Forensic Entomology

Forensic entomology (FE) involves the analysis of insects for legal and forensic investigations, with estimating the minimum postmortem interval (min PMI) as the most crucial medicolegal task. Postmortem interval (PMI) is the time between death and the discovery of the body. Rigor mortis and livor mortis are some of the natural processes associated with decomposition that can be used in PMI determination. However, these functions can be reciprocated, thus becoming inaccurate very quickly, and are limited to the first 72 hours after the death. On the other hand, insects are potent tools for estimating min PMI within these 72 hours and beyond. Furthermore, depending on environmental conditions and rate of accessibility, necrophagous insects colonize a carcass, blowflies usually being the first to arrive at the corpse. (Amendt et al., 2011).

The Blowfly Lifecycle

Blowflies are necrophagous insects belonging to the family Calliphoridae and order Diptera and are sometimes called Calliphorids. They rely on proteins for ovary maturation, oviposition, and offspring development. Calliphorids are primarily attracted by the odor released during the first stages of cadaver decomposition (Paula et al., 2018). After arrival (within two to three hours), the females lay eggs near the natural orifice or wounds that provide body fluids for feeding (Haglund & Sorg, 2001). The hatching of eggs depends on temperature and species, but it mostly happens within 6 to 48 hours. They hatch into larvae (maggots) characterized by three stages. The first instar is usually one to two millimeters long and molts into the second instar. During these stages, the larvae shed off their cuticle in an ecdysis process, which permits further growth.

After the second ecdysis, the third instar is formed. The larvae then enter a prepupal stage, migrating from the body to find a place to pupate. They will often shelter beneath stones, soil, or leaves (outdoor crime scene) or nearby furniture (indoor crime scene). They then form pupae within a hardened cuticle of the third instar called the puparium. Adult flies then emerge, marking the completion of the lifecycle. Larval development takes three to ten days, while adult emergence takes six to eighteen days (Campobasso et al., 2001).

Effects of Moisture on Blowfly Lifecycle

The moisture content of the decomposing material is an essential stimulus for oviposition. As such, dried and mummified bodies are considered unsuitable for blowflies to lay eggs on, as the molting larvae require moisture and air for successful development. It has been reported that a quiescence state (diapause) occurs during larval development when humidity is below 40% (Bauer et al., 2020). While the dry mass of emerging adults negatively correlates with moisture reduction, the development time, larval stage duration, the absolute size of larvae, and shape of growth rate are also negatively affected.

Consequently, moisture content significantly impacts the minimum time required to complete larval stages of blowflies. Treatments with 0% moisture content showed no larval eclosion, while larvae with 33% water content failed to thrive. As a result, the larvae either died or could not hatch due to structural changes in the eggs caused by desiccation. Conversely, larvae reared in treatments containing 50% and 70.8% moisture content showed a complete and successful development. Additionally, larvae reared on wet liver take a longer development time compared to those raised in substrates with 70.8% moisture (Bauer et al., 2020).

 Effects of Temperature on Blowfly Lifecycle

Besides humidity, temperature is another significant factor that affects a blowfly’s development time. Several studies have shown that high temperatures shorten the Calliphoridae lifecycle. Their poikilothermic nature can explain this, where increased temperature positively affects biochemical processes. To observe the effect of temperature on the lifecycle of necrophagous Calliphorids, Pruna et al. (2019) researched Lucilia sericata. “At 18°C, the average developmental time was 778 h (±38,6); at 25°C average time was 401 h (±33,5); and at 30°C average time was 288 h (±24,8)” (Pruna et al., 2019). These experiments indicate that larval developmental time is shortened with increased temperature. Additionally, ten °C is the temperature limit with no development, while the maximum is 35°C, where the larval development begins to decelerate. Subsequently, 18°C, 25°C, and 30°C were favorable temperatures for the egg to adult development.

Collection of Samples for Investigation

When the time of death is in question, sampling and how samples are collected become significant. Therefore, strict and standard guidelines for selection are followed for high-quality analysis.

Where to Look for Samples

Colonization occurs in different body regions under different sequence patterns. Natural orifices are usually the first regions to be colonized. These include the mouth, nose, eyes, and anal or genital areas. Any other mass colonization on other body regions indicates a wound. Subsequently, different areas could be infested by different insect species; therefore, they should be sampled separately for examination. In cases where the body is wrapped, the wrapping materials should be checked for insects.

Since immature insects usually migrate from the body to pupate, searching the surrounding area is necessary. For outdoor scenes, leaf debris and soil underneath and surrounding the body within a radius of two to ten meters should be checked for insects. For indoor cases, migrating pupae may be found under nearby furniture and in rugs, mats, or carpets. Additionally, nutrient sources other than the carcass should be checked as these may contaminate the insect evidence.

Where to Sample and How to Store Collected Insect Evidence

Various soil samples should be collected to at least a 10cm depth. To prevent further development of the insects, the pieces should be stored at cool temperatures, for instance, in fridges at 4°C. Further, insects at different development stages (eggs, small and large larvae, pupae, and adults) should be collected and stored in a separate vial. Present puparia are a clear indication of the first complete colonization; therefore, collection of empty puparia is also essential.

Sample sizes vary with the available number of larvae. However, all should be collected where there are fewer than 100 larvae and 1 to 10% where thousands of larvae are known. Adult or immature insects that die during collection should be preserved in 70-95% alcohol. The live specimen should be kept in vials and stored at cool temperatures of 4°C. Air circulation into the vials should be allowed; thus, coarse sawdust and paper towels would be helpful. Finally, the live samples should be transferred to a professional expert within 24 hours. The remaining specimen should be killed immediately. Hot water (?80°C) is recommended to kill fly larvae, while extremely freezing temperatures, usually -20°C, are recommended for beetle larvae. They should then be preserved in 70-95% alcohol. Storage of samples using formalin or formaldehyde should be avoided as these result in DNA degradation.

Additional Information

Photographic documentation to describe the body’s condition and the ecology of the scene would be helpful. It is also important to note the ambient temperature at the crime scene and to gather the area’s weather data from the nearest weather station. If possible, temperatures at the body’s position should be recorded at one-hour intervals for five to ten days after discovering the body. This is important as it allows accurate temperature estimates of the crime scene before the discovery of the body. Subsequently, this guides the investigator in establishing the period when the insects began to develop at the crime scene (Amendt et al., 2011).

The Goal of Forensic Entomology

Decomposition mediated by blowfly activity is a continuous process that can be measured, allowing accurate estimation of min PMI. Therefore, after gathering the samples, the forensic entomologist analyzes the blowflies on the body to calculate their age, which makes it possible to calculate the time of colonization when the blowflies first colonized the cadaver. Since blowflies are usually the first to inhabit a body, this data helps the entomologist estimate the minimum PMI. Furthermore, some samples are collected for toxicological analyses. Drugs and chemical toxins can be detected in larvae when the chemical’s rate of metabolism is exceeded by its absorption rate. Even though Diptera larvae still have limited quantitative value, identifying drug-mediated changes during their development is vital, as these changes can significantly alter PMI estimates.

 Effects of Blowfly Infestation on the Rate of Decomposition

Many biotic and abiotic factors influence decomposition. Necrophagous insects are among the biotic factors that affect carrion decomposition. They are attracted by odors and gases released during the postmortem decomposition of the carrion’s organic matter. The larvae destroy the organic matter within 7-30 days of exposure, depending on the area’s temperature. Pollution can be described in four stages: fresh (24 hours), bloated (2-10 days), decay (11-42 days), and dry (43-271 days). Subsequently, Calliphorids have been associated with early discoloration and bloating stages of decomposition.

In the fresh stage, the remains are not associated with any odor, and the first insects to arrive and oviposit are usually Lucilia illustris, followed by Phormia regina. As the temperature increases, larvae masses develop in the bloated stage. This decaying stage is characterized by color, odor, and swelling changes. Also, larvae feed on the carrion’s skin, thus breaking it. Temperatures continue to advance, and larvae migrate, searching for a place to pupate. Lastly, the dry stage is characterized by minimal larval activity, as the remains are associated with little or no odor. Insect activity stops entirely after the 271st day (Campobasso et al., 2001).

Other Insect Colonizers

Besides the Calliphoridae flies, other Dipterans that colonize the remains of a body include the Muscidae and Sarcophagidae. In addition to flies, beetles belonging to the order Coleoptera are colonizing insects. They have the Silphidae, Staphylinidae, Histeridae, Nitiduliae, Dermestidae, Cleridae, and Scarabaeidae. Calliphoridae and Muscidae are the first to arrive in 2-3 hours, followed by the Sarcophagidae. Colonization by Coleopterans starts at the beginning of the bloated decomposition stage. During this stage, the Silphidae are first to arrive, followed by Staphylinidae and Histeridae, predators of the Dipterans. The decay stage of decomposition is mainly associated with colonization by Nitiduliae insects, while the dry stage colonization is by Staphylinidae, Dermestidae, Cleridae, and Scarabaeidae.

Conclusion

Blowflies are potent tools that are applied in medicolegal death Investigations. They exhibit predictable sequences in their development rate, enabling forensic entomologists to determine the postmortem interval. Blowflies colonizing the body are sampled for examination and identification. Determining their age helps to estimate when they first occupied the body, which in turn helps to estimate the time of death.

References

Amendt, J., Richards, C., Campobasso, C., Zehner, R., & Hall, M. (2011). Forensic entomology: applications and limitations. Forensic Science, Medicine, and Pathology, 7(4), 379-392. https://doi.org/10.1007/s12024-010-9209-2

Bauer, A., Bauer, A., & Tomberlin, J. (2020). Impact of diet moisture on the development of the forensically important blow flies Cochliomyia macellaria (Fabricius) (Diptera: Calliphoridae). Forensic Science International, 312, 110333. https://doi.org/10.1016/j.forsciint.2020.110333

Campobasso, C., Di Vella, G., & Introna, F. (2001). Factors affecting decomposition and Diptera colonization. Forensic Science International, 120(1-2), 18-27. https://doi.org/10.1016/s0379-0738(01)00411-x

Haglund, W., & Sorg, M. (2001). Advances in forensic taphonomy. CRC Press.

Paula, M., Michelutti, K., Eulalio, A., Piva, R., Cardoso, C., & Antonialli-Junior, W. (2018). A new method for estimating the postmortem interval using the chemical composition of different generations of empty puparium: Indoor cases. PLOS ONE, 13(12), e0209776. https://doi.org/10.1371/journal.pone.0209776

Pruna, W., Guarderas, P., Donoso, D., & Barragán, Á. (2019). The life cycle of Lucilia sericata (Meigen 1826) was collected from the Andean mountains. Neotropical Biodiversity, 5(1), 3-9. https://doi.org/10.1080/23766808.2019.1578056

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Forensic Entomology

Forensic entomology (FE) studies insects related to criminal or civil investigations. In criminal cases, it is used to determine postmortem interval. In civil lawsuits, FE may be used because of the introduction of insects into a consumer product or the infestation of a structure. A blowfly is an insect commonly used to aid in the determination of postmortem interval.
Research the use of the blowfly in determining postmortem interval.
• Describe the stages of the life cycle of the blowfly.
• How long is its life cycle (hours/days/weeks/years)?
• How do temperature and moisture affect the blowfly’s life cycle length?
• When the time of death is in question, what is the process by which a forensic entomologist would collect samples?
• What other information must an FE gather at the scene during the investigation?
• What does the FE attempt to accomplish after gathering samples and information and returning the items to the laboratory?
• How does the infestation of the blowfly affect the rate of decomposition?
• After specific lengths of time, other insects infest the remains of a body. Provide examples of the other insects that do this. What are the time intervals in which infestation occurs?