Also known as medicocriminal entomology, forensic entomology is the use of insect evidence to aid legal investigations. There are roughly 700,000 known species of insect in the world, of which many have been known to play a role in a vast number of criminal and civil cases, as well as automobile accidents. The field can generally be broken down into three main areas. Medicolegal entomology focuses on the Necrophagous feeding insects that infest human remains, which will primarily be covered on this page. Urban entomology is involved with the insects that affect individuals and the immediate environment, such as their role in the likes of traffic accidents. Whereas store product pest entomology focuses on the role of insects in manufactured products, for example food products infested with insects.
Insect Life Cycles
Most insects undergo a complete holometabolous development, passing from egg to larva to pupa and finally adult. However some insects will deposit living larvae instead of eggs.
Being the most prevalent insect at a death scene, the blowfly’s lifecycle has been fairly well documented. The female will lay eggs, sometimes as many as 100, around the orifices and wounds, though fleshflies will immediately deposit living larvae. When the eggs hatch into larvae, usually within 24 hours, they will feed on the dead flesh, growing at a steady rate as they do so. As they grow the larvae moult, reaching the second and then third instar. Instar stage can be determined by the size and number of spiracles (breathing holes). Eventually the larvae reach the pre-pupal stage, where they prepare to moult again, move away from the body and form pupae. The process from egg to pupa takes between one and two weeks, depending on the species. If an adult has not yet emerged, the ends of the pupa will both appear rounded and features. However when the adult fly emerges and leaves, one end will appear as if it is cut off, revealing the hollow interior. After pupating for a number of days, the adult fly will emerge. An adult fly which has just emerged will often display crumpled wings for a while. A newly-emerged fly will usually stay on the ground for the first few days while its body hardens.
Larvae can often be very indistinguishable. Whereas beetle larvae will usually look very different from one species to the next, the larvae of blowflies are all very similar. However it is possible to distinguish between beetle larvae and blowfly larvae, as the larvae of beetles will have 3 pairs of legs, as oppose to none.
Certain factors will affect the rate at which insects infest a body. If the cadaver was frozen for a period of time, the arrival of insects will be severely delayed. Similarly, a buried or wrapped body will also attract insects at a delayed rate. Unusual blood spatter patterns may be explained by insect activity. Insects will frequently move through pools of blood, disturbing the original spatter.
Commonly Found Insects
Acari, or mites, are minute organisms that tend to appear in the soil underneath the body during the later stages of decomposition. They are usually transported to the body by other insects.
Aranea, or spiders, frequent corpses in order to feed on other insects. Their presence usually has no relevance to the post mortem interval. Diptera is the collective group of insects with a pair of wings, there being around 100,000 known species. Insects of this order that commonly visit a corpse include: Trichoceridea, or winter-gnats, whose larvae feed on decaying material. Stratiomydae, whose larvae will also feed on decaying matter later in the decomposition process. Phoridae, a large fly family. Syrphidae, also known as hover flies, whose larvae usually occurs in filthy water but sometime a dead body. Sepsidae flies, whose larvae feed on decaying flesh during the caseic fermentation period. Piohilidae, also known as the Cheese-skipper, have larvae that have been known to jump up when disturbed. Drosphilidae, or fruit flies, often appear at the scene of a corpse to primarily feed on fungi. Calliphoridae are the common bluebottles and greenbottles.
Numerous beetles may also infest the site of a cadaver, occurring at different stages of decomposition. Staphylinidae, or rove beetles, may arrive just hours after death and will often remain throughout the entire decomposition process. Dermestidae beetles are usually found in the later stages of decomposition. Histeridae beetles are usually discovered during the bloated stage of decomposition and the early parts of the dry stage.
Other insects will also appear, though perhaps less commonly. Clothes-moths may visit the scene to feed on the hair of the deceased during their larval stages. Some bees, wasps and beetles will appear to feed on carrion feeding insects.
However the species of insects present is dependent on the part of the world, the time of year, and general conditions of the environment.
Use of Insects
In criminal investigations, insect evidence has been most commonly employed when determining the post mortem interval of a victim. A wide variety of insects are attracted to the scene of a corpse, where they will move through their life cycles at a fairly predictable rate. The presence of specific insects alone can indicate a rough time of death estimate, as different species arrive at the scene of a body at different intervals. Some insects will prefer a fresh corpse, whereas others will arrive at a later stage of decomposition.
During the first three days after death the proteins and carbohydrates in the body will begin to break down. In this time blowflies will be the first to arrive at the scene to lay their eggs, usually in the orifices of the body. Over the next three days the body begins to decay further and the abdomen inflates. The eggs previously laid will begin to hatch, releasing larvae, and some species of beetle will arrive. Between days 8 and 18 more insects join the scene, including numerous flies, beetles, ants and cockroaches. Between days 19 and 30 the body enters the post-decay stage, eventually attracting further beetles and mites. As the body is drying out after this, dermestids, tineids, and mites will be the dominant insects present. As insects are cold-blooded and their temperature is dependent on the environment, their metabolic rate will be severely affected by the ambient temperature.
Insects at the scene may have toxicological benefits. The body may reach a stage at which it is not possible to extract any useful biological samples. In this case, any maggots present may be analysed in order to examine any human tissue they may have ingested. Insect evidence can be used to determine if a body has been moved, as certain species of insect are endemic, being unique to specific areas. For example, a body found in an urban environment would only hold rural insects if the corpse had been in a rural environment previously. This would suggest someone had moved it. Disturbances to insects’ life styles are also an indication of some post-mortem interruption.
Entomology has played a role in cases of child or senior abuse and neglect. Insects have been known to colonise wounds or unclean areas of the body, known as cutaneous myiasis. Insect evidence has also been known to help in the investigation of illegally imported goods and drugs. If insect specimens are found in such products, their native origins may help establish where the goods came from.
The presence of insects may also hinder an investigation. Many roach species have extremely strong mandibles that can produce post-mortem damage that will closely resemble abrasions or chemical burns. This damage must not be mistaken for injuries sustained prior to death.
Any crime scene involving insect evidence will be treated with the standard protocols, though additional information may be gathered. Environmental conditions are of particular importance at a scene involving insect evidence, including whether conditions, the location of the body, sun and shade conditions, open doors or windows, and temperatures of the air, ground, and between the body and ground.
Adult flies and beetles are the first to be collected, as they are fast moving and can quickly vacate the scene when disturbed. Collected flies are often immobilised with alcohol, whereas beetles tend to be placed straight in vials. Larvae are often stored in a similar fashion, though some may be boiled for preservation. A collection of larvae should be immediately preserved, in order for the entomologist to study them in the state they were found. However a selection should also be kept alive and allowed to develop in order to discover their species. Pupae should not be preserved in any alcohol, as they will then not develop any further. Any beetled recovered should be stored in individual containers, as they are cannibals. Live collected insects should be given a small amount of beef for them to feed on during transit.