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Forensic Entomology Blow Flies


Forensic Entomology

Blow Flies & Beetles

Here is some information about the development and appearance of some common forensically important insects. Insects have existed on earth for about 250 million years; comparatively humans have existed for about 300,000 years. Such an enormous amount of time has allowed insects to attain a wide diversity in both form and development. There are currently about 700,000 described species and it is estimated that there may be more than 10 million species of insects yet to be described. Some insects have evolved a gradual or “paurometabolous” development in which there is an egg that hatches into an immature or “nymph”, which resembles the adult form, but is smaller and lacks wings. In the forensically important insects, this is best represented by the cockroaches. However, most forensically important insects undergo a complete or “holometabolous” development. There is an egg stage (except for a few insects such as the flesh flies that deposit living larvae) which hatches into a larval form and undergoes a stepwise or incremental growth. This pattern is caused by the successive molts (shedding of the outer skin that has become too small) that the larva must undergo before it finally enters the inactive pupal stage. The pupa is simply the hardened outer skin of the last larval stage and the adult will develop inside of this protective skin.

Blow flies

In the insects that undergo complete development, the larval stages appear quite different from the adult form. The larvae of flies (order Diptera) that are commonly recovered from decomposing human remains lack functional legs, and the body of many species appears cream colour, soft-bodied, and quite “maggot-like”. For a picture of some of the maggots commonly found on human cadavers Once the larva or “maggot” is through feeding it will migrate away from the corpse in order to find a suitable site to form the pupal stage. The pupae of blow flies are often overlooked, as they closely resemble rat droppings or the egg case of cockroaches. The pupal stage is an extremely important stage to the forensic entomologist and a thorough search should be made for the presence of pupae at any death scene. If the adult insect has not emerged, the pupa will appear featureless and rounded on both ends. For a picture of an unmerged blow fly pupa If the adult insect has emerged, one end will appear as if it has been cut off, and the hollow interior will be revealed. For a picture of an adult emerging from the puparium Most adult blow flies appear a metallic green or blue and are easily recognizable. For a picture of two common adult blow flies

The red-tailed flesh fly, Sarcophagi haemorrhoidalis (adult female). Flies in the family Sarcophagidae deposit living first instar larvae. They “skip” the egg stage required by the blow flies. Therefore, large egg masses will not be visible on remains colonized by these flies. An adult female Sarcophagi fly deposits a fewer number of living first instar larvae than female blowflies do when depositing their eggs. Thus, in most cases, their overall abundance on decomposing remains is less than egg laying flies but their larger size makes them highly visible.

The Australian Cockroach

The nymph (or immature) is at the upper left, and the adult is on the lower right. Many roach species have strong mandibles that are capable of producing post-mortem damage to human skin that resembles abrasions or chemical burns. In many cases these abrasions appear as small pits in the skin. Additionally, roaches often consume human hair, and may frequently remove entire hair shafts during their feeding. The roach species most commonly encountered in forensic investigations are the American, Australian, and the German.

The secondary screwworm, Cochliomyia macellaria, (adult male). Although this species can be found throughout the United States, it numbers are being limited by the intense competition with the introduced species Chrysomyia rufifacies (see below).

One of the hairy maggot blow flies, Chrysomyia rufifacies (adult female). This species has been recently introduced into the United States. The larvae of this species are both predatory and cannibalistic. They often consume the larvae of other fly species and are commonly the sole species recovered from decomposing remains in the south-eastern United States.

Some common larvae or “maggots” found on human remains. Chrysomyia rufifacies (left), one of the hairy maggot blow flies, was introduced into the United States in the late 1980’s and has now established itself over the southern US from Florida to California. During the warm summer months, this species can be found as far North as Michigan. Hydrotaea aenescens (centre), the bronze dump fly, is common on decomposing remains when fecial material or exposed gut contents are present. Sarcophagi haemorrhoidalis (right), one of the red-tailed flesh flies, is a common occurrence on carcasses in the early and advanced stages of decomposition. This species typically arrives along with the early arriving green bottle blow flies, which they quickly outgrow. This species can be recovered throughout the year in the southern United States, and the larvae can live in moist semi-aquatic habitats that would be unsuitable to many other fly species.

The larvae shown here are from four different beetle families, and are representation of the body types that can be found in each of the respective families. The families are (from left to right) Dermestidae, Staphylinidae, Silphidae, Scarabaeidae. Larvae such as these are typically found later in the stages of insect succession, after the early arriving flies. The Dermestids are commonly called skin beetles and their larvae are commonly used in museums to clean bones of their associated tissues. Staphylinds (commonly called rove beetles) and Silphids (called carrion, sexton, or burying beetles) are both predaceous on maggots. Of lesser forensic importance is the Scarabs. Their larvae are commonly found on carcasses in the late stages of decay. While some species can be useful in post-mortem interval estimations, detailed developmental data is lacking for many species.

Death Scene Procedures: Scene observations and weather data Collection of insects from the body at the scene Collection of insects after body removal Shipment of collected insects to a forensic entomologist.

It is important to note that the collection of insects and other arthropods from a death scene may disturb the remains. Therefore, the forensic entomologist (or the crime scene personnel charged with making the collection) should contact the primary investigator and make plans for the collection of entomological evidence. Once a course of action as been determined, utmost care should be taken during insect collection so that the remains are disturbed as little as possible. Before collections are made notes should be taken as to the general habitat, ambient weather conditions, and location of the body. Observations should also be made to describe the microhabitat immediately surrounding the body.

Scene observations and weather data.

(1). Observations of the scene should note the general habitat and location of the body in reference to vegetation, sun or shade conditions, and its proximity to any open doors or windows if recovered within a structure. Locations of insect infestations on the body should be documented as well as noting what stages of insects are observed (such as eggs, larvae, pupae, or adults). It is also useful to document evidence of scavenging from vertebrate animals and predation of eggs and larvae by other insects such as fire ants. Observations such as these can be noted on the Death Scene Form.

(2). Collection of climatological data at the scene. Such data should include:

(a). Ambient air temperature at the scene taken approximately at chest height with the thermometer in the shade. DO NOT EXPOSE THERMOMETER TO DIRECT SUNLIGHT!

(b). Maggot mass temperature (obtained by placing the thermometer directly into the larval mass centre).

(c). Ground surface temperature.

(d). Temperature at the interface of the body and ground (simply place the thermometer between the two surfaces).

(e). Temperature of the soil directly under the body (taken immediately after body removal).

(f). Weather data that includes the maximum and minimum daily temperature and rainfall for a period spanning 1-2 weeks before the victims disappearance to 3-5 days after the body was discovered. Such information can be gathered by contacting the nearest national weather service office, or your state climatologist.

Collection of insects from the body at the scene

The first insects that should be collected are the adult flies and beetles. These insects are fast moving and can leave the crime scene rapidly once disturbed. The adult flies can be trapped with an insect net available from most biological supply houses. They are inexpensive and readily obtainable. Once the adult flies have been netted, the closed end of the net (with the insects inside) can be placed in the mouth of a “killing jar” (which is a glass container with cotton balls or plaster soaked with ethyl acetate, or common fingernail polish remover). The jar is then capped and the insects will be immobilized within a few minutes. Once they are immobile they can be easily transferred to a vial of 75% ethyl alcohol. Beetles can be collected with forceps or gloved fingers and placed directly into 75% ethyl alcohol. Proper collection vials (4 dram size) can be obtained here.

It is extremely important that the collected specimens are properly labelled. Labels should be made with a dark graphite pencil, NOT IN INK. The label should be placed in the alcohol along with the specimens, and alcohol can dissolve the ink from the paper! However, pencil is not affected by alcohol and should be used for labelling purposes. The collection label should contain the following information:

1). Geographical Location

2). Date and hour of collection

3). Case number

4). Location on the body where removed

5). Name of collector

Once the adults have been collected the collection of larval specimens from the body can begin. First the investigator should search for the presence of eggs, which are easily overlooked. After this step, the larvae should be readily apparent on the body. Generally speaking, the largest larvae should be actively searched for and collected. Additionally, a representative sample of 50-60 larvae should be collected from the maggot mass. These insects can be placed directly into a killing solution or ethyl alcohol. However, the specimens are better preserved if they are placed in boiling water for about 30 seconds. Obtaining boiling water at a scene is difficult, so boiling of the larvae upon returning to the proper facility is satisfactory. If the larvae are boiled with about 48 hours of initial preservation, a good specimen should result. It is important to note that some forensic entomologists prefer not to have the submitted larvae boiled. Therefore, the investigator should discuss preservation techniques with their cooperating entomologist. In any case the exact preservation techniques should be documented and forwarded to the forensic entomologist. If the body has more than one area of colonization (more than one maggot mass) each site should be treated separately.

Once the preserved collections have been made, duplicate samples should be made for live shipment. Living specimens can be placed in specimen containers or Styrofoam cups with tight fitting lids along with some moist paper towelling, or most preferably a food substrate such as beef liver or pork meat. Tiny air holes should be poked in the lid using an ice pick or similar instrument. This cup should be placed into a slightly larger container that has about 1/2 inch of soil or vermiculite in the bottom to absorb any liquids that may accumulate and leak. This entire container should be enclosed in an appropriate shipping container and shipped overnight to a forensic entomologist.

Collection of insects from scene after body removal

Many of the insects that inhabit a corpse will remain on, or buried, in the ground after the body has been removed. The steps listed above should be followed when collecting insects from the soil (i.e. both a preserved and a living sample should be taken). Soil and litter samples should also be taken both immediately under where the body was positioned, and from the immediate surroundings. It is not necessary to dig deeply. A good technique is to collect the leaf litter and debris down to the exposed upper surface of the soil, and then make a separate collection from about the first two or three inches of topsoil. Each soil collection area should be about 4-6 inches square, and be taken from underneath the head, torso and extremities. All soil samples should be placed in a cardboard container for immediate shipment to a forensic entomologist. These collections should be labelled and forwarded to the forensic entomologist along with the insects collected from the body.

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