Carrion Insects and Semiochemicals

Different species of carrion insect have developed different seasonal life-cycle development times, variations in habitat, and preferences in carcass size, host species, and decomposition stage due to the high levels of interspecific competition (Fisher et al. 1998). Some, like the blowfly Lucilia sericata (Diptera: Calliphoridae), are able to develop on live sheep and other warm-blooded vertebrates as well as in carrion (Fisher et al. 1998). Such strategies greatly increase the chance of survival of their offspring.

Carrion odour contains a wide range of chemicals. Volatile molecules appear almost immediately after death but human olfactory perception is too insensitive to detect such short-term degradation. However, carrion insects have evolved to detect olfactory stimuli from the corpse, even at the early stages of decomposition. Those semiochemicals emitted from decomposing bodies are classified as 'apneumones'. These are chemicals which are emitted by non-living material, such as a corpse, and evoke a behavioural reaction from the receiver (see Fig. 11.2). The word apneumone was derived from the Greek word a-pneum, meaning breathless or lifeless. This category was originally described by Nordlund and Lewis (1976) and has since become increasingly important when discussing carrion insects.

It is likely that chemicals emitted from a decomposing body may provide information about its location and suitability as a host and may even provide signals to their predators. It is well documented that insect succession occurs on a decomposing body and different species of insect are attracted at different stages of decomposition. While the production of volatile chemicals is caused by the decomposition of bodies due to bacterial and enzymatic activity, many of the volatiles released are as a result of the action of carrion insects on the corpse. While this is still in the early stages of investigation, other areas of research have revealed important findings that could be relevant to forensic entomology. For example, there are many examples in plant-insect interactions where mechanical damage caused by insect colonisation or feeding can alter the semiochemical profile of the host plant (Dewhirst and Pickett 2009). When plants are attacked by aphids (Hemiptera: Aphididae) the volatile profile of the plant is altered, and this profile can even be specific to a particular aphid species (Du et al. 1998). In such cases, plants release herbivore-induced signals that can alert and attract predators (e.g. parasitoids) to the plant, which indirectly protect plants against herbivory. An example of this can be seen when the pea aphid, Aphis pisum, attacks the broad bean, Vicia faba. Levels of attractive volatile chemicals (including 6-methyl-5-hepten-2-one, linalool, (E)-^-farnesene and £)-ocimene) increase following aphid feeding (Du et al. 1998). Similarly, lima bean plants infested with spider-mites Tetranychus urticae (Prostigmate: Tetranychidae), release an odour which attracts the spider-mites' natural predator, the predatory mite Phytoseiulus persimilis (Mesostigmata: Phytoseiidae) (Sabelis and van de Baan 1983; Dicke and Sabelis 1988). Similar instances of predation or parasitism are also witnessed on decomposing bodies. For example, the parasitoid wasp, Alysia manducator (Hymenoptera: Braconidae), is a common parasitoid of the blowfly, Calliphora vicina (Diptera: Calliphoridae), and will lay eggs on the fly larvae during specific stages of their development (Reznik et al 1992). Nasonia vitripennis (Hymenoptera: Pteromalidae), is another common parasitic wasp which comes to a decomposing body to lay eggs on larvae of the Muscidae (Order: Diptera). Their evidential value as indicators of time since death has been explored by Grassberger and Frank (2003) with some degree of success. Others, such as the yellow jacket wasp (Hymenoptera: Vespidae), the beetle Necrodes littoralis (Coleoptera: Silphidae) are predators of the adults blowflies and many Dipteran larvae, respectively (LeBlanc 2008). However, in these instances, the chemicals being released, attracting the predators and parasites, or their sources have not yet been identified. Whether it is volatiles from the decomposing body or those produced by the insects themselves which are used by the insects is still unknown.

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