The presence of blowflies larva of 10 - 18 mm, and flesh flies larva of 27 mm tells us that the animal has been dead for 4 to 6 days, approximately.
Explanation:
Forensic entomology is the insects' study to establish the time of death or postmortem interval.
There are two methods to determine the time of death through the insects´ study: one of them uses the age of larvae and the development rate, the other method uses the insects´ succession during the body decomposition.
During the first weeks of death, studies focus on the size of the larvae of the first colonized species of carcasses, since a larger size of the larva indicates more days of the insect's life and, therefore, a longer time of death.
Flyes from the Calliphoridae family and Sarcophagidae family, both of them necrophagous, are the firsts in colonizing the carcass.
Calliphoridae, known as blowfly, lay their eggs of approximately 2 mm long. Sarcophagidea, known as flesh-flye, do not lay eggs, but larvae. The egg stage usually lasts between 24 and 72 hours, always depending on the species. The analysis of the egg´s development state might help to determine the time elapsed since oviposition and with it the time of death.
The firsts decomposition weeks correspond to different states:
1) Putrefaction and decomposition
2) Chromatic state
Named like this after the color acquired due to bacterial action. It occurs between 24-72 hours after death. There are blowflies and flesh flies. 24 hours after death eggs are laid. The eggs' embryonary states last between 24 and 72 hours. 48 hs after death, larvae are in phase I, equivalent to a larvae size of 0.5 to 5.0 millimeters long.
3) Emphysematous or gas period
It happens between the 3rd and 5th days after death. 72 hours after death, larvae are in phase II of their development, reaching 4.0 to 10.0 mm long.
4) Advanced emphysematous period
96 hours after death, larvae are in phase III of development. At this point blowflies, larvae might reach 10 to 25 mm long. Coleopters adults reach the body.
5) Collusion or liquefaction period.
6, 8, or 12 days after death the insects are in prepupa or pupa state. Between 8 and 20 days after death, insects are in the adult phase.
6) Skeletal reduction period
All these periods are affected by different factors that might accelerate or delay the decomposition process. These are humidity, temperature, soil, insects, among others. In general terms, the higher the temperature is the faster insects develop, and the body decomposes.
70 degrees F is enough for the flies to develop and pass from one stage to the other, which means that at 82 degreed F larvae develop even faster. The presence of blowflies larva of 10 - 18 mm, and flesh flies larva of 27mm tells us that the animal has been dead for 4 to 6 days, approximately.
