Creepy Crawlies
For the files of murder mystery and horror story writers.
Benny was feeling luckier than he had ever felt before as he carefully picked his way through dark woods skirting a long-abandoned logging road. After weeks of effort to gain the trust of a major drug dealer, he was about to meet and make a buy from one of the dealer’s runners. The returns he would get when he distributed the drugs would finance a legitimate business he had been planning to establish for years. Profits to be made in the dope trade could be high, but so could attending risks. Benny was about to learn just how high the risks could be.
He heard a rustle of leaves behind him, but the sound of a gunshot probably had no time to register in his brain as a bullet tore through the back of his skull. A cash-filled briefcase he was carrying hit the ground just before his body did. A gloved hand reached down and retrieved the briefcase. Unhurried feet crunched through dry leaves as they casually made their way toward the road. There was a sound of a car stopping on the road, a door opening and closing and then the fading sound of the car as it drove away. The killing site was quiet. The killer had gone; and, although his body remained, Benny had gone too.
The Sun was just clearing the woods of a thin morning mist when the first insect, a blow fly, arrived at the body. Of the many insects that ultimately appear at such a site, blow flies are usually the first (often within a few minutes of death) and most numerous to arrive and begin colonizing a body. Blow flies have a keen sense of smell and are attracted by odors associated with decay, the odors primarily being the result of actions of bacteria on dead tissues. Blow flies are also the most studied because they usually provide the most accurate information with which to estimate a minimum value for what is known as a post-mortem interval, or PMI. From a minimum PMI, one can estimate a time of death, often referred to on criminal investigation shows as a TOD. Determining a minimum PMI is the primary purpose of forensic entomology. Such information can often be used to narrow a field of murder suspects. The relatively few persons who specialize in studying insects in relation to dead bodies are known as forensic entomologists.
Forensic entomologists are not usually involved in cases that are less than 72 hours old. Prior to that time, other techniques are equal to or more accurate than those involving insects. If a death has occurred more than three days before, however, insects provide the most accurate and often the only method of estimating a minimum PMI. Taking the temperature of a body that has not reached the temperature of its surrounding environment can provide useful information, but its usefulness declines as body temperature drops. Also, since the rate of decreasing body temperature varies from one individual to another, the estimated minimum PMI can vary by as much as two hours.
After about two days, putrefaction begins and body temperature will begin to rise. Chemical reactions of cells in a living body function both aerobically (needing oxygen) and anaerobically (needing no oxygen). Cells that function anaerobically produce lactic acid, which, in a living body, can be reconverted as oxygen is inhaled. In a dead body, of course, this cannot happen; and the lactic acid rises to high levels in muscle cells. Actin and myosin fuse to form a gel, which causes a stiffness known as rigor mortis. It begins about three hours after death, reaches maximum stiffness after about 12 hours, and then gradually dissipates. The latter process is known as resolution of rigor. High environmental temperatures can decrease the reaction time and running or other aerobic exercises before death can produce higher initial levels of lactic acid. In a body submerged in cold water, even after several days, rigor mortis might not begin until the body has been removed from the water. All these factors must be considered when estimating minimum PMI.
During their life time, blowflies pass through four distinct stages. They begin as eggs, of which some 250 might be laid by a fly in body openings such as eyes, noses, mouths and wounds. They then move on to become larvae (maggots), pupae and finally adults. Large numbers of larvae tend to hatch at the same time. They maneuver about a body as a mass, disseminating bacteria and secreting enzymes that enable them to consume nearly all soft tissues of a body.
Time intervals marking the first three of the four stages are relatively predictable, although they vary as a function of factors that include temperature and available food (in the maggot stage). Size and body configuration are the time indicators of maggot stages, known as instars. At a temperature of 70 degrees Fahrenheit (about 22 degrees Celsius), a black blow fly would typically spend 23 hours as an egg, 27 hours as a first instar larva, feeding on the corpse, before molting (shedding its outer layer, or cuticle, to allow growth), 22 hours as a second instar larva before molting, 130 hours as a third instar larva before its outer layer hardens into a puparium, and 143 hours as a pupa before hatching as a fly. The instar stage of a larva can be determined by the number and size of its breathing holes, which are known as spiracles. (Before we take leave of the word spiracles, of interest, but of no particular relevance to the subject, is the method bees use to kill some species of wasps. Rather than stinging them, which often kills the bees themselves, they simply plug their spiracles.)
Surrounding temperatures, and cloud conditions during the periods of insect development must be estimated from records of the nearest weather stations. At warmer temperatures, and with more food available, growth is accelerated. As maggots mass together, their metabolic activity can produce an increase of temperature between 5 and 20 degrees Celsius above that of a surrounding environment. This, of course, represents another variable that must be considered when estimating a minimum PMI.
If there are traces of such toxins as cocaine and heroin in a body, they can increase the growth rate of larvae. If clothing is permeated with such items as oil, paint or fuel, they can slow the colonization rate of insects in a body. Certainly, if a body has been buried, wrapped or refrigerated, that can also slow the growth rate of larvae. Even a lack of insect evidence on a body can be of value. If a body is obviously several days old, and it is summer, the absence can indicate that the body was kept somewhere free of insects until recently.
In addition to the body itself and its clothing, ground beneath it and surrounding it must be examined for insect activity. While in the last instar of majestic maggotry, a full-grown larva eventually stops eating and crawls off, sometimes up to 50 feet, to find a cool, dry place to pupate. It then loosens itself from its outer skin, which hardens (tans) into a hard shell that becomes its puparium. Within the puparium, the now pupa continues to develop until it emerges as an adult. The emerging fly has an interesting system of egress from its puparium. On its head the fly has a sac, known as a ptilinum, that it alternately inflates and deflates to hammer through an end of the puparium.
Other insects also covet dead bodies, each insect having its own cycles to follow. Larvae sometimes have to be reared to adulthood to identify their species. This is important because some insects come at different phases of body decomposition. This is known as insect succession, and it also can provide clues to help estimate minimum PMI. Some insects prefer bodies in sunlight, some in shade, some in open meadows, and some in thickets and dense woods. Different insects are active in different areas in different seasons. All such facts must be considered as potential providers of clues useful in estimating the minimum PMI.
Incidentally, insects can provide useful information concerning more than just PMI. Wounds are sometimes obscured by the effects of decomposition. Since insects usually lay eggs in body openings, activity at a different spot can indicate a possible wound site. In fact insects usually colonize wound sites first. An example is wounds on the palms of hands, which could be indicative of defense wounds.
Since insects also colonize wounds in living persons, they can be used to help estimate when such wounds were inflicted, for example, on an abused child. Maggot presence can also be used to estimate when diapers were last changed on a neglected child or an infirmed adult.
Even if there is insufficient tissue left to determine drug presence, since maggots bioaccumulate, an analysis of their bodies can determine what drugs might have been present in a corpse before death.
If a body has been moved, identifying insects on the body that are native to the original site, where the murder might have occurred, but not to the second, can help locate the original site.
If a killer returns to a murder scene, it often disturbs the insect cycles; and not only might the minimum PMI be determined, but also the date the killer revisited the site.
Insects have also been useful in placing a suspect at a crime scene. A rapist was tied to a rape site using an insect found inside a cocklebur.
As the foregoing attempted to demonstrate, the job of a forensic entomologist involves collecting and evaluating a stupendous amount of information under far less than pleasant conditions.
Two last bits of potentially interesting information: (1) The number of plants pollinated by flies reportedly equals or exceeds the number pollinated by bees (in fact, trees having fruit seeds from which chocolate is ultimately produced are pollinated by flies); and (2) If you open your violin case and find the bow strings ravaged, look for a fly or two. Some flies find horse hair delicious.
