The U.S. National Research Council issued
a report in 2009 that heavily criticized the forensic
sciences [20]. In numerous problems encountered in
forensic science, the PMI is of central importance and
yet difficult to estimate. Up to now, no objective and
accurate method is offered for the estimation of PMI,
especially for late stage after death and decomposed
corpse. The fact that insects and other arthropods
contribute to the decomposition of corpses and even
may help to solve killings is known for years. By
calculating their developmental stage, flies are useful
in estimating the mPMI, however, we must keep in
mind that this mPMI is not the actual PMI.
The real PMI can be divided into 2 periods—
precolonization interval (pre-CI) and postcolonization
interval (post-CI) [10]. In this case, pre-CI includes
periods I and II, while post-CI corresponds to period
III. Compared with common cases, the situation here
was somewhat different and more difficult to interpret,
because the body had been under water for some time
and surfaced in a different physiological state than
would have occurred if the victim had just been killed
minutes earlier and never exposed to water. In addition
to water temperature, other factors which affect
the rate of decomposition in water include bacterial
content and salinity [1]. A corpse submerged in fairly
warm and eutrophic water such as the River in this
case will decompose more rapidly than a corpse in
relatively cooler water with a lower bacterial count
[14]. Of course, the weight of the body and the clothes
should also be taken into consideration.
Under different environmental conditions,
the pre-CI stage lasts several minutes up to hours, or
even days. There is only a number of assumption and
guesses for this period [21]. The understanding of preCI is still superficial, so the estimation of period Ⅱis
currently problematic and we have to overlook it in this
case. In fact, in this case the period of insect activity
more appropriately indicates the minimum length of
time the body resurfaced (Period II and III). Water is
one of the main factors to prevent access of the body
to insect, oviposition and subsequent development of
insect larvae. During floating, if the corpse is close to
the land it can be easily predated by terrestrial insects
as in this case study. These observations have been
well described in previous studies [22, 23]. However,
if the pre-CI stage has to be overlooked in every case,
we can’t deny that there will be severe deviations
from the true PMI in some cases. Therefore, how to
establish an objective, reproducible, scientific method
for the estimation of pre-CI stage become another
difficult problem of forensic entomology to be solved.
The Period Ⅲ is the period of insect activity,
which will reflect the minimum period of time since
death or PMI but will not precisely determine the
time of death. We can get to a comparatively exact
determination about flies’ developmental stages and
calculation of the developmental duration before this
stage to estimate PMI, based on the developmental
features of flies and environmental conditions [24].
Accurate identification of an insect specimen is
usually a crucial first step in a forensic entomological
analysis. Although B. peregrina can be identified by
morphological keys, species-diagnostic anatomical
characters are not known for the immature stages of
many forensically important insects, and an existing
key may be incomplete or difficult for non-specialists
to use. The 272-bp fragment of the mitochondrial
COI gene in this case displayed that DNA-based
method can be used as a supplemental means for
morphological method in identification of the common
Chinese forensically important flies.
Generally, calliphorid species are the first to
locate and oviposit onto corpses, and are among the
dominant species found on corpses in China [25].
However, the B. peregrina, which is the most widespread
sarcophagid specie associated with cadavers in China,
was the dominant specie in this case according to
the entomological evidences. The potentiality of
sarcophagids for PMI estimations has been well
demonstrated in many Studies [26, 27]. In this case,
although the female’s back was completely wrapped
up by her blue T-shirt, female flesh flies of B. peregrina
deposit living first instar larvae on decomposition and
the larvae could arrive to the tissues of back in a short
time after they were deposited. Moreover B. peregrina
can fly under inclement conditions that would prevent
the Calliphoridae, thus in this case, it may be the first