Decomposition: fly life cycle and development times

The presence of insects in a corpse is a critical clue towards estimating the time of death for bodies dead for longer periods of time.

Because flies rapidly discover a body and their development times are predictable under particular environmental conditions, the time of death can be calculated by counting back the days from the state of development of flies living on the corpse.

Eggs

• present in clumps of up to 300
• laying to hatching takes 1 day

Larva - 1st instar

• initially feeds on fluid exuded from the body
• migrates into body
• hatching to first moult takes 1 day

Larva - 2nd instar

• moves around in maggot mass
• first moult to second moult takes 1 day

Larva - 3rd instar

• still moves in mass
• greatly increases in size
• second moult to pre-pupa takes 2 days

Pre-pupa

• migrates away from the corpse seeking a suitable pupation site, (usually in soil)
• does not feed
• transforms into pupa
• pre-pupa to pupa takes 4 days

Pupa

• resides within puparium
• undergoes transformation from larval body form adult fly
• does not feed
• pupa to emergence takes 10 days

Adult fly

• mates on emergence from pupa
• feeds on protein from body fluids
• lays eggs on corpse
• emergence to egg laying takes 2 days

These development times are generalised. They vary depending on the species and the temperature.

The larva, or maggot, is the main feeding stage of the fly. On hatching, first-instar larvae are roughly 2 mm long, growing to about 5 mm before shedding their skin. The second instar larvae grow to around 10 mm before they shed their skins to become third-instar larvae. Third-instar larvae grow to between 15 mm and 20 mm before wandering off as pre-pupae.

Apart from the change in size, the overall form of fly larvae varies little between instars. The most distinctive feature for separating larvae of different instars is the structure of the posterior spiracles, though which the larvae respire.

Some flies produce predatory maggots that feed on other maggots. The predatory maggots of Chrysomya rufifacies are covered with spiny protrusions which deter other predators.

Maggots (fly larvae) are remarkable eating machines. Their front ends are armed with mouth hooks with which they rake in decaying flesh, shredded from the corpse. Their rear ends consist of a chamber, in which their anus and posterior spiracles are located. (They also have anterior spiracles). Spiracles are used for breathing, and the possession of spiracles in a posterior location means that maggots can breath feeding 24 hours a day.

Between their heads and their tails is a muscular, segmented body, a simple intestine and a pair of very large salivary glands. They wriggle easily through a corpse, secreting digestive enzymes and spreading putrefying bacteria which help create their soupy environment.

Maggots are gregarious animals and travel around in 'maggot masses'. Their digestive activities are so intense that the corpse heats up in the vicinity of a maggot mass, sometimes reaching 53 celsius. It can get so hot inside a maggot mass, that centrally located maggots have to migrate to the edge to cool down. However, the heat is a bonus, because it increases the rate of putrefaction, and the rate of digestion.

Maggots moult twice during their development and can grow from 2 mm to 20 mm in length in four days. Having acquired the necessary nutrients to make a fly, they retire into their puparia where the transformation occurs.

A female blowfly lays up to 300 eggs at one time, and with numerous females visiting a corpse, the number of maggots can be immense. For example, 48,562 maggots were found on a 156 g piece of meat after 24 hours exposure. However, because this was insufficient food to sustain them, only 231 flies finally emerged. In warm weather, conducive to fly growth, maggots can consume 60 per cent of a human body in less than a week.

This table shows the approximate development times of some Australian fly species (in hours) at 20°C. The life cycles of Australian flies are poorly known - a great area for further research.