Adventive and Incidental Species

In addition to those insects listed above, a myriad of other insect and arthropod species may be associated with decomposing carrion. These organisms are using the carrion for shade, shelter, or as a vantage point for predation. These arthropods could just as easily use any object in the environment, and do not have any intrinsic forensic significance. However, their presence may indicate an extended PMI (e.g., the presence of a spider web on animal carrion indicates an extended

PMI). As a group, the organisms that may be found on carrion but do not have any direct forensic application are called adventive species (Braack, 1986; Smith, 1986).

Any mobile terrestrial or airborne arthropod may accidentally land on or in the vicinity of carrion. These specimens may be collected along with the forensically important organisms, but do not have any forensic value due to their unpredictable arrival and accidental nature. These organisms are called incidental species as a group and are often identified and ignored during a forensic case (Smith, 1986; Byrd & Castner, 2010).

Important Insects in Decomposition

Diptera are commonly known as the true flies and are one of the largest and most diverse orders. The order name literally means “two-winged,” and refers to the fact that the hind pair of wings is greatly modified and reduced. There are approximately 120,000 species worldwide, with around 20,000 found in North America. Diptera are considered the most important order to human and animal health, due to their vector capabilities, and are the most important in forensics, since they often compose the bulk of the first sere.

The Diptera exhibit a complete metamorphosis, meaning they start out as eggs, hatch into the first of several larval instars, pupate, and eclose into adults. Most dipteran females lay eggs and are termed oviparous. Others retain the eggs internal until they hatch, and deposit early instar larvae into the environment. This is called ovoviviparous or larviparous. A very few groups retain the developing larvae in their bodies until the larvae are ready to pupate. These are called pupiparous flies.

The larvae tend to inhabit aquatic or semiaquatic environments during these immature stages. The number of larval stages, termed instars, varies with species. In general, flies have three or four larval instars before pupation. Once the larval stage has finished feeding, the immature fly will change in form and turn into the pupae. The pupal stage can last for a long while, and eventually the fully mature adult will emerge.

There are several families within the true flies that may lend important information to a case. These include Oestridae, Calliphoridae, Sarcophagidae, Muscidae, and Phoridae.

Oestridae

The Oestridae are commonly known as bot flies. They are obligate parasites of mammals, and show a high degree of host specificity, parasitizing only one species or a small group of related hosts. They also show a marked level of site specificity in a normal host, and the site of invasion by the first instar bot maggot generally is not the site of maggot development. Members of this family may be grouped based on their preferred feeding site, and fall into the New World skin bot flies, the Old World skin bot flies, the nose bot flies, and the stomach bot flies.

The larval stage, or the maggot, of Oestridae is the invasive form, and is a thick, grub-like organism covered with moderate to heavy spines (Figure 5.11). These larvae are obligate parasites and need a living animal to survive. They cannot complete their life cycle in a dead host; if the host dies, so does the bot fly. The exact time it takes a bot fly to die after host death has yet to be quantified, however. When exploiting the skin of a host, the bot fly produces an inflamed and painful swelling of the flesh called a furuncle. This furuncle is generally sterile, does not attract myiasis-producing flies, and is not usually a site for maggot development.