Page 13 - Natures Witness Entomology

take to pass through the various stages through to use it as a food resource. A sign that this is
pupation. Then, when a crime scene is investigated, occurring is the presence of younger larvae of
the forensic entomologist compares the insect one species (often flesh flies) with older larvae of
species and their distribution of larval stages to the another species (often blow flies) that colonized
database to estimate the time of death. A key piece the cadaver earlier. Cadavers decompose as
of data which must be experimentally determined bacteria and the body’s own cellular enzymes join
is the time required for the different larval stages. forces to break down tissues, a process assisted
by insects and other scavengers. Taphonomy is the
The adult female blowfly, for example, lays her science which studies the natural process of plant
fertilized eggs on the carcass in a single batch, but and animal decay.
she may return to lay eggs several times during
her reproductive life (two to three weeks). The In addition to succession, these larval
eggs begin to hatch in 12 to 24 hours, producing development rates help forensic entomologists
small (approximately 2 mm) first stage larvae. estimate the PMI. This is challenging since
Because the outer ‘skin’, or integument, of insect insects are cold-blooded animals and their larval
larvae cannot expand to accommodate growth, the growth rate increases as the environmental
larvae molt their outer covering to keep growing temperature increases until they reach a lethal
and developing. The first larval stage, or ‘instars’, point. Researchers rear insects at a constant
become the larger second instars after they molt. temperature and calculate the time it takes
The second instars feed and subsequently molt to for an insect to develop from one life stage to
become third instars. The feeding third instars are another. By comparing growth rates at a variety
very active and grow rapidly to a length of 14 to 18 of temperatures, entomologists have calculated
mm. They then develop into post-third stage larvae Degree Hours required for the insect to develop
which stop feeding, migrate away from the cadaver, from one stage to another. The number of hours
and burrow into the soil. They become inactive to reach a stage is multiplied by the standard
and the integument hardens into a pupa. After six rearing temperature during that time period. The
to eight days the adult fly emerges from the pupa, Degree Hours needed to complete an insect’s
crawls to the soil surface, the wings harden, and it development does not vary. If larvae take 40
flies away to begin the process anew. Flies survive hours at 25 degrees C to develop to the next live
over winter in the pupal stage and emerge in the stage, this is 1000 degree hours. If the larvae
following spring when temperature conditions are kept at 20 degrees, they will take 50 hours
become favorable. The process for fleshflies is to reach the same stage. When investigators
similar, with the exception that eggs hatch within can get accurate weather reports for an area,
the body of the female, and she deposits live first they calculate Accumulated Degree Hours and
instar larvae. estimate the hour when larvae hatched from the
eggs. The temperatures for the days preceding
Insect species are attracted to lay their eggs on the discovery of the body and the growth and
a corpse at different times. The regular pattern development rate of the fly species in degree
of development of the larvae or maggots on the hours must be known. By adding the incubation
corpse can be used to estimate the number of time for the egg, the entomologist can estimate
days since the eggs were laid for each species. the time of initial oviposition, which is an estimate
Each new species replaces an earlier species in of the time of PMI.
this succession since the cadaver is going through
a process of decay and attracts new insects able Additionally, there are exceptional circumstances

THE MYSTERY OF LYLE AND LOUISE 13