1. Introduction
1.1 Forensic entomology
1. Introduction
1.1 Forensic entomology
Forensic entomology is a discipline which dedicate to use arthropod as an
evident in the crime scene. Insect is the important target to collect in the crime
scene, which can help the investigator to acquire accurate and precise conclusion
in the investigation (Catts and Goff, 1992). In forensic entomology, it divided
into three different subdomains, which are urban entomology, stored-product
entomology and medico-criminal forensic entomology (literature cited in Catts
and Goff, 1992). Forensic entomology in narrow sense usually indicate
medico-criminal forensic entomology, so medico-medico-criminal forensic entomology will be
referred to as forensic entomology hereinafter.
Insect can provide numerous useful information. The advantage of utilizing
necrophagous insects in crime scene investigation are as followed (Catts and Goff,
1992). (1) Insect usually can detect the corpse and arrive the crime scene soon
after corpse been exposed. (2) Investigator can understand the condition of the
corpse by examining the insects gathering on the corpse, life stages or status of
insects and different composition of the fauna. For example, blow flies usually
will gather and oviposit on the opening wounds or natural orifices (Bourel et al.,
2003). Another example is that, if some insects showed on the corpse where did
not belong to their natural habitat, it may indicate that the corpse had been moved
(literature cited in Keh, 1985). (3) Because the corpse represents a limit and
temporary resource (Beaver, 1977), many insects will compete for colonizing on
the corpse and utilizing the corpse as food resource or oviposition site (Thompson
et al., 2013). Along the decomposition of the corpse, it will go through several
stages which the body will emitted different chemical cues to attract different
insects, the fauna around the corpse in each decomposition stage is predictable.
(4) The fauna around the corpse usually can provide us us eful and vital
information, but usually ignored by the investigators.
The earliest forensic entomology study document is in thirteenth century of
China, Sung Tzu’s The Washing away of Wrongs. One section in the book, the
investigator used the evidence of gathering flies on a blood tainted sickle, to find
out the real suspect (literature cited in Catts and Goff, 1992). Beside to find out
the killer, lethal weapon and reason of death, another vital information that
investigator need to figure out is the PMI (Post-Mortem Interval).
The definition of PMI is the interval between the time when victim died to
the moment when the corpse been discovered (Catts, 1992). It can be obtained by
estimating the maximum PMI and minimum PMI. In traditional pathology, PMI
can be estimated by different features showed on the corpse, like rigor mortis,
livor mortis or algor mortis, however, along the time passed by, using early
postmortem changes on corpses to estimate PMI will become tougher (Bourel et
al., 2003; Goff, 2010). In definition, maximum PMI is the time when the victim
was last seen alive. Compared with traditional pathology, using insects to
estimate minimum PMI may obtain a more precise estimation (Bourel et al., 2003;
Greenberg and Kunich, 2002). In traditional pathology, one can only estimate
accurately and precisely for the first two or three days after death (Villet et al.,
2011). However, to analyze and calculate the age of immature insects (maggots)
can precisely estimate the PMI from the first day to several weeks (Amendt et al.,
2004). Using insects to estimate minimum PMI becomes an alternative approach.
Using accumulated degree hours (ADH) to estimate minimum PMI, by recording
the ambient temperature in micro-environment to figure out the larval
developmental data with specific species in the early stage of decomposition
(Greenberg, 1985; Lord et al., 1986). In the later process of decomposition, the
major targets of estimate minimum PMI have shifted from the larval
developmental data to the faunal composition (Smith, 1986). The error of
minimum PMI acquired from developmental data of larvae can be less than an
hour (Gennard, 2007). For applying larval developmental data, three different
models have been used, which are isomorphen diagrams (Grassberger and Reiter,
2001), isomegalen diagrams (literature cited in Amendt et al., 2011) and thermal
summation models (Higley and Haskell, 2010). These developmental models use
weights or lengths of larvae to estimate how long do maggots take to development,
in order to estimate the minimum PMI.
Along the corpse decomposition, the corpse will go through several different
changes. Although the changes of corpse are continuous events which do not have
a significant boundary in decomposition process (Goff, 2010), it is easier for data
collecting and observation if researchers separate the process into difference
stages. Megnin (1894) propose a concept that decomposition of corpses would
experience eight stages, each stage has pathological differences, and insects
faunal composition presence on the corpse also change in each stage. Nowadays
forensic entomologists usually divide decomposition process into four or five
stages (Hall and Huntington, 2010). Four stages model in decomposition process
are fresh, bloated, decay and remains (Catts and Goff, 1992). Beginning with
fresh, which will go through several pathological feature (rigor mortis and livor
mortis), no obvious odors emitted from the corpse in this stage, some ants and
blow flies will attracted to the corpse several hours after death, depends on the
temperature and environmental condition. Microbe continuously metabolize and
activate, cause huge amount of gas produced inside the abdomen, accompany
with slightly odors, numerous blow flies or flesh flies appears, which is the
beginning stage of bloated. In the stage of decay, large feeding masses of
Dipteran larvae will presence. Larvae will penetrate the skin and cause the body
deflated. Some Coleoptera also presence in this stage as predator of larvae. In
decay stage, strong and stink odors appears. The last stage is remains
(skeletonize). Most of the soft tissues on the corpse has been eaten, left the bones,
connective tissues, hair behind. Most of the Diptera larvae already leave the
corpse for puparium, only few individuals are left on the remains. Coleoptera are
the dominant species in earlier phase of this stage, for consuming those hard
tissues. Remain stage can last for years.
In the each stage of decomposition process that researchers established, it is
not hard to see the participation of insects. Insects play an important role in the
decomposition process, in early stage after death, decomposition is process by
the aerobic microbe from the environment fall on the corpse surface, and the
anaerobic microbe which already locate inside the body (Thompson et al., 2013)
After hatching, maggots will gather to form maggot masses. Maggot masses will
penetrate the epidermis, create an entrance for insects and microbe to invade
inside the corpse, enlarge exposure area to the outer environment. The ventilation
through the entrance make the aerobic microbe and anaerobic microbe to
re-distribution on the corpse (Thompson et al., 2013). Corpse condition would
change along with the different microbe or insects colonized, create an
environment that may attract or inhibit other insect in the succession, which
believe is the main reason to the different insect faunal co mposition through
decomposition. Most of the soft tissues on corpse are consumed by the insects,
and usually is consumed by dipteran larvae.
Upon all the insects attracted by the corpses, we can divided them into four
groups as following (Catts and Goff, 1992).
1. Necrophages: Species that use the corpse as main resources to feed, and also
produce offsprings on the corpse. These insects usually can find the corpse in
a very short period, like most of the blow flies (Calliphorids), flesh flies
(Sacrophagids), and some beetles. The larvae of the blow flies are also an
important tool to estimate the PMI.
2. Omnivores: Species that utilize corpses and the faunae around the corpse as
food resources. Some insects also use corpse as an oviposit location. Ant, rove
beetles and some other coleopterans are belonged to this category. Large
numbers of species in this category could retard the process of succession by
decreasing the amount of other necrophagous insects (Early and Goff, 1986).
3. Parasites and predator: Species that are predators or parasitoids to faunal
members around the corpse, including some hymenopterans (parasite wasps,
yellow jackets, etc.), coleopterans as well as dipteran larvae. Some species
can be both necrophage and predator, such as Chrysomya rufifacies. First
instar larvae of C. rufifacies are necrophagous, but begin from second instar,
they become as the facultative predators, which may use their spine to strangle
and capture other larvae as preys (Byrd and Castner, 2009). Some mites and
nematodes are also belonged to this category. Smith (1986) considered species
in this group are the secondary important members in forensic entomology.
4. Incidentals: Species that accidentally pass by or live in the place where are
overlapping with corpses. They use the corpse as a shelter or extension of
habitats, like species of springtails, spiders, centipedes or mites (Goddard and
Lago, 1985). Some herbivore insects (Hemiptera) will also present on the
corpse by accidentally droping from the plants nearby. Different distributions
of insects in different environments may provide the information of movement
of body, which are also the vital evidences in the crime scene investigations
(Catts and Goff, 1992).
Tomberlin et al. (2011b) proposed a framework for study forensic
entomology and the insect colonization on the corpse. They use the beginning
time of insect colonization to separate the process into pre-colonization stage and
post-colonization stage. The latter one, the post-colonization stage can be further
divided into two phases, started with the insect colonization on the corpse, ended
up in the insect dispersal from the remains. First phase of the post-colonization
stage is the consumption phase, which is the feeding process of arthropods and
its offsprings. When arthropods finish their life cycles from the corpse, or there
is no more resources can be utilized on remains, arthropods begin to disperse
from the remains, which is the second phase of the post-colonization stage,
dispersal phase (Tomberlin et al., 2011b). In post-colonization stage, which is the
studies and researches were most dedicated in and have more data been
documented. However, pre-colonization phase is usually be ignored by the
forensic entomologist (Tomberlin et al., 2011a). The importances of the
pre-colonization stage is not less than post-pre-colonization stage because it will affect
the time gap between actual death and the time when corpse been colonized by
insects. The duration between these two events will strongly influence the
accuracy of PMI estimation. Which Tomberlin has suggested that researchers
should put more efforts in this field.
The pre-colonization stage begins from the happening of death, The first
phase in pre-colonization stage is the exposure phase. In exposure phase, corpse
will emit some chemical cues to attract arthropods, however, insects haven’t
receive those chemical cues yet. Then is the beginning of the detection phase.
Detection phase contains two steps, they are the activation step and searching
step. Activation step started when chemical cues was detected by the arthropods,
and the searching step activates when the response of arthropods to search for the
actual location of the corpse. Acceptance phase begins when arthropods locate
the corpse and have the first contact to the corpse. Arthropods will evaluate the
media and make decision to colonize or not. Pre-colonization stage ends up with
arthropods’ actual colonization of consumption and oviposition. Discussion on
the acceptance stage in pre-colonization phase is rare (Tomberlin et al., 2011a).
What condition could make blow flies take more time than usual, to evaluate and
make decision to oviposit on the media, or make blow flies avoid to lay eggs are
interesting questions.
In the death scene investigation, the time when victim dead is the major
concern that researchers want to know, and this is what minimum PMI estimate
for. Because the time of death is acquired from the time period of maximum PMI
and minimum PMI, the longer the period between maximum PMI and minimum
PMI, the lower the accuracy we acquire. To estimate the PMI correctly, we are
strongly relied on a key assumption which the blow flies can detect the corpse
and laid eggs on corpse soon after death. So the period between the time when
victim die and the time of insect colonization should be as short as possible (Catts
and Goff, 1992). But this is often invalid, especially in the indoor, sealed
environment or under the extreme weather condition (Amendt et al., 2004; Catts,
1992; Catts and Goff, 1992). In the situations which we mentioned above, insect
cannot invade the corpse immediately since the lack of physical mechanism to
break through the barriers to the corpse, the decomposition is mostly process by
bacteria and fungi only. Microbe that occupied inside and outside of the corpse
carry on metabolism, cause the nutrition and water loses. These situations are
seldom happened in the natural condition but frequently found in homicide cases
which the natural succession patterns were usually changed artificially. So how
the primary flies like blow flies will react when the corpse condition does not
follow the natural succession pattern? We were curious about the reaction of
oviposition behavior of blow flies when they encounter different decay -aged
treatment individually. If fresh pork liver is a better medium for oviposition and
larval development, will blow flies have higher frequency to lay eggs on the fresh
treatment? Will blow flies make a quicker decision when they encountering a
more suitable media? On the other hand, if the condition of oviposition media is
no longer suitable for oviposition, will the frequency of oviposition decline? Will
the blow flies take longer time to evaluate the media for oviposition or not? In
addition, we were wondering if oviposition performance in the laboratory is
similar to that in the field? To answer those questions, we prepared medi a of
different decay-aged pork liver for oviposition and preference test.