The Decomposition Process

Decomposition commences almost immediately after death and it is believed that the semiochemicals utilised by carrion insects are produced at the onset of decomposition (Vass et al. 1992; Dix and Graham 2000; Vass et al. 2002; Dent et al. 2004). The cells begin to die and enzymes digest the cells from the inside out, a process called autolysis or self-digestion. This action causes the cells to rupture and release nutrient-rich fluid (Dix and Graham 2000; Vass 2001). Tissues containing more digestive enzymes, such as the liver, are digested at a faster rate than those containing fewer enzymes. Putrefaction occurs as the bacteria, already present in the large intestine, destroy the soft tissues resulting in the production of liquids and various gasses (hydrogen sulfide, carbon dioxide, methane, ammonia, sulfur dioxide, and hydrogen) (Vass 2001). These bacteria gain access to the vascular system and spread throughout the body. There is often a green discolouration associated with these changes (Williams et al. 2001). As the blood begins to break down within the blood vessels and the skin loses pigmentation, the dark stained blood vessels can be observed through the skin producing an effect called marbling. The outer layers of skin begin to slip off the body while fluid under the slipping skin form blisters. Trapped gasses cause the body to become 'bloated'. The body swells, primarily within the abdomen, and decomposed blood or faecal matter may be 'purged' from the lungs, airways, or rectum. Once the trapped gasses have escaped, a more active stage of decomposition can be observed. Volatile compounds derived from the decomposition of materials, such as proteins and fats, are subsequently produced (Vass et al. 2002; Dent et al. 2004). The greatest physical changes to the cadaver occur at this time. The organs degrade and become unrecognisable forming a grey exudate within and beneath the body.

The odour associated with putrefaction is mainly caused by the release of sulfur-containing compounds and various inorganic gases that are produced in the bowel. However, bacteria, fungi, protozoa and even insects aid in the breakdown of soft tissues of the body during putrefaction and this results in the production of various gases including CO2, H2S, CH4, NH3,SO2, H2 and a variety of volatile organic compounds (Statheropoulos et al. 2005). Following this active decomposition the body may become skeletonised leaving behind just dry leathery skin and bones, depending on the environment in which the body has been resting (Clark et al. 1997; Dix and Graham 2000). The body can go through several phases and rates of decomposition and these are highly dependent on weather, temperature, humidity, and the environment, i.e. indoors, outdoors, buried, under water, wrapped or concealed (Clark et al. 1997; Dix and Graham 2000).

In the initial stages of decomposition, there are no visual or odour effects obvious to humans at this time, however, some insects are able to detect the decomposition immediately (Anderson 2001). Blowflies are most often the first insects to oviposit on a carcass and it is likely that a number of factors initially attract these insects to the body, including volatile semiochemicals. Throughout decomposition, bodies constantly change and emit hundreds of chemicals (Vass et al. 2002). It is currently unclear which semiochemicals are detected by the different carrion insects that are found on a decomposing body at the different stages of decomposition. However, chemical ecology research is unravelling this complex interaction between insects and decomposing corpses. This could potentially be exploited to develop a more accurate time of death, alongside the insect identifications relating to succession.

For the purposes of forensic entomology, the process of decomposition is divided into five visually distinct stages. These are the fresh, bloated, active decay, advanced decay, and dry stages, originally described by Payne (1965) and Anderson and VanLaerhoven (1996). The descriptions of these stages are based on physical condition, odour, and at times varying insect activity (Payne 1965). The "fresh" stage refers to the period immediately after death and continues until the body is bloated. Chemical breakdown occurs during this stage; however, few morphological changes are observed. There is no obvious odour to humans. The "bloated" stage becomes evident when an accumulation of gasses from the activity of anaerobic bacteria produce a swollen, bloated appearance. There is an obvious odour present at this time. The "active decay" stage is recognisable by the deflation of the carcass due to the gases escaping from the body, often due to the insect activity occurring on the body. There is a very strong putrid odour that can be detected -this is when the strongest odours are detected. During the "advanced decay" stage, a large amount of the flesh has been removed; however, there is still some moist tissue present. The odour is less obtrusive than in the previous stage, but it is still quite noticeable. The "dry" stage, also at times refered to as "skeletonisation", has been reached when the carcass has been reduced to bones, cartilage, and dry skin. At this stage, only a slight odour is present.

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