Attachment device of male diving beetles: functional morphology and dynamics

• Main Authors: Ying Chen, 陳瑩
• Other Authors: Kai-Jung Chi
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/71735204142965784366

碩士 === 國立中興大學 === 生物物理學研究所 === 100 === Male diving beetles have specialized adhesive setae to adhere firmly on the elytra of female for underwater mating without using glue or muscular control. Two types of setae are found in the palettes of Dytiscid beetles: sucker-like circular setae, and spatulate setae with proximal sucker from which parallel channels extended distally. Survey of museum specimen suggests that palette size increases with body size, but at a give body size, those with spatulate setae are smaller. To examine whether spatulate setae have better adhesive performance to compensate for smaller total contact area, we examined and compared the adhesive ability and functioning mechanisms of circular setae from Hydaticus pacificus and spatulate setae from Cybister rugosus. In either shape of setae, both adhesive force and shear resistance increase with load. The increase of adhesive force by the spatulate setae is more sensitive to load than circular ones. Though spatulate setae generate four times of adhesive force per unit area that by circular ones, total adhesive force relative to body weight provided by a diving beetle with spatulate setae is only 30% that with circular ones. Our observations of the attachment-detachment process reveal that the stalk of spatulate setae has mobile joints allowing sliding motion along the direction parallel to the channels, but that of circular setae does not permit rotation and directly transfers external force to the setal surface. We propose a “spring model” to describe the difference of two types of setae, and predict that with smaller spring constant and longer detaching time of sliding, less external force could be transferred to the surface of spatulate setae. We also propose a “pipe flow model” to explain the results that faster detaching velocity leads to greater adhesive forces. Consequently, the males could resist fast swinging of the females while detach easily with slow peeling motion. Lowest shear resistance toward proximal direction guides the seta to return to position for efficient detachment. In conclusion, we found the following features of spatulate setae: (1) greater adhesive force per area in the sucker part; (2) mobile stalk joints to reduce force transferred to the setal surface; (3) velocity control to adjust the adhesive force. Therefore, the less known spatulate setae of male diving beetles use special structures and mechanisms to improve attachment performance and detachment efficiency, so that even with smaller palette size the diving beetles with spatulate setae could still succeed in nature.