles)
Dermestidae (Larder and Carpet beetles)
Trogidae (Hide beetles)
Pyralidae (Pyralid moths)
Tineidae (Clothes moths)
Dermestes spp., Anthrenus spp., Attagenus spp.
Tinea pellinonella, Tineola bissekkiella
FACTORS AFFECTING THE RATE OF DECOMPOSITION
The decomposition process can be influenced by factors of different nature concerning the body itself and also the external environment, which can increase or decrease its rate. These factors include the person's age, body size and weight, cause of death, and trauma (including wounds).
Clothing can also slow down post-mortem body cooling and favour the onset of the putrefaction process.
In addition, many environmental conditions have been claimed to have a significant on the decomposition process. (Mann 1990, Haglund 1997, and Haglund 2002)
Among the environmental influences; the most important variables are temperature, followed by humidity and the season of death. According to Mann et al (1990) access for invertebrates to the dead body in particular flies is the second most important factor after temperature affecting decomposition of a body.
A basic guide to determine how fast a body will skeletonise is given as Casper's Law which includes:
"When there is free access of air a body decomposers twice as fast than if
immersed in water and eight times faster than if buried in earth."
These variables can each have a different effect on the decomposition rate of exposed human remains and may combine in different ways at a particular crime scene. The effect of the various factors mentioned above will be discussed in the following section.
Among the cadaveric factors, is the size of the corpse; this has a profound effect on the rate at which it breaks down and the carrying capacity for fly larvae. Small corpses may offer limited food and affect the survival of the insects feeding and breeding on it (Kuusela and Hanski 1982).
Kuusela and Hanski (1982) found that the size of the corpse did not make a significant difference in the species of flies attracted to the corpse. However, the researchers did observe a positive correlation between the size of the corpse and the number of flies bred.
Some authors claim that obese corpses decompose more rapidly due to the greater amount of liquid in the tissues whose succulence favours the development and distribution of bacteria. (Campobasso 2001)
However not much research has been done on this ............
Clothing is also a factor that has a significant impact on the decomposition process as it can slow down post mortem body cooling and favours the onset of the putrefaction process.
Dead bodies are often found wrapped in some material so as to facilitate handling and to prevent discovery.
This slows down decomposition process partially by blocking decomposing bacteria and other organisms from gaining access to the body.
Specific sites of trauma on the body.............
Cause of death.............................
Temperature is a known variable that has a significant impact on the rate of decay of exposed corpses (Mann, Bass and Meadows 1990; Rodriguez and Bass 1983) as it interferes with insect activity and even more with the rate of insect development.
Warmer temperatures increase the number and type of insects found in association with the body, and insect activity produces faster degradation.
A corpse exposed during spring and summer generally has a richer and different fauna compared to another exposed in winter. Therefore, based on entomological evidence the death of advanced decay bodies can be easily related seasonally.
Studies by Rodriguez and Bass (1997) showed that bodies buried in the summer exhibited a greater rate of decomposition than bodies buried in the winter.
The effects of temperature are a crucial concern in the calculation of the post mortem interval, and if it is ignored, an inaccurate value will result.
Humidity and season..................
In warm weather,total skeletonization has been observed to occur in as little as six days (Morton and Lord 2000:156).
Invertebrates in particular fly larvae can consume almost all the soft tissue of a dead body (Lord 1990, Payne 1965).
ESTIMATING THE POST MORTEM INTERVAL (PMI) BASED ON INVERTEBRATES DVELOPMENT RATES
Post mortem interval (PMI) estimation is necessary in every suspicious death investigation in order to reconstruct events and circumstances of death, to link a suspect to the victim, and to ascertain the credibility of statements made by witnesses.
PMI estimations are based on the body decomposition, faunal evidence analysis and the environmental influences (Figure 1). (Hall 1990)
Figure1: A generalized death and decomposition scenario relating to the calculation of post mortem interval (PMI) using the analysis of faunal evidence (modified from Catts 1990).
Insects found associated with corpse are predominantly used as one of the indicators in estimating PMI. When a body is found at a crime scene, the presence or lack of invertebrates can determine time of death and can provide many clues about the cause of death or events just prior to death. However, after three days, insect evidence is often the most accurate and sometimes the only method of determining elapsed time since death. (Anderson 1996)
In determining a post mortem interval, which is defined as the time elapsed between death and discovery, insects provide evidence by two main procedures. The first procedure to PMI estimation requires analyses of the time needed for the insect species to develop to the growth stage encountered at the death scene. The majority of carrion insects rarely deposit offspring on a live person, therefore the age of a larva provides a minimum time since death. The second is the succession patterns of insects found in the body, which has the potential of providing a rough approximation of the PMI in the late post mortem stage. Succession stages are represented by the variety of insect species present at a particular time. This diversity is then compared to known succession pattern for that geographic area.
The time needed for the diversity of species to become as established as they were at the time of discovery represents the estimated post mortem interval. (Eberhardt and Elliot 2008)
In general, determining the stage of immature stages of insects found on a corpse is helpful when death occurred less than a month prior to discovery, and the succession pattern is important when a corpse has been dead for several months. (Greenberg and Kunich 2002)
A great diversity of insects arrive at decomposing human bodies, both to feed and to lay their eggs, but blowflies (Diptera: Calliphoridae) are usually the first group of insects to arrive after death. Therefore, blowflies represent the insects of greatest forensic importance (Arnaldos et al 2005) since they can arrive within few minutes (Payne 1965) or even few seconds (DeJong 1995) following corpse exposure, they have the potential to give the most accurate evidence of the PMI.
Blowfly infestations of human bodies are a natural outcome of the flies' task in the environment as primary decomposers. The larval infestations are an essential component of the natural recycling of organic matter and, on human bodies; they can provide vital evidence to the timing and cause of death.
Adult blowflies are well adapted to sensing and locating the sources of odours of cadivers decomposition, the eggs are usually laid in dark and moist places such as the eyes, mouth and open sores of the body. The eggs then quickly hatch into first instar larvae which feed rapidly on the corpse, and shed their skin twice to pass through second and third instars until they finish feeding, or once the food resource has become unavailable. The larvae wander away from the corpse in order to find a safe place to pupate.
It is important to identify how long it takes for the insect to arrive on a corpse, the stage of decay to which it is attracted, its life cycle and its rate of development. (Turner and Wiltshire 1999, Dadour et al 2001)
LIFE CYCLE OF BLOWFLIES
Blowflies have been most comprehensively studied in relation to their forensic importance as they are usually the first group to colonize a body after death, often within minutes, and are very useful in determining the time of death.
Like other insects, blowflies' exhibit different stages as they develop from egg, larva stages, pupa to adult (Figure 2).
Gravid female flies feed on body secretions, especially around the dark and moist places of the body such as the eyes, mouth, nose, or open wounds and then lay eggs at these sites.
According to Gunn (2009) blowflies colonize corpses that are fresh or at the early stages of decay and although the adult flies may feed on a body that is dried out or skeletonised they would not lay their eggs upon it. Individual females may deposit as many as 300 eggs (Mullen 2009), the eggs then quickly hatch into first instar larvae which feed on both the tissues of the corpse and the microbes that grow on it. The larvae also release enzymes and other substances that help to break down the underlying substrate. The larvae then moult to the second instar; again these feed, grow and moult to the third instar.
After the fully grown third instar larvae stop feeding and show no further response towards food, depending on the species the larvae leave in search of a suitable pupation site. They may move many meters before burrowing into the soil.
The larvae then contracts and the cuticle harden and darken to form the puparium, within which the pupa transforms into an adult fly. When the fly emerges, the empty pupae case is left at the scene as evidence of the blowflies' development.
Figure 2: The blowfly life cycle; showing the different developmental stages
However, many factors affect insect development including temperature, soil moisture content, soil compaction, as well as the effect of pre burial and high density.
Because life cycles are affected by variations in the daily environmental conditions, all the mentioned factors need to be considered when determining the PMI, insects cannot provide an exact time of death, only a close estimate.
FACTORS AFFECTING INVERTEBRATE SUCCESSION
When a corpse is found weeks, months, or more after death, insect evidence is the only method available to determine reliably the time of death. Insects colonize in a predictable sequence, with some species being attracted to the remains very shortly after death; others are attracted during the active decay stage, and still others being attracted to the dried out skin and bones. (Anderson 1995)
When the insects leave the remains, they always leave evidence of their presence behind, as they complete every stage of their life cycle on the remains. (Putman 1983)
However, insect colonization on a corpse is impacted by many variables is dependent on many factors, including geographical region, exposure, season, etc.
One of the most important factors is the geographical location in which the corpse is found. The diversity of insects found on the corpse can vary from region to region. Certain groups will colonize first, such as blow flies and flesh flies, but the species involved will vary.
Times of colonization of insect species and groups also vary greatly with geographical region. In many areas, dermestid beetles (Cleoptera: Dermestidae) are regarded to be very late colonizers, normally arriving when only dried out skin and bones remain, sometimes months after death (Rodriguez and bass 1983).
Decay proceeds much faster in the tropics, where conditions are both hot and humid, and slower in cold or dry conditions.
EFFECTS OF SEASON
In general, corpse deposition during the spring and summer will yield far more active and numerous insect colonizers, while the converse is true of fall and winter (Smith 1986).
EFFECTS OF BURIAL
Burial effectively isolates the corpse from many of the unusual insects, in particular from blowfly species which have a significant effect on the rate of decomposition. Furthermore, many of the insects associated with above ground carrion are restricted from colonizing a buried corpse (Goff 1992). Even a soil layer of just 2.5 cm can significantly delay decomposition, because blowflies don't normally lay their eggs on the soil surface but the corpse itself.
Rodriguez and Bass (1985) reported the following observations on buried human bodies. Bodies buried at 1.2 m for one year showed a good state of preservation with skeletonisation limited to the head, hand and feet. However, bodies buried at 0.6 m for six months showed little decomposition; the genital area was completely decomposed and the body appeared to be brown in colour. Whereas, bodies buried at 0.3 m for 3 months were decomposed, skeletonisation of the arms and legs were complete and small traces of fungi were visible on the trousers.
The results indicate that buried remains are still colonized by insects, but burial influences the time required for insects to reach the remains, the sequence of colonization, the species involved, and the rate of decomposition (Rodriguez and Bass 1985)
EFFECTS OF TEMPERATURE
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In conclusion, since invertebrate carrion feeders perform a valuable recycling of organic matter in the ecosystem, the PMI estimates cannot be separated completely from the examination of corpse decomposition.