Forensic DNA Analysis

Determining Minimum Post-Mortem Internal by Evaluating the Blow-flies Life Cycle and Surrounding Environmental Factors


Forensic entomology is the study of insects and other arthropods in a legal context. The Encyclopedia Britannica defines forensic entomology as a specific field that assists police in determining time of death of an individual (Encyclopedia Britannica 2009). Insects can infest a corpse at a very predictable rate. Certain insects immediately invade the body to feed or to lay eggs, while others will not approach the body until it has reached a more advanced stage of decomposition. Thus, the type of insects or eggs present in a corpse indicates how long the victims have been deceased.

The most common practice to determine time of death is calculating the minimum time since death also known as the minimum post-mortem interval. Identifying the age of the insects present on a human corpse can provide a relatively precise estimate in circumstances where pathologists may only be able to give a broad approximation (Hall and Brandt 2006). The fundamental assumption is that the body has not been dead for longer than it took the insects to arrive at the corpse and develop into a certain stage. If you can conclude the age of the oldest insects on the body you can then determine the minimum post-mortem interval (Hall and Brandt 2006).

There are two major groups of insects that are attracted to cadavers and provide the majority of information in forensic entomology investigations (Byrd and Castner 2001). Those are beetles and flies. There are over 750,000 beetle species and 86,000 fly species worldwide (Byrd and Castner 2001). Of the two, flies are usually the first to come in contact with the cadaver.

One fly in particular is considered a valuable tool in forensic entomology due to its high predictability if ambient temperature is known (Byrd 1998a). That is the blow-fly. An analysis of research involving blow-flies reveals the blow-flies' life cycle stages and environmental factors are essential in the determination of the minimum post-mortem internal.

The blow-fly, from the order Diptera, family Calliphoridea, has the ability to detect dead animal matter from up to 16 km away (Rogner 2004). The blow-flies' maggots prefer feeding on moist corpse which makes it easier for them to chew. This is why the blow-fly is known to be the first insect to arrive within hours or even minutes after an animal's death (Ogg 2007).

Many other species of fly, beetle, wasp and moth are also associated with cadavers, resulting in a succession of insects arriving at the body, but as they tend to arrive after the blow-flies, they are less useful in establishing a minimum post-mortem interval (Hall and Brandt 2006).

Blow-flies are distributed worldwide and were first noted in the southern United States to include California, Arizona, Florida, Texas and Louisiana due to its warmer temperature (Byrd 1998b). With the recent changes in climate this species has increased its range throughout the United States. The blow-fly has migrated in even the northern United States. There have been reports of blow-flies as far north as Michigan (Zurawski, Benbow, Miller and Merritt 2009). Due to its expanding distribution and its behavioral characteristics, the blow-fly is particularly important for forensic entomology investigations in the southeast, central and southern portion of the United States (Catts and Goff 1992).

The blow-fly larvae have a shorter development than other species and their predaceous nature can alter entomological-based post-mortem interval estimations, which are found on their prey (Byrd 1998b). The blow-fly must have access to decomposing carrion or fetid meat (rotted meat) in order to complete its life cycle (Byrd 1998b). The life cycle of the blow-fly is unique in comparison to other insects due to its precise transitional timeframe but it still goes through the typical four stage process: egg, larvae, pupa and adult.

The eggs are approximately 2 mm in length and are laid in a loose mass consisting from 50 to 200 eggs at a time (Byrd 1998b). Group ovipostion by several females results in large masses of thousands of eggs that may completely cover the decomposing carcass. Usually during the first eight hours there is little sign of development unless the air temperature is warmer (Catts and Goff 1992). If so, the eggs can hatch approximately around that eighth hour (Byrd 1998b). If they did not hatch by then a drastic change occurs after the eighth hour period. During this timeframe the larvae go through the chorion of the egg (Catts and Goff 1992). The egg stage will typically last a day or so (Catts and Goff 1992). This is the end of the egg stage and the beginning of the larval stage.

The larval stage of the blow-fly has three instars. The time it takes to reach the different instars depends very much on the temperature and humidity (Catts and Goff 1992). At 70 degree ferinheight each stage in a blow-flies life takes a known amount of time to complete (NMU 2006).

The first instar is approximately 5mm long after about 1.8 days. The second instar is approximately 10mm long after 2.5 day. The third instar is approximately 17mm long after 4 to 5 days (Catts and Goff 1992). The larvae then feed on carrion until they obtain maturity. Evaluating the larvae during this timeframe makes it easy to determine the instar stage based off of the size of the larvae, the size of the larva's mouthparts and the morphology of the posterior spiracles (Catts and Goff 1992). Identification can be done based on the remaining mouth parts of the third instar larvae (Brazoria County Sheriffs Office 2009). At the end of the third instar the larva becomes restless and begins to move away from the body to begin the prepupae stage (Catts and Goff 1992).

Once matured, the larvae will migrate away from the carrion to search for a suitable pupation site. Pupation usually occurs within the first inch of topsoil or under leaf litter, rocks, or fallen limbs.

During the pupation timeframe, the larval skin shrinks and hardens to form a dark brown puparium (Byrd 1998b). The area infected will gradually be emptied for blood and the fat body will gradually obscure the internal features of the larvae. At this stage the larvae has become a prepupa which can be approximately 12mm long (Catts and Goff 1992). This stage could last as long as 12 days or as little as seven to eight days after oviposition depending on temperature.

The prepupa will gradually become a pupa. The pupa is approximately 9 mm in length and is apparent 18-24 days after oviposition. During this transition a distinct characteristic is formed and the pupa will gradually become darker with age. Identifying the presence of an empty puparia should indicate to a forensic entomologist that the person in question has been dead for more than 20 days. Once the blow-fly is at the adult stage they can live up to six weeks (Byrd 1998b).

Once entered into the adult stage the blow-flies do not fly much for at least a day or two while their body is hardened. If an unhardened adult blow-fly is found this is a good indication that the fly developed on the site. Once fully hardened the blow-fly will mate and the cycle begins again (Bullington 2008). The development of the blow-fly is highly predictable if the ambient temperature is known.

An analysis of research involving blow-flies reveals the blow-flies' life cycle stages and environmental factors are essential in the determination of the minimum post-mortem internal. The Blow-flies ability to detect dead animals so quickly, its worldwide distribution and their precise and timely life cycle patterns makes this species an optimal insect for forensic analysis. Using this information can determine the minimum post-mortem interval of the victim. To calculate this information you must determine the oldest larva of each species present and measure the ambient temperature (Byrd 1998a). From these values you can determine the earliest possible date and time for deposition of the larvae. This will give you an approximation of the time and date of the death.


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Zurawski, M., Benbow, M. E., Miller, J. R., and Merritt, R. W. 2009. Examination of Nocturnal Blow Fly (Diptera: Calliphoridae) Oviposition on Pig Carcasses in Mid-Michigan. Journal of Medical Entomology. 46 (3):671-679.

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