Rough Guide to Forensic Entomology

This title may be more confusing to some of you, to be honest the first time I encountered the word 'entomology' was in the webcomic XKCD (which I 100% recommend to anyone with a healthy interest in any area of science). Entomology is the study of insects, and so forensic entomology is the study of insects in the context of criminal science. The most common application of entomology in forensics and the one which I'm going to be discussing in this article is determining how old a corpse is by the insects on or in it, but will also touch on the fact insects can be used to determine whether the body has been moved after death.

Forensic entomology is most useful on corpses which have been discovered soon after death. Knowing your insects and their life cycles is vital for this. Within minutes of death the body will be colonised by the true flies (Diptera), specifically flesh flies (Sacrophagidae) and blow flies (Calliphoridae). They lay egg and deposit larvae extremely quickly in the orifices of the corpse, or in the wounds. Blow flies are generally more useful to the forensic pathologist as they have more stages in their development, laying eggs whereas the flesh flies deposit live young on a corpse.

The life cycle of blow flies can be used to determine rough amount of time the corpse has been there, this is a relatively simple process for anyone with a knowledge of entomology as blowfly stages of development are pretty distinct:

Blow flies are surprisingly pretty insects

1. Eggs: Look for tiny, yellowish eggs. The presence of nothing but eggs shows that the corpse has been there for around 23 hours or less. 

2. Instar 1 : Small larvae, look for tiny (around 5mm) fleshy covered maggots. The presence of only extremely small larvae (and maybe eggs) will show the corpse has been there for 23 - 50. 

3. Instar 2 : Medium sized larvae, exactly the same as the instar 1 stage but significantly bigger (around 10mm). The presence of no larvae bigger than instar 2 larvae shows the corpse has been there for 50 - 77 hours. 

4. Instar 3: Anatomically exactly the same as instar 2 but significantly larger (around 15mm). The presence of instar 3 larvae but no pupa indicates the corpse is 77 - 207 hours old. 

The larvae look anatomically identical at any stage, it's the size that counts!

5. Pupa: More difficult to find and sometimes found not on but in the vicinity of the corpse. Pupa resemble small dark red husks. The presence of pupa indicates that the corpse 207 - 350 hours old.

Pupa are more difficult to find, especially around the corpse as they are often mistaken for animal droppings.

 After about two weeks (the end of the pupa stage) the copse will probably be stripped to skin, bone and cartilage and the flies will move on. Thus the presence of no blow flies in any stage of development shows that the corpse is older than two weeks. 

This is a very easy to apply method of estimating the age of a corpse however you must take into account environmental factors in your estimations. All of these time measurements are given for 23ºC , so higher temperatures will mean your times must be adjusted to be lower and colder will increase the length of time.

Insects also provide evidence of whether the body has been moved, for example the insects found in urban areas differ greatly from those found in rural areas. This difference can be used to find out for example if a body has been killed in a city and then dumped in a hedge in a rural area.

Forensic Entomology's uses even stretch to toxicology. corpses which have decayed to the point where they have lost most of the tissue are extremely difficult to test for poisons (almost all non-radio active poisons affect major internal organs, nerves and tissues but not the bones). However, as the larvae feed on the dead tissue (technical term is necrophagous) they mimic almost precisely the concentrations of toxins in the body of the human as they decayed, meaning a sample of larvae is a decent substitute for a toxicologist to analyse.

That concludes the quick guide, all in all entomology is as useful as you make, the more you learn about specific life cycles and geographical locations of specific insects will let you gather more, more accurate data on corpses. However with just this information you can make a quick estimate of how long a corpse has been there, a great trick for any aspiring detective.