Environmental effects on forensic entomology

Forensic entomology, more specifically its medicocriminal branch, has evolved since the first days of its implementation. Yet, the field is still growing and daily, new information and techniques are being attempted to help entomologists more accurately report vital information such as post mortem intervals. There is a standard accepted theory that outlines eight distinct faunal successions, which will be eluded to later in the article. However, what happens when during decomposition the maggot mass found on the body differs from what should be expected? It is the forensic entomologist’s job to determine the level of difference from the norm and to scientifically account for the deviation.

Arguably the most important reason behind medicocriminal entomology is to accurately determine a post mortem interval. PMIs, in conjunction with insect behavior, serve as the bridge to connect the work done by forensic entomologists and detectives. Below is an attempt to shed light on the various factors that could contibute to an innacurracy in degree days or an abnormal colonization of a maggot mass.

Climate

Moisture levels

Rain and humidity levels in the area where the body is found can affect the time for insect development. In most species, large amounts of rain will indirectly cause slower development due to drop in temperature. Light rain or a very humid environment, by acting as an insulator, will permit a greater core temperature within the maggot mass, resulting in faster development.

Bodies of Water

M. Lee Goff, a noted and well respected forensic entomologist, was assigned to a case involving the discovery of a decomposing body found on a boat half a mile from shore. Upon collection of the maggot mass, only one insect, Chrysomya megacephala, was discovered. He concluded that the water barrier accounted for the scarcity of other flies. He also noted that flies will not attempt to trek across large bodies of water unless there is a substantially influential attractant.

Sun exposure

"Because insects are cold-blooded animals, their rate of development is more or less dependent on ambient temperature." Bodies exposed to large amounts of sunlight will heat up, giving the insects a warmer area to develop, reducing their development time. In one experiment conducted by Bernard Greenberg and John Charles Kunich with the use of rabbit carcasses to study accumulation of degree days found that with temperature ranging in the mid 70s to high 80s the amount of deveopmental time for maggots was signifigantly reduced.

In contrast, bodies found in shaded areas will be cooler, and insects will require longer growth periods. In addition, if temperatures reach extreme levels of cold insects instinctively know to prolong their development time in order to hatch into a more accepting, viable climate in order to increase the chance of survival and reproduction.

Air Exposure

Hanged bodies can be expected to show their own quantity and variety of flies. Also, the amount of time flies will stay on a hanged body will vary in comparison to one found on the ground. A hung body is more exposed to air and thus will dry out faster leaving less food source for the maggots.

As the body begins to decompose, a compilation of fluids will leak to the ground. In this area most of the expected fauna can be found. Also, it is more likely that rove beetles and other non-flying forensically important insects will be found here instead of directly on the body. Fly maggots initially deposited on the body may also be found below however. As they travel from their feeding site to digest their meal they fall to the ground because the body is suspended and they have nothing to cling to.

Climate measurement

Important concepts in forensic science are threshold temperature and degree days.

• For each type of insect, development will not occur unless the surrounding temperature is above the minimum threshold for that insect.
• A given amount of thermal energy is needed to develop from one stage of an insect life cycle, such as egg laying or egg hatching, to the next. This is measured as degree days. The degree days in a specific location over a specific period is calculated as the mean of the minimum and maximum temperature, minus the threshold temperature, mulitplied by the number of days.

Observations of the life stage of the insects present, in combination with the calculated degree days, can be used to estimate the post mortem interval.

Geographic distribution

According to Jean Pierre Mégnin's book La Faune des Cadavres there are eight distinct faunal successions attracted to a corpse. While most beetles and flies of forensic importance can be found worldwide, a portion of them are limited to a specific range of habitats. It is forensically important to know the geographical distribution of these insects is order to determine information such as post mortem interval or whether a body has been moved from its original place of death.

Calliphoridae is arguably the most important family concerning forensic entomology given that they are the first to arrive on the corpse. The family’s habitat ranges into the southern portion of the United States. However, while Chrysonoma rufifaces, the hairy maggot blow fly, is part of the Calliphoridae family and is widespread, it is not prevalent in the Southern California, Arizona, New Mexico, Louisiana, Florida, or Illinois regions.

Flesh flies fall under the family Sacrophagidae and generally arrive to a corpse following Calliphoridae. However, as previously mentioned they are capable of flying in the rain. This key advantage enables them to occasionally reach a body before Calliphoridae overall effecting the maggot mass that will be discovered. Flesh flies are globally distributed including habitats in the United States, Europe, Asia, and the Middle East.

Beetles are representative of the order Coleoptera which accounts for the largest of the insect orders. Beetles are very adaptive and can be found in almost all environments with the exception of Antarctica and high mountainous regions. The most diverse beetle fauna can be found in the tropics. In addition, beetles are less submissive to temperatures. Thus, if a carcass has been found in cold temperatures, the beetle will be prevalent over Calliphoridae.

External links

• Forensic entomology - use of insects to help solve crime
• A Forensic Entomology Case from the Amazon Rain Forest of Brazil
• Run models and calculate degree-days
• Growing Degree Days
• Grounds Maintenance magazine