Insect Olfaction and Decomposition 1121 Insect Olfaction

Insects must locate food sources in order to sustain life, obtain energy and gain nutrients required for the production of offspring. This is achieved by means of efficient sensory processes and behavioural mechanisms that are mediated by external and internal stimuli (Agelopoulos and Pickett 1998). Insects use chemical signals to navigate through their environment. They are able to quickly process the information within an odour plume coming from a source, such as a decomposing body. Generally, insects use different sensory perceptions to locate food, a mate, an oviposition site, and detect danger (Cragg and Cole 1956; Borror et al. 1989; Castner 2001). The cues can be visual, auditory, olfactory, gustatory, and physical and each is likely to play a role in the induction of a series of behaviours, leading to the successful location of the source of interest. The most dominant cues used by insects is considered to be olfactory stimuli. Most insects have a highly developed olfactory system and use it to detect volatile chemicals.

The capacity to detect and respond to volatile chemicals present in the environment exists in nearly all living creatures; however, this ability is particularly important in insects (Vickers 2000). Insect olfactory organs involved in the response to volatile chemicals are located primarily on the antennae (Borror et al. 1989). Often in nature the morphology and position of chemosensory appendages, such as the antenna, may help determine its importance and efficiency in capturing chemical cues. For example, most insects possess long, movable antennae which provide greater capacity to detect volatiles without requiring the insect to re-position its body frequently to detect an odour (Vickers 2000). These evolutionary features indicate that chemical cues play an important role in insect behaviour and survival.

Insect antennae are covered with a large number of sensillae (Castner 2001; Shields and Hildebrand 2001). Each sensillum houses olfactory receptor neurones (ORN) which detect volatile chemicals (Shields and Hildebrand 2001). The chemicals enter through pores on the sensilla where they are transported across the sensil-lum lymph by odorant binding proteins to the dendrites of the olfactory neurones (McIver 1982) (Fig. 11.1). The sensory neurones input information directly into the central nervous system and this induces a behavioural response in the insect

Fig. 11.1 Sensillum. V = volatile compound, BP = binding protein, R = receptor

(Hansson 2002; Zhou et al. 2004). Most insects respond not only to single compounds, but also to mixtures of compounds. With the correct combination of sensory inputs, animals or plants are recognised as hosts (Bruce et al. 2005). Insects also detect chemical gradients, giving them vital information about the location of an odour source (Vickers 2000). They detect volatile chemicals that indicate host suitability and also the presence of potential predators or competitors (Pickett et al 1998; Shields and Hildebrand 2001). As this chapter explains, even the state of decomposition of a body is revealed through the volatiles released. These volatile "signals" are also called semiochemicals.

11.2.2 Semiochemicals

The word "semiochemical" is derived from the Greek word simeon, which means 'sign' or 'signal' (Agelopoulos et al. 1999). Semiochemicals are volatile in nature and when airborne, they can be detected from long distances and potentially perceived by a number of other organisms of the same or different species (Agelopoulos and Pickett 1998; Selby 2003). Semiochemicals convey information between organisms and can be classified into two groups, pheromones and allelochemicals, according to the effect produced on the receiver or emitter (Nordlund and Lewis 1976; Blight 1990). Pheromones are chemicals which cause interactions between individuals of the same species (intra-specific) such as those that initiate behaviours

Fig. 11.2 Classification of semiochemicals (Nordlund and Lewis 1976; Howse et al. 1998)

such as mating; while allelochemicals create interactions between different species (inter-specific) (Agelopoulos et al. 1999) (Fig. 11.2). Semiochemicals are often perceived by the receiver beyond its visual range (Gikonyo et al. 2003) and a behavioural response can be triggered with only very small quantities of chemicals (Cork et al. 1990). Some volatiles are released in such small quantities, in fact, that they are barely detectable by the most advanced analytical techniques (Zumwalt et al. 1982); however, these can still be detected by insects.

The successful location of a plant or animal host by an insect is reliant on its ability to detect semiochemicals that give information about host suitability or physiological state (Pickett et al. 1998). For example, female mosquitoes (Diptera: Culicidae) detect odours such as carbon dioxide, ammonia and lactic acid from their animal or human hosts and these chemicals are of major importance in the successful location of an appropriate host in order to obtain a blood meal (Blackwell et al. 1992; Takken and Knols 1999). Semiochemicals may also be used to locate a suitable oviposition site or mate. For example, volatiles released during decomposition of a body allow blowflies to find the carcass, thereby increasing its chance of finding a suitable oviposition site, a mate, and food for their offspring (Smith 1986). Additionally, olfactory stimuli may function in combination with other stimuli, for example, oriental fruit moths, Grapholita molesta (Lepidoptera: Tortricidae), are unable to remain orientated when placed in a visually diminished "blank" environment containing a chemical attractant, implying that a visual cue is vital for site location (Vickers 2000). Similarly, the blackfly, Simulium arcticum (Diptera: Simuliidae), relies heavily on visual cues, such as shape and colour, to locate a host at close range and, therefore, responds not only to the CO2 being released by the living host (Sutcliffe et al. 1991).

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