The antennae of the silkmoth

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Insects posess a pair of antennae that are appendages on the head. In most insects, the antennae are very important as they bear sensory organs detecting almost any non-visual modality: odor, taste, mechanical stimuli, temperature, and humidity. Antennae have a bewildering diversity of shapes adapted to the requirements of different insect species and can also display sexual dimorphism. The antennae are composed of three parts, the scapus, pedicellus, and flagellum. Usually, the distal part, the flagellum is most prominent. In the silkmoth, the flagella are comb-shaped (Figure 2A) and the antennae of the male are somewhat larger than those of the female. The shape can be thought of as an adaptation to optimise odorant catch. It has been shown that 27% of the odorant molecules from an air stream passing though the combs are adsorbed on the surface of the flagellum（Kaissling, 1987).

Sensory ogans on the antennae are called sensilla. Especially the flagellum is covered by many types of sensilla of which an example is shown (Figure 1B, 2). A sensillum contains sensory neurons and 3 types of sheath cells that surround the internal structures of the sensillum (Figure 1C). In the sensory neurons, a stimulus generally converted into an electrical event at the level of the dendrite, attached to the cell body that often lies outside the sensillum proper. The sensory neurons are bipolar, the other process emanating from the cell body being the axon, that projects all the way into target areas in the brain.

Sensilla can usually be classified by their external appearance. For example, olfactory sensilla generally have small pores or slits in their cuticular envelope. These opening provide an interface between the inside and the outside to allow the exchange of odorant molecules. On the silkmoth antennae, five different types of sensilla have been identified: olfactory s. trichodea, s. basiconica, s. coeloconica (Kaissling et al., 1978; Pophof, 1997), putative gustatory s. chaetica (Steinbrecht, 1988) and s. styloconica, that are though to be thermo- and hygroreceptors (Steinbrecht, 1989). Most sensilla on silkmoth antennae are olfactory, in the male, about 70% of these are sensitive to sex pheromone. The s. trichodea sensitive to pheromone contain two sensory neurons of which one is sensitive to bombykol while the other is sensitive to bombykal (Kaissling et al., 1978). In females, the most abundant type of olfactory s. trichodeum contains two receptor cells highly responsive to two plant odours, linalool and benzoic acid (Heinbockel and Kaissling, 1996). Other olfactory sensilla also respond to plant odours (personal communication, but from whom?).

　　　　　　　　

Figure 1: External morphology of silkmoth olfactory sensilla (s. trichodea) by means of scanning electron microscopy (SEM). (A) External aspect of the head capsule and the antennae. (B) Structure of a branch of the flagellum bearing numerous sensillar hairs. Scale bar 25μm. ( C) Schematic representation of the structure of an olfactory sensillum trichodeum. [LEGEND DOES NOT FIT FIGURE]

　　　　　　　　　　　

Figure 2: Scanning electron micrograph of a long sensillum trichdeum from the flagellum of a male silkmoth (Bombyx mori) and specificity of the receptor cells. The female pheromone blend is composed of of the major component bombykol [(E, Z) -10,12-hexadecadien-1-ol] and the minor component bombykal [(E, Z) -10,12 -hexadecadien-1-al]. Bombykol alone induces full mating behaviour while bombykal has an inhibitory effect. The two sensory cells in the pheromone-sensitive sensilla show high specificity for only one molecular species each: bombykol or bombykal. They are mostly unresponsive to other compounds.


References

Pophof, B. Olfactory responses recorded from sensilla coeloconica of the silkmoth Bombyx mori. Physiolosical Entomology, 22, 239-248 (1997).

Hunger, T. and Steinbrecht, RA. Functional morphology of a double-walled multiporous olfactory sensillum: the sensillum coeloconicum of Bombyx mori (Insecta, Lepidoptera). Tissue & Cell, 30, 14-29 (1998).

Steinbrecht, RA. An anomalous sensillum chaeticum with a double set of cilia and outer dendritic segments in Bombyx mori L. (Lepidoptera: Bombyxcidae). Int. J. Morphol. Embryol., 17, 83-87 (1988).

Steinbrecht, RA. The fine structure of thermo-/hygrosensitive sensilla in the silkmoth Bombyx mori: Receptor membrane substrate and sensory cell contacts. Cell Tissue Res. 255: 49-57 (1989).

Kaissling, K.-E. R. H. Wright Lectures on Insect Olfaction, ed. Colbow, K. (Simon Fraser Univ., Burnaby, 1987).

Kaissling, K.-E, Kasang, G., Bestmann, H.J., Stransky, W. and Vostrowsky, O. A new pheromone of the silkworm moth Bombyx mori: sensory pathway and behavioral effect. Naturwissenschaften 65, 382-384.

Heinbockel, T. and Kaissling, K.-E. Variability of olfactory receptor neuron responses of female silkmoths (Bombyx mori L.) to benzoic acid and (±)-linalool. J. Insect Physiol. 42, 565-578.