We're not up to our knees in carcases. You may have already noticed this but I thought I'd mention it anyway. When one animal dies, it doesn't take long before others dispose of it. And in most cases, they don't care whether it's a fish, bird or human. It's all food.
Although large scavengers* can make a mess of a corpse, the small ones are the most efficient at waste disposal. Within moments of death, the first wave of flies moves in. From then on invertebrate carrion-feeders arrive in a predictable sequence (succession). An entomologist familiar with this sequence and with knowledge of insect development rates can estimate with accuracy the time since death.
Flies are eager arrivals. Calliphorids materialise from nowhere**. These are the same dull blue and metallic green species that make a nuisance of themselves at summer barbecues. What they're looking for in a corpse is a suitable site for a nursery. They lay their eggs in the dampest spots. When the maggots hatch, the food is already there.
The second group of flies includes the sarcophagids. These grey-striped and chequered insects don't lay eggs but produce lively larvae. Because the maggots don't hang around in eggs, they can soon catch up with the calliphorids that arrived earlier.
Among the flies that turn up much later are the tiny Phoridae and Piophilidae. Both have larvae that feed on seriously decayed material. Several species of phorid are known as coffin-flies—for obvious reasons. It's slightly less obvious why piophilids are called cheese-skippers. You need to see the maggots at work before the common name makes sense. When disturbed, they curl up, grabbing their ... ahem ...anal papillae with their mouthparts. And then they let go, springing several centimetres into the air. Boo!
Succession gives a rough idea of the time since death. But entomologists can make much more accurate estimations by looking at maggot growth rates. The growth rate of maggots (and other insect larvae) depends on temperature—the higher the temperature, the faster the growth. If an entomologist knows the site temperature and the species of fly, it's a relatively simple matter to measure the maggot and calculate its age. That gives the minimum post mortem interval. (Growth rate at different temperatures is represented in a graph called a isomegalendiagram.)
What's not so simple is identifying the species in the first place. Young maggots are particularly tricky to identify, so entomologists often rear them through to adulthood. (A friend did a lovely small-scale study on arthropod succession in North Queensland. Deidre*** set up thirty samples and collected all the insects from one sample each day for a month. To make sure she could put the right name on the flies, Deidre reared the maggots in the insect room. All of them. Towards the end of the study, the insect room became quite unnerving ...)
That's not the end of the necessary entomological expertise. Other factors to be considered are the geographic locality, habitat, weather and time of day. Some flies are fussy about shade or rain. Most don't fly at night. And so it goes on. Unfortunately relatively little is known about Australian fly ecology and behaviour, especially in areas away from the major cities. And there just aren't enough dipterologists to go around.
*By large, I don't just mean vultures, hyenas and wild dogs. Anything with a maximum of four legs is large by the standards of this blog.
**Not literally. We're no fans of spontaneous generation.
Some favourites from my library:
Erzinçlioğlu, Z. (2002) Maggots, murder, and men. Thomas Dunne.
Goff, M.L. (2000). A fly for the prosecution. Harvard University Press.
Smith, K.G.V. (1986). A manual of forensic entomology. British Museum (Natural History.)