9.4. Insect — carrion interactions
In places where ants are important components of the fauna, the corpses of invertebrates are discovered and removed rapidly, by widely scavenging and efficient ants. In contrast, vertebrate corpses (carrion) support a wide diversity of organisms, many of which are insects. These form a succession — a non-seasonal, directional, and continuous sequential pattern of populations of species colonizing and being eliminated as carrion decay progresses. The nature and timing of the succession depends upon the size of the corpse, seasonal and ambient climatic conditions, and the surrounding non- biological (edaphic) environment, such as soil type. The organisms involved in the succession vary according to whether they are upon or within the carrion, in the substrate immediately below the corpse, or in the soil at an intermediate distance below or away from the corpse. Furthermore, each succession will comprise different species in different geographical areas, even in places with similar climates. This is because few species are very widespread in distribution, and each biogeographic area has its own specialist carrion faunas. However, the broad taxonomic categories of cadaver specialists are similar worldwide.
The first stage in carrion decomposition, initial decay, involves only microorganisms already present in the body, but within a few days the second stage, called putrefaction, begins. About two weeks later, amidst strong odors of decay, the third, black putrefaction stage begins, followed by a fourth, butyric fermentation stage, in which the cheesy odor of butyric acid is present. This terminates in an almost dry carcass and the fifth stage, slow dry decay, completes the process, leaving only bones.
The typical sequence of corpse necrophages, saprophages, and their parasites is often referred to as following “waves” of colonization. The first wave involves certain blow flies (Diptera: Calliphoridae) and house flies (Muscidae) that arrive within hours or a few days at most. The second wave is of sarcophagids (Diptera) and additional muscids and calliphorids that follow shortly thereafter, as the corpse develops an odor. All these flies either lay eggs or larviposit on the corpse. The principal predators on the insects of the corpse fauna are staphylinid, silphid, and histerid beetles, and hymenopteran parasitoids may be entomophagous on all the above hosts. At this stage, blow fly activity ceases as their larvae leave the corpse and pupate in the ground. When the fat of the corpse turns rancid, a third wave of species enters this modified substrate, notably more dipterans, such as certain Phoridae, Drosophilidae, and Eristalis rat-tailed maggots (Syrphidae) in the liquid parts. As the corpse becomes butyric, a fourth wave of cheese-skippers (Diptera: Piophilidae) and related flies use the body. A fifth wave occurs as the ammonia-smelling carrion dries out, and adult and larval Dermestidae and Cleridae (Coleoptera) become abundant, feeding on keratin. In the final stages of dry decay, some tineid larvae (“clothes moths”) feed on any remnant hair.
Immediately beneath the corpse, larvae and adults of the beetle families Staphylinidae, Histeridae, and Dermestidae are abundant during the putrefaction stage. However, the normal, soil-inhabiting groups are absent during the carrion phase, and only slowly return as the corpse enters late decay. The rather predictable sequence of colonization and extinction of carrion insects allows forensic entomologists to estimate the age of a corpse, which can have medico-legal implications in homicide investigations (section 15.6).