Awram and Free (1987a) noted that when the open mandibles were held either side of the leaf edge along which a male was progressing, the brush of hairs along the posterior edge of each mandible contacted the substrate and so may be especially important in pheromone transfer.
Usually at each site, it is possible to identify one or more objects or parts of objects which are visually conspicuous and visited by route ﬂying males. However, when a site is being scented such a focal point of visitation receives no more attention than much of the less conspicuous background against which it is located (Awram and Free 19872). For example the focal points for visits of B. hortorum males were usually bare ground between tree roots and irregularities at the base of tree trunks yet they scented leaves of plants near the bases of trees, and the leaves and bark of trees up to 3 m above ground. The focal points of visitation for B. lucorum and B. terrestris were conspicuous parts of trees, usually the stems of the branches, but they applied scent to the margins of leaves and to twigs which were often up to 2 m from the focal points. Learn more about pheromones at EzineArticles.com.
Haas (1946, 1949a) reported that males scent marked objects located in an increasing spiral away from the site visited and in the direction of the next site. Although it is agreed that scent deposition more or less surrounds the focal point, the presence of actual scent spirals has not been conﬁrmed by other workers (Svensson and Bergstrom, 1979; Awram and Free, 1987a). Indeed it is difﬁcult to envisage how a spiral pattern could be maintained in turbulent wind conditions and most sites appear to be physically unsuitable for tracing a complex pattern. Furthermore, males ﬂy in different directions when leaving a site so such a pattern would be superﬂuous as an indication of ﬂight route.
Chemical composition of site marking pheromone
The compositions of the pheromone used by males for site marking have been studied intensively, and components of the pheromone from no less than 33 species of Bombus and 8 species of Psithyrus have now been identiﬁed (e.g. Stein, 1963; Bergstrom et al., 1968; Calam, 1969, Kullenberg et al., 1970; Svensson and Bergstrom, 1977 and 1979; Bergstrom et al., 1981, Cederberg et al., 1983). Find out do pheromones work at http://pheromones-4u.com.
The pheromones consist of mixtures of fatty acid derivatives and/or isoprenoid components (terpenes). The blend of components present is species speciﬁc and each species usually has one very dominant component, sometimes one or a few major ones, and usually several minor ones (Tables 16.1 and 16.2).
Species belonging to the same sub-species tend to show greater chemical similarity than less closely related species. Indeed the distinction between pheromones of taxonomically allied species (e.g. B. lucorum and B. magnus; B. alpinus and B. polaris) sometimes appear to be based mostly on the different proportions present of the same components. The two different forms of B. lucorum differ in the proportions of dominant components; the main component of the dark form is ethyl dodecanoate and of the light form is ethyl tetradecenoate although each species produces both components (Bergstrom et al., 1973). However, in contrast the marking pheromones of morphologically allied species may sometimes have a largely dissimilar chemical composition; indeed, the chemical composition has even helped to clarify the taxonomic status of the different species (Bergstrom, 1981). Check out pheromones for men at Pheromones-Experts.com.
Despite the wealth of chemical knowledge that has been made available, comparatively few ethological studies of their functions have been made, and in no case is the exact use of all the different components in a secretion known. Some components may be releases of behaviour and others may merely decrease the volatility of the releaser substances.
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