Prodpran Thakeow, Sergio Angeli, and Stefan Schütz
5.4.3. Distribution of the sensillar types
Sensilla distribution was measure all over the antennal surface, although more examinations were done over the last three antennal segments, since they bear a diversity of dense sensilla, their surface is larger then any other segments and strongly contribute to the odour coding of the species. No significant difference between males and females was observed in the density of any type of sensilla.
Short s. trichodea were regularly distributed on all the antennomers with increasing frequency on the last three antennomers. By counting on the microscope screen the numbers of sensilla on the ventral and dorsal surface of the antenna was determined to be approximately 60, 80, and 100 of the short s. trichodea for antennomer 8, 9, and 10, respectively.
Long s. trichodea were present only on the last three antennomers, occurring on the upper part of each antennomer, surrounding each octopus-like cluster. Often some of these long curved pegs were found to cover the octopus-like clusters as it visible in Figure 5-2C.
S. basiconica were arranged in clusters in the position described above as illustrated in Figure 5-5. No other basiconic sensilla were found on the entire surface of the antenna.
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Figure 5-2 Sensilla trichodea. Scanning electron micrographs displaying A: the sensillar distribution on the last segments of Cis boleti antenna. ST: short s. trichodea; LT: long s.
trichodea. B: basal part of a short sensillum trichodeum. WP: wall pores on the sensillar surface. C: sensilla on the apical part of the 9th segment showing a octopus cluster (OC) surrounded by the long sensilla trichodea (LT). D: high magnification of the terminal part of a long sensillum trichodeum (LT) showing the small longitudinal ridges on the surface.
BA: basiconic sensillum. Bar in A = 20 µm, in B = 1 µm, in C = 3 µm, and D = 1 µm.
5. Antennal morphology of C. boleti - 117 -
Figure 5-3 Sensilla basiconica. Scanning electron micrographs displaying the octopus-like clusters. Each cluster is formed by 9-14 basiconic sensilla. A: antennal apex showing the 4 octopus-like clusters (OC) on the distal part. B: distal part of the 9th antennomer showing one octopus-like cluster. C: distal part of the 8th antennomer showing one octopus-like cluster. D and E: high magnification of basiconic sensilla showing dense pores on the external wall surface. Bar in A and in B = 10 µm, in C and D = 1 µm, and in E = 300 nm.
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S. coeloconica were the less abundant sensilla, being present in few units on the upper part of the last three antennomers above or nearby each octopus-like cluster. However, it was difficult to number all the coeloconic sensilla, since they are relatively short and often hidden behind other sensillar types.
Figure 5-4 Sensilla coeloconica. Scanning electron micrographs displaying (A) the distal part of the 9th antennomer, (B) the high magnification of the 2 coeloconic sensilla located nearby octopus-like cluster. BS: basiconic sensilla; CS: coeloconic sensilla; LT: long trichodea sensilla; OC: octopus-like cluster. Bar in A and in B = 1 µm.
5. Antennal morphology of C. boleti - 119 -
Figure 5-5 Map representation of the distribution of four sensillar morphological categories found on the antennal surface of the adult of Cis boleti (Coleoptera: Ciidae): short trichoid sensilla, long trichoid sensilla, coeloconic sensilla and octopus clusters formed by 9-14 basiconic sensilla. . The distribution of the different sensilla was intended to reflect the real distribution on the antenna, e.g. the high concentration of figures reflects the high concentration on the antenna. Antennal form represents the direct proportion and shape among the different segments, where the entire antenna is approximately 840 µm long.
I II III
IV V VI VII VIII
IX X
dorsal ventral
short trichoid sensillum
long trichoid sensillum
coeloconic sensillum
octopus-like cluster
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5.5. Discussion
Here we report the first morphological investigation of C. boleti antenna. Prior to this study, no report on morphological structure of the adults of C. boleti antennal sensory organ has been done, to our knowledge. An investigation was performed on the sensory organs of the antennae and mouthparts of the larvae of C. boleti, in a comparative study of larval sensory organs of several beetle species (Alekseev et al. 2006). However, the larval sensilla are morphologically and functionally very different from the adult sensilla, therefore, no comparison is possible between our study and this previous one. According to a recent phylogenetic study among Coleoptera, the Ciidae belong to the Tenebrionoidea superfamily (Hunt et al. 2007). Therefore, we decided to compare C. boleti antennal structure with the one on Tribolium castaneum (Coleoptera: Tenebrionidae), which has a similar gross antennal morphology and a comparable body size. In the antenna of adult T.
castaneum only two types of sensilla were described: s. trichodea and s. basiconica, both supposed to mediate olfactory signals (Roth and Willis 1951). The distribution of the s.
trichodea of T. castaneum was comparable with the one found for the short s. trichodea of C.
boleti. Interestingly, the basiconic sensilla of T. castaneum were also found to be restricted to the apical ends of the club segments, although in this case they were arranged in a ring around a dense patch of slender trichoid sensilla. Coeloconic sensilla of the antenna of C.
boleti were very rare and no wall-pores were visible. In other insect species it is believed they have olfactory function (Steinbrecht 1999; Keil 1999), or they are considered as hygro- and thermoreceptors (Roux et al. 2005).
The most peculiar sensillar structures of C. boleti antenna are the octopus-like clusters.
These multi-sensillar plaques of basiconic sensilla have never been discovered before, to the best of our knowledge. The basiconic sensilla of C. boleti are likely comparable with the basiconic sensilla of T. castaneum, since they share similar topographic distribution (but the latter ones are not organised in clusters) and have a similar external morphology. The number and the position of these sensilla were found to vary among different species of Tribolium. In some cases two or three basiconic sensilla were found to diverge or to branch from the same cuticle structure, in analogy of our finding in C. boleti (Roth and Willis
5. Antennal morphology of C. boleti - 121 - 1951). There is a high diversification in coleopteran species, but we suggested that basiconic sensilla of T. castaneum and C. boleti may have a similar specialise function in odour coding. In other groups of insects, as for example in Lepidoptera, it has been reported that long trichoid sensilla respond to the insect’s pheromone rather to plant odour, while basiconic sensilla respond to the so-called general odours or green volatiles (Steinbrecht et al. 1996). C. boleti is not know to produce pheromones, but the presence only in the males of a gland setiferous organ on the 1st visible abdominal sternite may indicate that sex pheromones are secreted (Faustini and Halstead 1982). On the other hand, the lack of differences between the sense organs of males and female negate the possibility that the trichoid sensilla may act as sex-pheromone receptors. Determining which sensillar types are used by C. boleti to code host fungivorous volatiles, like the highly attractive 1-octen-3-ol, and pheromone components will require further electrophysiological research.
Acknowledgement
This experiment was carried out with kind help of Mr. Matthias Hahn, Institut für Materialphysik and Johann-Friedrich-Blumenbach-Institut für Zoologie & Anthropologie, Georg-August University of Göttingen.
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