9.2: Post-Mortem Changes in Body Condition

No discussion of forensic entomology would be complete without first discussing the postmortem (after death) changes that occur to the condition of a body following death. Death, decomposition, and insect activity are entwined in a complete cycle of life and death. The forensic science student often finds this topic the most difficult with which to cope. It is best to think of the body of one who is no longer living as an empty shell or an item of evidence. There are three processes in the human condition: maturation, declination, and decomposition. Maturation is the process of human growth from infancy to adulthood. This involves physical, mental, and hormonal changes that begin at birth and progress through puberty and end in mature adulthood. Sadly, there is a period of declination when cognitive and physical abilities begin to be impaired or lost. The end result, however, is decomposition. Decomposition is the breaking down of the body owing to the degeneration of the cells and organs after death and the growth of bacteria. Although the rate of decomposition may vary owing to such variables as environmental conditions, obesity or emaciation, light or heavy clothing, or the surface upon which the body lay, the means of decomposition remains the same and involves two processes: autolysis and putrefaction. Shortly after death, the cells in the body begin to breakdown because of the absence of oxygen and the end of blood circulation. During this stage of decomposition, enzymes within the cells are released. These enzymes begin what can only be described as digestion. This process is called autolysis. As the cells and organs break down, the beneficial bacteria that is already present in the body and have been starved of the nutrients upon which they have become accustomed to but were deprived following the death of the host, become overwhelmed with a new source of food: the body. As the bacteria feast on the available food source, especially in the digestive system (bacterial flora), they release gas biproducts that infiltrate into the body tissues and organs. This begins the putrefaction process. The gases produce bloating of the skin; hydrogen sulfide invades the blood vessels making the veins and arteries making them appear darker and more pronounced called marbling; and the skin blisters and peels as skin slippage. Once this occurs, the gases are released from the body, in particular the foul-smelling diamines cadaverine and putrescine, into the atmosphere. It is the release of the sulfur-based and ammonium-laced chemicals associated with decomposition that attract the attention of insects and beetles. Among these insects and beetles are those that are only interested in feeding on the body; those that are feeding on the body and other insects and beetles; and spiders, isopods, and hexapods who use the body for shelter and an expansion of their environment. The species that are most important to forensic entomology are those that feed on the body and those that feed on the body and other insects and beetles. These are the flies (Diptera) and beetles (Coleoptera).

Diptera

The most common species of Diptera are the blow flies (Calliphoridae), common house flies (Muscidae), and flesh flies (Sacrophagidae). These flies may be the first to arrive on the scene following a death; especially if it associated with a blood-letting event. It is estimated that the common house fly can smell blood at a distance of up to four miles. Although the blow flies and flesh flies waste little time in joining the feast, house flies tend to wait for the bloating stage of decomposition to begin their egg-laying. That egg-laying is the beginning of the colonization period of the fly. The colonization will be an aggregation of eggs laid in one area of the body. Areas of trauma, natural openings of the body, hair, or folds of skin are prime locations for egg-laying. It is important to know the eggs will be deposited near the sight of trauma, but not within the wound itself as the eggs need to breathe. The preferred areas of egg-laying on the body that are the first to be colonized are the nose, eyes, ears, and mouth. After mating, the female fly will lay batches between 75 and 150 eggs. This egg-laying process is known as oviposition. Each egg measures approximately 1/16th of an inch (1.2mm) and is yellowish white in color. Depending on temperature and humidity, it takes an average of 12 to 20 hours for the egg to hatch into a larvae (maggots). The larval stage of development consists of a period of eating and growing that can take several weeks to a month. Maggots are small, legless and worm-like, will measure between 1/3rd and 1/10th of an inch (3mm to 12mm), and are brownish white in color. During this period, the maggots will shed their skin (molt) several times. Colonization during this period will always be from the head of the decomposing body downward, or from the area of trauma downward. Clothing does not interfere with or alter this distribution. Maggots cannot penetrate skin, so there must be an opening in the skin or an area of trauma. It is possible to extract human DNA from maggots during the larval stage, which is very important to forensic scientists. Maggots can also be a good source to determine if the victim had drugs or toxins in their system; however, insect toxicology cannot determine if the victim had over-dosed. This examination is referred to as entomotoxicology. Once the maggot has fully developed, it will move to a drier part of the decomposed body and will pupate. As the maggot pulls itself out of its current shell into a pupa, it will purge liquids that will stain the body and destroy nearby plants. This stage of development is often called the “wandering stage” (also migratory or post-feeding) as just before they pupate, the maggots will move towards light, whereas in the larval stage, the maggot is repulsed by light. This wandering allows the maggot to find an area that is away from predators and close to soil where they can instantly burrow to facilitate the transformation into pupal form. The pupal stage of development is the transformational period for the maggot to develop into an adult fly. This period lasts between a few days and a week depending on the temperature and environment. The transformed adult fly will exit the pupal shell, which will remain in the environment and will look like a small greyish brown to brown ribbed capsule. This entire cycle takes between seven to ten days depending on the environment and climate. If sufficient food source from the decaying body remains, the cycle will be repeated. It is therefore necessary to collect a large enough sample of the oviposition detritus. This will be discussed later in this chapter.

Figure \(\PageIndex{1}\): Life cycle of the common house fly. – Wikimedia

Coleoptera

Although ants would appear to be the most prolific insect in the world, the beetles are definitely on their heels. It is estimated that there are between 400,000 and 600,000 species of beetles on the planet, perhaps even more that are yet to be identified. The main varieties of beetles are the rove beetles (Staphylinidae), scarab (Scarabaeidae), clown beetles (Histeridae), carrion beetles (Silphidae), ground beetles (Carabidae), checkered or bone beetles (Cleridae), and skin beetles (Dermestidae). For the purpose of this chapter, we will discuss only skin beetles and bone beetles. Skin beetles are small, oval shaped beetles measuring less than ½” (12mm). In fact, the larvae of skin beetles can be longer than the actual beetle itself at 15mm. They are generally grey or greyish brown in color and may be covered in hairs or scales. They voraciously consume decaying flesh and can clean the skin of a decomposing human body within a week or less depending on the size of the colony and the environment, leaving only skeletal remains. The female skin beetle can lay up to 150 eggs at one time. The eggs are tiny (.02” or .5mm), white or cream in color, and are characteristic by small spine-like projections and ridge-like lines that resemble a skeletal back bone. The eggs usually take one or two weeks to hatch into the larvae. The larvae are elongated and taper to one end, rust or dark brown in color, and are hairy. There are three instar phases of larval development, each characterized by a growth in size. The larval stage of development lasts between five and six weeks. The pupa is slightly smaller than the larvae, is white to light brown in color in the early stages and will appear darker once the encapsulated beetle begins to take adult form. Depending on the species of skin beetle, the pupa will take approximately one week to develop into an adult beetle and shed its capsule. The chewing pattern of the skin beetle is very unique with the edges of the holes that are created having jagged or uneven edges. Bone beetles are attracted to the latter-stage of the decomposing body when fly larvae provide a good food source, as well as the dry remains of the body such as hair and skin. Bone beetles are active in cold weather when fly activity is diminished. Bone beetles can feed on the dry remains of the body long after other insects have departed. Therefore, they are useful to forensic entomologists in cases involving mummified bodies and skeletal remains.

Figure \(\PageIndex{2}\): The life cycle of a beetle. Courtesy Bugboy52.40 Wikimedia

Frass is the term used for the fecal matter of beetles and other insects. It resembles sawdust, but can be white, off-white, or brown in color. Frass is a good source of victim DNA if collected and analyzed within two weeks following death.

Figure \(\PageIndex{3}\): Beetle Frass - Courtesy Jim Tassano, Foothill Sierra Pest Control

Other Considerations

Plastic sheeting covering the body will produce a barrier against flying insects; however, once the plastic begins to deteriorate the flying insects will enter through the gaps. As previously mentioned, cold weather will slow fly colonization but not stop it. In snowy conditions, flies may still lay eggs, but the eggs will likely overwinter until the weather gets warmer. Alternatively, temperatures in excess of 110-degrees Fahrenheit are lethal to flies. Therefore, in the event of a burned body, the flies will not colonize the body until temperatures have dropped below that which is lethal. Ants will inhibit fly activity as they like to feed on fly eggs and maggots. Ants will also feed on the skin of the decedent, producing small, cluster patterned holes. Scavenging by ants may also resemble cigarette or acid burn marks. Ants deposit formic acid as they chew; although this will not cause a reaction on the swelling or reddening reaction common to ant bites on persons who are still living. Ants typically do not eat hair because they cannot digest the shaft. They also tend to feed on the boney prominences of the body. Roaches will do the same thing except they do eat hair, eyebrows, eyelashes, and hairs in the pubic region. In fact, a forensic entomologist can tell the species of roach by examining the cut in the shaft of the hair. Silverfish (Lepisma saccharina) are common to North America, Europe, China, and even the Hawaiian Islands. They tend to lay their eggs in dark areas like the folds of skin on a decomposing body. The food source for the silverfish will include the clothing of the victim as well as the skin and the bodies of dead insects. Therefore, the presence of silverfish on and near the body should be documented.

Insect colonization may occur in the anal or genital areas of victims of sexual trauma or assault. Therefore, an inspection of these areas should be made regardless of the assault or trauma being obvious or apparent.