The number of stages determined depends on the size of the cadaver, and the time of year in which the body is exposed.

The following stages are found:
1. Fresh
2. Bloated
3. Decay
4. And lastly remains.

The fresh stage lasts up until bloating from anaerobic bacteria occurs. This is dependent on the conditions in which the body is present.

Usually Bloat starts at greater than 48 hours after death. Autolysis, or cell breakdown, accompanies this bloating stage.

Decay is the putrefactive stage, involving anaerobic bacteria, and can be subdivided into early, mid and advanced. Flies will lay eggs into the body openings and at wound sites if they are exposed and environmental conditions are suitable for fly activity. Maggots will consume the flesh and disseminate bacteria throughout the body. After the majority of the flesh is removed, the dry remains stage begins. The remaining dried flesh, skin and tendons will be removed by certain families of beetles and moths. The entire process of decay will depend on the environment, and may not occur at all if flies are excluded from the body. In this case mummification and adipocere, a waxy substance created from the fat layer in the body, formation may occur.
Each stage of decomposition will have certain insects involved. This will be from flies through to beetles and finally moths. The blowflies are the first to find the body and lay eggs. Then come the houseflies and flesh flies. Flesh flies lay larvae directly onto the body, the eggs hatching inside the adult female fly.

One of the first groups of insects that arrive on a dead vertebrate is usually blowflies (Diptera: Calliphoridae). Usually the female oviposits within two days after death of the vertebrate. Then the blowfly goes trough the following stadiums: egg, 1. instar larvae, 2. instar larvae, 3. instar larvae, prepupae, pupae within puparium, imago.

If we know how long it takes to reach the different stadiums in an insects life, we can calculate the time since the egg was laid. This calculation of the age of the insects can be considered as an estimate of the time of death. But even if the estimate of the insect age is correct, the death of the victim (usually) occurred before the eggs were laid. This period is quite variable and depends on temperature, time of day the death occurred, time in year the death occurred, whether the corpse is exposed or immersed in soil or water. As a general rule insects will lay eggs on a corpse within two days after the corpse is available for insects.

How to estimate age of blowfly eggs, larvae, pupae and adults.

Eggs:
When blow flies oviposit, their eggs has come very short in their embryonic development. The eggs are approximate 2 mm in length. During the first eight hours or so there is little signs of development. This changes after that, and one can see the larvae through the chorion of the egg at the end of the egg stage. The egg stage typically lasts a day or so.

Larvae:
The blowfly has three instars of larvae. The first instar is approximately 5 mm long after 1.8 days, the second instar is approximately 10 mm long after 2.5 days, the third instar is approximately 17 mm long after 4-5 days. Identifying the right instar is the easiest part, and is done relatively easy based on size of larvae, the size of the larva's mouth parts and morphology of the posterior spiracles. The time it takes to reach the different instars depends very much on microclimate, i.e. temperature and humidity.

Prepupae:
At the end of the third instar the larva becomes restless and starts to move away from the body. The crop will gradually be emptied for blood, and the fat body will gradually obscure the internal features of the larvae. We say that the larva has become a prepupa. The prepupa is about 12 mm long, and is seen 8-12 days after oviposition.

Pupa:
The prepupa gradually becomes a pupa, which darkens with age. The pupa which are about 9 mm in length are seen 18-24 days after oviposition. The presence of empty puparia should therefore tell the forensic entomologist that the person in question has been dead in more than approximately 20 days. Identification can be done based on the remaining mouth parts of the third instar larvae.

TABLE 1 : VARIABLES AFFECTING DECAY RATE OF THE HUMAN BODY

Variable Effect on decay rate*

Temperature 5
Access by insects 5
Burial and depth 5
Carnivore/rodent access 4
Trauma (penetrating/crushing) 4
Humidity/aridity 4
Rainfall 3
Body size and weight 3
Prior embalming 3
Clothing 2
Surface body rests on 1

* Subjective criteria, with "5" the most influential factor

What happens after the Blowflies?

Other species become important as time passes. Eggs of the cheese skipper (Piophila casei) show up three to six months after a victim's death. Beetles that thrive on bones arrive after the bone is exposed. Other beetles begin preying on the maggots feeding on the flesh. In all, it's a complex ecosystem that will generously betray its secrets to the informed entomologist.

At three to six months other insects are attracted to the decaying object. The fats and proteins begin to break down releasing volatile fatty acids and attracting two new groups of insects. Cheese skippers, a species of the Diptera order and the family Piophilida are one of those insects and the other is Necrobia rufipes or red-legged ham beetles of the family Cleridae.

At seven to twelve months, yet more insects find their way to the decomposing carcass. As fluids evaporate and the body becomes dessicated, carrion beetles of the genus Silpha move in, and certain species of mites (tiny arachnids) thrive in the soil impregnated with the final drops of seepage from the body.

After one year, when the body is totally dessicated, a new group moves in to feed on the dried out tissue, tendons and ligaments. Included in this group are the various species of hide beetles of the family Dermestidae, which eat a wide variety of products including furs placed in storage, furniture or carpet material, hides, and insect collections. Also, species of skin beetles, members of the subfamily Troginae, beetles of the family of Scarabaeidae that thrive on skin, hair and dessicated tissue attached to bones, make their way to the remains. Lastly, Clothes moths or other small moths (Lepidoptera) of the family Tineidae, whose larvae feed on hair, woollens and clothes, come to take their share of the corpse.

At the end of three years, a last group finishes off the remaining waste, empty puparia and exuviae. Ptinus bruneus, yellow and dark mealworms, spider beetles (a beetle of the family Ptinidae), are among this final wave of insects to infest the corpse.

These metallic blue flies (genus Calliphora, family Calliphoridae) can be as large as a honey bee. They prefer outdoor shade, making them often the first fly to reach corpses in deep woods. At normal summer temperatures, the maggots take about three weeks to develop into adults.

Blowfly Egg Masses

Blowfly maggot. Notice that you can see the mouth hooks at the front end.

Blowfly Pupa

Red-legged ham beetle

Carrion Beetle