Insects and Decompositonal Stages of the Body
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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:
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.
TABLE 1 : VARIABLES AFFECTING DECAY RATE OF THE HUMAN BODY
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.
Red-legged ham beetle