Insect Growth as a Natural Clock

By Ryan McGillivray

Imagine yourself in the shoes of a forensics investigator just arriving at the scene of a crime. A body has been found in the middle of the woods. There are no witnesses, no cameras, and seemingly no eye witnesses to what happened or when it happened. Your first matter of business is to determine a time of death, but with no witnesses, how can this be done? Suddenly, you notice a fly buzzing on the body. To most people, this sight further adds to the gruesomeness of the scene, but to you it is a godsend. By this simple insect, you now have evidence to build your case. You now know the time of death, how long the body has been undisturbed, and potentially many other clues, all from a simple, gross little bug. Using insects as natural clocks may sound strange, but they are actually very useful in measuring increments of time in the field of crime scene forensics. Insects and their eggs found on bodies give trained investigators vital clues about how long a body has been dead, whether this was the scene of the death or not, and how long since the body has been placed in its current location.

When using insects as natural clocks to determine a time of death, many factors and variables must be known. First, one must know what insect arrives on a body at a given stage after death. The lifespan of these insects, and their reproductive time cycles must also be known in order to further control the clock. Insects may also tell whether a body was moved or not. For example, if a body is found in a region with insects in it that are not native to that particular area, there is a very good chance that the body was moved after it was already dead. This may also help determine where the actual death took place. A good example of the natural clock process is shown with the blow fly. Blowflies are usually first on the scene of a deceased body. The blowfly life cycle is known, so examining the state of the fly gives the age of that particular specimen. Below is an illustration of the life cycle of the average blow fly.

If a blowfly lays a given amount of eggs in a given amount of time, and that time is known, then a simple count of the number of eggs and larvae present on a body can calculate a time of death. Other factors, such as number of generations are also taken into account while calculating the most accurate date as possible. The information about the life cycles of several forms of blowflies are shown in the table below. The table shows the lifecycles as the fly develops. It also shows the time period between each instar, or each stage of life between molting. With a table such as this, if the species of fly is identified, calculating the time of death consists of a simple count and algebra equation.

Development Data on Different Species of Blowflies (Calliphoridae) and Fleshflies (Sarcophagidae)
Life histories of 11 species of blowflies and fleshflies reared at 27 degrees Celsius, and 50 percent relative humidity.
  Number of Generations Egg (Hrs) First instar (Hrs) Second instar (Hrs) Third instar (Hrs) Prepupa (Hrs) Pupa (Days) Total Immature (Days)
Sarcophaga cooley 29 -- 24 18 48 96 9 16
Sarcophaga shermani 28 -- 22 16 48 104 8 14
Sarcophaga bullata 18 -- 26 18 54 112 12 17
Phormia regina 23 16 18 11 36 84 6 11
Protophormia terranovae 27 15 17 11 34 80 6 11
Lucilia sericata 29 18 20 12 40 90 7 12
Eucalliphora lilaea 27 22 22 14 36 92 6 13
Cynomyopsis cadaverina 17 19 20 16 72 96 9 18
Calliphora vomitoria 5 26 24 48 60 360 14 23
Calliphora vicina 5 24 24 20 48 128 11 18
Calliphora terranovae 4 25 28 22 44 144 12 20

There are several drawbacks to this method of dating time of death. First, using insects may yield an approximate time of death within days. This method is ineffectual with smaller time scales where it is useful to know the time of death to within a few hours or even minutes. On the opposite side, this method becomes useless with extremely old bodies that are completely decomposed with no insects left. The ideal time frame for the insect clock is between a few days and up to three months. Perhaps the greatest drawback to using insects to calculate time of death is that the process is rendered useless in winter, cold regions, or other areas where insects are not present. When used in conjunction with many other methods, insects can prove to be extremely useful clocks that can help immensely in unraveling many of the mysteries crime scenes often weave. These unlikely detectives have done more then their part to crack countless cases which would have been very hard to solve without the help of insects.