An introduction to Forensic Entomology


In its broadest meaning Forensic Entomology refers to any matter in which insect evidence is used in legal matters, this can be anything from authenticating the presence of woodworm in an insurance claim to using the insects present on a dead body to establish the time since death or post mortem interval (P.M.I), and it is this last example which is most commonly considered "Forensic Entomology".

Forensic Entomology is, first and foremost, a branch of applied entomology, and as such a core understanding of Insect taxonomy and ecology must be obtained before such knowledge can be applied to legal matters, to quote the fictional detective Sherlock Holmes in the adventure of the Copper Beeches "Crime is common. Logic is rare. Therefore it is upon the logic rather than upon the crime that you should dwell"!

The moment of death, is when viewed somewhat dispassionately the start of the decomposition process, the lack of oxygen means that the individual cells of the body begin to expire, for example Brain cells begin to die within 3-7 minutes. The body ceases to generate metabolic heat and begins to cool down to equal that of surrounding temperatures, the lack of pulse causes the blood to settle in the body and the skin discolors. Also the body muscles contract in the phenomena known as Rigor mortis. Also the Bacteria contained with in the gut along with digestive enzymes begin the break down internal organs, the gases produced by the bacteria results in bloating of the body.

However a body does not normally decompose by itself. Various invertebrates specialize in the decomposition of animal carcasses. On a warm day Blowflies (Diptera: Calliphoridae) will appear on a body almost instantly, these flies lay their eggs around the natural orifices of a body, as newly hatched maggots are unable to pierce flesh and thus hatch close to the softer tissue of the orifices. Other primary colonizers of carrion include the fleshflies (Diptera: Sarcophagidae) and the Houseflies and there closest relatives (Diptera: Muscidae) these species and other species which feed upon and parasitise them occupy the carrion from fresh to odor development and beyond. Within days, Ground beetles (Carabidae) and Rove beetles (Staphylinidae) as well as Wasps (Vespidae) will be noted on around and under carrion, weather permitting. As body breaks down by bacterial means as well as by insect activity and odour of decay develops the fats of the carrion becoming rancid, and then developing a cheesy smell, this brings the Skipper flies (Piophilidae) Fruit flies and others to the carrion. The body begins to dry and becomes the domain of the beetles, the mites and the moths before becoming completely skeletonised.

How ever there are many things which can affect this progression of nutrient recycling. For example the size of the carrion, personal experience has shown that rat sized carrion does not normally last past the primary colonizers due to its small size, also on smaller carrion Burying Beetles (Silphidae) will arrive at much the same time as the primary colonizers and bury the carrion as part of their life cycle (see Putman 1983, for details on the life cycle of Burying Beetles), whereas Smith (1973) notes that on Human Corpses they arrive approximately a year in to decomposition.

Season and climate also plays an important part in the decomposition of carrion and corpses, insect metabolism is dependent on the external temperature, so much so that correct determination of P.M.I. depends on obtaining accurate climate data at the crime scene, and some of the predatory insects on carrion have a yearly life cycle, so may or may not be present purely if it's the wrong time of year rather than for some other more sinister reason.

Knowledge of the ecology of these insects and the ones which succeed them on a corpse can be used to determine how long the corpse has been dead for.