What information can a forensic entomologist provide at the death scene?

Forensic entomologists are commonly called upon to determine some aspect of the postmortem interval or “time since death” in homicide investigations.  More specifically, the forensic entomologist may estimate a portion of the postmortem interval based on the age of the insect present.  This entomological based estimation is most commonly called the “Time Since Colonization”.  Based on the factors in a particular investigation, this may, or may not, closely approximate the entire postmortem interval.  In other cases, the forensic entomologist may take into consideration insect succession and provide a time estimation that would be termed the “period of insect activity”.  In many cases, entomological evidence may assist in providing information on portions of the postmortem interval utilizing one of several methods.

The forensic entomologist can use a number of different techniques including species succession, larval weight, larval length, and a more technical method known as the accumulated degree hour technique which can be very precise if the necessary data is available.  A qualified forensic entomologist can also make inferences as to possible postmortem movement of a corpse.  Some flies prefer specific habitats such as a distinct preference for laying their eggs in an outdoor or indoor environment.   Flies can also exhibit preferences for carcasses in shade or sunlit conditions of the outdoor environment.  Therefore, a corpse that is recovered indoors with the eggs or larvae of flies that typically inhabit sunny outdoor locations would indicate that someone returned to the scene of the crime to move and attempt to conceal the body.

Similarly, freezing or wrapping of the body may be indicated by an altered species succession of insects on the body.  Anything that may have prevented the insects from laying eggs in their normal time frame will alter both the sequence of species and their typical colonization time.  This alteration of the normal insect succession and fauna should be noticeable to the forensic entomologists if they are familiar with what would normally be recovered from a body in a particular environmental habitat or geographical location.  The complete absence of insects would suggest clues as to the sequence of postmortem events as the body was probably either frozen, sealed in a tightly closed container, or buried very deeply.

Entomological evidence can also help determine the circumstances of abuse and rape. Victims that are incapacitated (bound, drugged, or otherwise helpless) often have associated fecal and urine soaked clothes or bed dressings.  Such material will attract certain species of flies that otherwise would not be recovered.  Their presence can yield many clues to both antemortem and postmortem circumstances of the crime.  Currently, it is now possible to use DNA technology not only to help determine insect species, but to recover and identify the blood meals taken by blood feeding insects.  The DNA of human blood can be recovered from the digestive tract of an insect that has fed on an individual.  The presence of their DNA within the insect can place suspects at a known location within a definable period of time and recovery of the victims’ blood can also create a link between perpetrator and suspect.

The insects recovered from decomposing human remains can be a valuable tool for toxicological analysis.  The voracious appetite of the insects on corpses can quickly skeletonize the remains.  In a short period of time the fluids (blood and urine) and soft tissues needed for toxicological analysis disappear.  However, it is possible to recover the insect larvae and run standard toxicological analyses on them as you would human tissue.  Toxicological analysis can be successful on insect larvae because their tissues assimilate drugs and toxins that accumulated in human tissue prior to death.