Sex Pheromones in the Insects
Sex pheromones are chemicals or odours given off by an individual in order to invoke a sexual response or behaviour change on individuals of the same species (Shorey 1973). These chemicals can be released by males and/or females, depending on the species (Gieselhardt et al. 2008, Ayasse et al. 2001). Pheromones generally consist of a carbon backbone of between 10 and 20 carbon atoms (in Lepidoptera), these are however arranged to form multitudes of different compounds, ranging from methyl groups and alcohols to fatty acids and acetates. These include isomers and stereoisomers of the same compounds allowing for a large diversity and chance for high species specificity (Raina 1993, Ayasse et al. 2001, Roelofs and Brown 1982, Lanier 1970). Sex pheromones have been studied in greater detail over recent years, but most historical work was done on Hymenoptera and Lepidopteran species. However, work has been done on Coleoptera, Dictyoptera and Mantodea(Perez 2005, Ayasse et al. 2001). Pheromones are released from many anatomical sites of the insect body. Some of the more obvious and common places include in pheromone glands between the 8th and 9th segments of the lepidopteran abdomen and the aedaegal glands of most insects (Gieselhardt et al. 2008, Roelofs and Brown 1982, Ayasse et al 2001). There are many other, more unexpected, locations at which pheromones are released or stored. These include the antennae of many parasitic wasps (Isidoro et al. 1996), the cuticle of insects in the form of lipids (Jurenka et al. 2007), venom glands of ants and mandibular glands of some flightless bees (Ayasse et al. 2001). Detection and tracking of pheromones
Other location mechanisms include the orientation of an insect towards the source point of a pheromone. In the Desert Tenebrionid Beetle, once the male detects the sex pheromone of the female, he stops and walks in a circle in what seems to be an orientation to the source of the scent (Geiselhardt et al. 2008). This behaviour seems to be a relatively common method for determining pheromones in walking insects. Flying insects have another way of locating the source of a sex pheromone. This method entails an insect flying in a zigzag pattern towards the source of the pheromone, narrowing the track of the zigzag as it approaches the source. This has been documented in many moth species and most recently in the Cowpea weevil by Kuenen and Rowe (2006). In order to track a source point of a pheromone, recent research by Baker et al. (2008) postulates that pulsing release of pheromones, a common observation in many species, is necessary in order for receptor mechanisms not to be over run by large amounts of a particular stimulus. This could be corroborated by the research done on half life of certain sex pheromones (mainly bombykol) by Vogt et al. (1985), showing that there is a decrease in half life of bombykol once it has come into contact with degrading enzymes on the antennae of moths. Should there, however, be a constant flow of pheromone, especially from distance, there could be a loss of orientation of an individual in a “cloud” of pheromone. Effects of sex pheromones
Sex pheromones can have many different effects on conspecifics. These however can be separated into two major groups; those that aggregate the opposite sex around the pheromone releasing individual, and those that start sexual courting behaviour. Attractant pheromones are released, generally in higher quantities than the courting ones, to cause either an aggregation of mates around the source point, to go hand in hand with calling behaviours or attracting mates in lower population density areas (Baker et al.2008). With courting pheromones, the smaller amounts of pheromones are released and change the behaviour of the receiving insect (at a shorter distance) to a more sexual behaviour, initiating courting behaviour and eventually...