Amoret Whitaker Martin Hall Forensic Entomology: Using insects to solve crime Most people avoid contact with insects and are Succession of insects repulsed by the sight of maggots. In this article, When a person or animal dies, the body starts to Key words decompose immediately and gives off odours. The Amoret Whitaker and Martin Hall of the forensic chemical composition of these odours changes over entomology Natural History Museum in London look at time, as does the physical state of the body, which the positive way in which insects can be used as goes from fresh to active decomposition, through insects evidence in criminal investigations. to dry and finally skeletal. At different stages during decomposition the decomposition process, different types of insect succession are attracted to the body to feed and lay eggs or larvae on it, so their offspring can also feed. The type of insects follow a predictable pattern, with blowflies (Figure 1) coming in at the earliest stage, orking in the Entomology Department followed by many other groups of arthropod such at the Natural History Museum is not as carrion beetles, ants, mites and moths. W just about identifying and sorting our 28 million preserved insect specimens. We Arthropod: regularly receive telephone calls from Crime Any animal which Scene Managers when a body has been found in has an exoskeleton, suspicious circumstances, asking if we can collect a segmented body the live insects and work out how long the body and jointed limbs, has been dead. such as crabs, spiders, insects and Forensic entomology is the study of insects and woodlice. other arthropods in a legal context. It covers many different areas, such as pests in stored food products and insect infestations in human habitations. However, the most high-profile use of forensic entomology is in estimating the time since death, or post-mortem interval (PMI), in cases of Figure 1 The blue-blottle blowfly, Calliphora vicina, unexplained or sudden death. feeding on a dead animal, with larvae. September 2008 1 Blowfly life cycle Estimation of post-mortem interval Because the blowflies (blue- and green-bottles) are It follows that, if blowflies are found associated with the first insects to be attracted to a body, within a body, you can estimate the post-mortem interval hours or sometimes minutes of death, they are the if you can find out: a) what the species of blowfly most useful insects for estimating PMI. The adult is, b) what stage the blowflies are at, and c) under flies will come to feed on the body and the females what temperatures they developed. It is important will oviposit, laying batches of 50-100 eggs in a to know the exact species of blowfly as different single egg mass. These eggs hatch out into tiny species, even closely related ones, may develop larvae, called 1st instars, which as they feed and at different rates. The stage may be fairly easy to grow, will moult and go through two more stages, recognise, e.g. eggs, instar stage, post-feeding or called the 2nd and 3rd instars. At the end of the 3rd pupal, but the length of the larvae can also be instar stage, the larvae finish feeding and usually measured to gain greater accuracy, and pupae move away from the body to pupate, at which stage can be dissected or X-rayed to gauge the stage of they are called post-feeding larvae. Once a suitable development (Figure 3). Finally, the temperature at place has been found to pupate, the outer skin of which they developed can be estimated by obtaining the larva constricts and hardens, becoming the the weather data from the nearest meteorological pupal case, or puparium. Inside the case the larva station to where the body was found. metamorphoses into an adult fly, which eventually A more accurate estimation can be made by breaks out of the pupal case, completing the life placing an electronic datalogger at the place where cycle (Figure 2). the body was found for a period of time (typically 7-10 days) and comparing these temperatures with those of the local meteorological station over the same period of time. This is called a regression analysis, and the resulting equation can then be c used to estimate what the temperatures at the b deposition site would have been in the period before the body was found, i.e. during the time a when the blowflies were developing on it. d f e Figure 3 Pupa of the bluebottle fly, Calliphora vicina, dissected from its puparium. Larval (or maggot) masses When the larvae reach the end of the 2nd instar, and throughout the 3rd instar stage, they congregate Figure 2. Stages of blowfly development:a eggs, together to feed in large masses (Figure 4). Their b 1st instar larvae, c 2nd instar larvae, d 3rd instar combined feeding activity generates heat, so the larvae, e puparia containing pupae, f adults. larval mass may be many degrees higher than the ambient (surrounding) temperature (Figure 5). In Effect of temperature colder climates, the larval mass temperature can be The rate of development of insects is almost up to 20°C or more above the ambient temperature, entirely dependent upon the temperature at which resulting in a faster rate of development of the they develop, i.e. the warmer it is the faster they insects. However, there is an upper temperature will develop, and the colder it is the slower they will threshold of about 42°C, above which the larvae develop. Therefore in a typical UK summer, when will die. If the blowflies found on the body have the temperatures are averaging 15°C, blowflies reached, or gone beyond, the late 2nd instar stage will develop from egg through the larval and pupal of development, then larval mass temperatures stages to adult flies in approximately 15 days. In a must be taken into consideration, or the PMI is typical UK winter, when the temperatures are well likely to be overestimated. before the body was below 10°C, dropping to below 0°C at night, it found, i.e. during the time when the blowflies were may take many weeks for blowflies to develop. developing on it. 2 Catalyst 44 42 40 38 44 Ambient 36 42 44 Right Head 4034 Temp 42 3832 Left Head 40 Ambient 3630 (ºC) 38 Right Head Right Side 34 Ambient Temp 28 36 32 Left Head Right Head Left Side 34 Temp3026 Right Side (ºC) 32 Left Head Genitalia 2824 30 Left Side (ºC)26 Right Side 22 28 Genitalia 24 Left Side 20 26 22 Genitalia 24 2018 22 18 20 8:00 8:30 9:30 9:30 9:00 9:00 8:00 8:00 8:30 8:30 8:30 8:03 17:00 18 16:00 17:00 16:15 16:30 16:00 16:00 16:00 16:00 16:00 8:00 8:30 9:30 9:30 9:00 9:00 8:00 8:00 8:30 8:30 8:30 8:03 17:00 16:00 17:00 16:15 16:30 16:00 16:00 16:00 16:00 16:00 07- 08- 08- 09- 09- 10- 10- 11- 11- 12- 12- 13- 13- 14- 14- 15- 15- 16- 16- 17- 18- 19- 07- 08- 08- 09- 09- 18:00 0- 10- 18:30 1- 11- 19:30 2- 12- 19:30 3- 13- 19:00 4- 14- 19:00 5- 15- 18:00 6- 16- 18:00 7- 18- 18:30 9- 8:30 8:30 8:03 17:00 16:00 17:00 16:15 16:30 16:00 16:00 16:00 16:00 16:00 Jul-JuJl-ul- JJuull- JJuul-l- JulJ-ulJ-ul-JuJlu-l- JJul- JJuul- l- JulJ-ulJ-ul- JuJul-l- Juul-l- JuJl-ul-Jul-JuJl-ul- JuJul-l- Jul-l- JuJl-ul-Jul-JuJl-ul- JJuull- Jul- Jul- Jul- Jul- Jul- 07- 08- 08- 09- 09- 10- 10- 11- 11- 12- 12- 13- 13- 14- 14- 15- 15- 16- 16- 17- 18- 19- 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04Jul- 0Ju4l- J0ul-4 Jul0- 4Jul- 0J4ul- J0u4l- Ju0l-4 Jul-04Jul- 0J4ul- J0u4l- Ju0l-4 Jul-04Jul- 0J4ul- J0u4l- Ju0l- 4Jul- 04Jul- 0Ju4l- J0ul-4 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 04 Date/TDimaete/Time Date/Time Figure 4 Larval mass, feeding on a body. Figure 5 Larval mass surface temperatures recorded on a human cadaver used during research study. Compare the blue, green and brown lines (larval Other factors to be considered masses) with the red line (ambient temperature). The difference between When we estimate PMI, we are actually estimating ambient and larval mass temperatures is greatest at 8:30am on 09-Jul-04. the time when flies oviposit on the body and, therefore, the minimum PMI. In other words, we estimate the minimum amount of time since the person has died, because the blowflies would not have been present prior to death, and as we don’t know exactly when the eggs were laid, we cannot give an actual PMI. There are many other circumstances within which the body is deposited, which will also affect the speed of oviposition and the speed of development of the insects. For instance, if a body is buried, submerged, wrapped or enclosed in some way, it may be either totally or partially inaccessible to adult flies which may delay or inhibit their ability Figure 6 Insect evidence in sealed forensic evidence bags.