Forensic Entomology is the science of using insect evidence to uncover circumstances of interest to the law, often related to a crime. The time of death can usually be determined using insect evidence gathered from and around a corpse, provided the evidence is properly collected, preserved and analyzed by an appropriately educated forensic entomologist. The broad field of forensic entomology is commonly broken down into three general areas: medicolegal, urban, and stored product pests. The medicolegal section focuses on the criminal component of the legal system and deals with the necrophagous (carrion or dead flesh) feeding insects that typically infest human remains. The urban aspect deals with the insects that affect man and his immediate environment. This area has both criminal and civil components as urban pests may feed on both the living and the dead. The damage caused by their mandibles (mouthparts) as they feed can produce markings and wounds on the skin that may be misinterpreted as prior abuse. Urban pests are of great economic importance and the forensic entomologist may become involved in civil proceedings over monetary damages. Lastly, stored product insects are commonly found in foodstuffs and the forensic entomologist may serve as an expert witness during both criminal and civil proceedings involving food contamination.

     Insects can most importantly reveal the time of death of a person. By determining what stage of development the insects found on the dead body appear in, a time of death can be established. In the insects that undergo complete development, the larval stages appear quite different from the adult form. The larvae of flies that are commonly recovered from decomposing human remains lack functional legs, and the body of many species appears cream colored, soft-bodied, and quite "maggot-like". Once the larva or "maggot" is through feeding it will migrate away from the corpse in order to find a suitable site to form the pupal stage. The pupae of blowflies are often overlooked, as they closely resemble rat droppings or the egg case of cockroaches. The pupal stage is an extremely important stage to the forensic entomologist and a thorough search should be made for the presence of pupae at any death scene. If the adult insect has not emerged, the pupa will appear featureless and rounded on both ends. If the adult insect has emerged, one end will appear as if it has been cut off, and the hollow interior will be revealed. Most adult blowflies appear a metallic green or blue and are easily recognizable.

      Beetles will also appear on a rotting corpse. They are one of the largest groups of animals and they also undergo complete development. Because of their development the larvae appear very different from the adult form. Although the larvae or "maggots" of a large number of blow fly species may look almost identical; the larvae of beetles may look very different from one species to the next. Beetle larvae recovered from corpses can be easily differentiated from maggots as they have three pairs of legs and the maggots found on decomposing remains will not have any legs. Once a larva as been identified as that of a beetle, further field identification can be accomplished because of the wide diversity of larval forms. The bodies of beetle larvae may range from almost white, robust, and hairless to dark brown, slender, and quite hairy. Others may appear almost black and have armored plates on their back. Although the number and appearance of adult beetles that can be found on human remains, it is much too diverse to show even a representative sample. 

     Forensic entomologists are most commonly called upon to determine the postmortem interval or "time since death" in homicide investigations. 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 ante mortem 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.