Forensic Entomology

SIS_2_38-61_RZ 08.07.2006 13:18 Uhr Seite 49 Science topics Forensic entomology Are you a biologist with a mission? Do you want to fight crime with science? Martin Hall and Amoret Brandt from The Natural History Museum in London, UK, introduce the fascinating (and smelly) field of forensic entomology. orensic entomology is the study of insects and other arthropods in Fa legal context. The applications are wide-ranging, but the most frequent is to determine the minimum time since death (minimum post-mortem interval, or PMI) in suspicious death investigations. This is done by identi- fying the age of the insects present on a human corpse, which can provide a relatively precise estimate in circum- stances where pathologists may only be able to give a broad approxima- tion. 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. Thus, the age of the oldest insects on the body determines the minimum PMI. Female (left) and male (right) adults of the common bluebottle blowfly, Calliphora vicina (Diptera: Calliphoridae) Two examples of this are: eggs had been laid on the body six 1. A body discovered in the summer days before. Witnesses subse- in southern England had suffered quently testified that the fatal fire extensive burns, making the was observed on the night before pathologist’s interpretation of the the estimated day of egg-laying. conventional post-mortem 2. A body discovered in late winter in changes in the body very difficult. northern England was well pre- Ageing the blowfly larvae on the served because of the cold temper- body indicated that the first fly atures, and pathological evidence www.scienceinschool.org Science in School Issue 2 : Summer 2006 49 SIS_2_38-61_RZ 08.07.2006 13:18 Uhr Seite 50 Adult blowflies are well adapted to If the ambient temperatures during sensing and locating the sources of the period of development are odours of decay, so cadavers are known, then, in theory, the minimum quickly found. Eggs are usually laid PMI can be determined. in the natural orifices (e.g. eyes, nose, However, there are many compli- mouth, ears) or other dark and moist cating factors that affect the rate of places, such as the folds of clothes or development of larvae on a body: just under the body. Eggs hatch into · Temperature (which can depend on first instar larvae that grow rapidly, geographical location, indoor or moulting twice to pass through sec- outdoor exposure, sun or shade, ond and third instars until they finish time of day and season) feeding. Depending on the species, · Heat generated by the maggot Life cycle of a calliphorid fly (clockwise they pupate on the body or move mass from bottom left): adults, eggs, first instar away to find a suitable site. They may Food source (tissue type, e.g. liver, larvae, second instar larvae, third instar · larvae, puparia containing pupae move many metres before burrowing heart, lungs) into the soil or under objects such as · Contaminants and toxins (external rocks and logs or, if indoors, under and internal) suggested that the person had carpets and furniture. The larva then · Burial or other obstructions (e.g. died two to three weeks before. In contracts and the cuticle hardens and plastic sheets, water) that hinder contrast, insect evidence suggested darkens to form the barrel-shaped access and egg-laying by adult that the person had died more puparium, within which the pupa insects. than two months before the body metamorphoses into an adult fly. All of the above need to be consid- was discovered. This was consis- When the fly emerges, the empty ered when estimating a PMI, yet for tent with other evidence and was puparial case is left behind as long- many of them, little information is accepted by the coroner’s court. lasting evidence of the insect’s devel- available. For example, the elevated The insects of greatest value to opment. temperature due to the maggot mass forensic entomology are blowflies The rate of development of all can be readily appreciated by any (family Calliphoridae), because they insects is directly dependent on the angler reaching into a bowl of bait are usually the first insects to colonise ambient conditions, particularly tem- maggots, but quantifying its effect on a body after death, often within perature. Between upper and lower larval development still requires hours. Because of this, the age of the thresholds, which vary between detailed study, for example using oldest blowflies gives the most accu- species, the higher the temperature, novel thermal imaging techniques rate evidence of the PMI. Many other the faster the insects will develop; the (see box). species of fly, beetle, wasp and moth lower the temperature, the slower The degree to which a forensic ento- are also associated with cadavers, they will develop (see graph below). mologist is involved in a case can resulting in a succession of insects arriving at the body, but as they tend to arrive after the blowflies, they are less useful in establishing a PMI. Days from egg-laying to adult emergence Blowfly infestations of human bodies are a natural outcome of the flies’ 70 Graph showing the rate of development role in the environment as primary 65 of the immature decomposers. The ubiquity of fly lar- 60 55 stages of blowflies vae on carrion is clear to anyone who with temperature, 50 comes across the dead body of a based on published hedgehog or rabbit while walking in 45 data for the bluebot- the country. The larval infestations 40 tle blowfly Calliphora might look gruesome, but they are a 35 vicina (Diptera: Calliphoridae) vital component of the natural recy- 30 cling of organic matter and, on 25 human bodies, they can provide vital 20 clues to the timing and cause of 15 70 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 death. Temperature (ºC) 50 Science in School Issue 2 : Summer 2006 www.scienceinschool.org SIS_2_38-61_RZ 08.07.2006 13:18 Uhr Seite 51 Science topics Thermal imaging Larval mass on the piglet, shown with thermal imaging Still-born piglet Scientists at The Natural History appears untouched by insects in a Temperature of individual blowfly Museum, London, UK, are just normal photograph, but thermal larvae, measured using thermal beginning to explore the thermal imaging indicates an active larval imaging dynamics of larval masses using mass in the throat and chest infrared photography. The temper- regions (top right) that causes an ature of individual blowfly larvae increase in temperature where the can be measured (bottom right). larvae are feeding. BACKGROUND The still-born piglet (above) vary. The entomologist may attend ed by the police, for example, ideally the crime scene personally to collect after telephone consultation with the the insect specimens from the body or entomologist. Photographs of the its surroundings. This is ideal, scene and the post-mortem examina- because he or she can use knowledge tion will then be shown to the ento- of insect biology and behaviour to mologist. make sure that as many specimens as When investigating a suspicious possible are collected and to help death, the main questions which need interpret the results. Alternatively, the to be answered by the forensic ento- entomologist may collect insect speci- mologist are: mens during the post-mortem exami- Which species of blowfly are present on Author, Martin Hall nation as well as viewing photos of the body? The collected specimens the crime scene or visiting the scene must be correctly identified, so that all insect species can be used. This ques- after the body has been removed. of the relevant information on physi- tion is answered by taxonomy, one of Finally, the specimens may be collect- ology, behaviour and ecology of that the most neglected of the natural sci- www.scienceinschool.org Science in School Issue 2 : Summer 2006 51 SIS_2_38-61_RZ 08.07.2006 13:18 Uhr Seite 52 ences but the foundation for all others. Which are the oldest specimens of blowfly? They may still be feeding on the body; they may have left the body to pupate elsewhere; or they may have already emerged as adults and left behind their empty puparial cases. How old are the oldest specimens? Estimating age involves detailed mor- phological study of the insects under a binocular microscope, to determine their stage of development and to Searching soil samples from a compare that with data from standard gravesite for insect speci- databases relating developmental mens. The initial search at the scene is made on a plastic stage to age at different temperatures sheet, before bagging the (see next question). Third instar blowfly larvae feeding on a human samples for further analysis What were the ambient temperatures at body, many head down, in a maggot mass at a tem- in the laboratory. Note the the scene while the flies were developing perature approximately 15 ºC above ambient use of full protective clothing on the body? An electronic temperature Interview with a forensic entomologist After a zoology degree search for puparial cases. The problem was that the and a masters in taxono- crime was two years old, and no research has been my and biodiversity, done on how long empty puparial cases remain intact Amoret worked on vari- in the soil. So I started a study where I buried a large ous groups of insects, number of empty puparial cases and over a period of such as hoverflies and three years I dig them up. The results should be able wasps, and spent three to tell us the rate at which the puparial cases degrade years rewriting The over time, at least over a three-year period.

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