UMR CNRS, Université Paris Diderot, France.
J R Soc Interface. 2010 Dec 6;7(53):1745-52. doi: 10.1098/rsif.2010.0075. Epub 2010 Apr 28.
Pads of beetles are covered with long, deformable setae, each ending in a micrometric terminal plate coated with secretory fluid. It was recently shown that the layer of the pad secretion covering the terminal plates is responsible for the generation of strong attractive forces. However, less is known about the fluid itself because it is produced in an extremely small quantity. We present here the first experimental investigation of the rheological properties of the pad secretion in the Colorado potato beetle Leptinotarsa decemlineata (Coleoptera, Chrysomelidae). Because the secretion is produced in an extremely small amount at the level of the terminal plate, we first developed a procedure based on capillary effects to collect the secretion for rheological experiments. In order to study the collected fluid (less than 1 nl) through passive microrheology, we managed to incorporate micrometric probes (melamine beads) that were initially in the form of a dry powder. Finally, the bead thermal motions were observed optically and recorded to determine the mechanical properties of the surrounding medium. We achieved this quantitative measurement with the collected volume, which is much smaller than the usual 1 µl sample volume required for this technique. Surprisingly, the beetle secretion was found to behave as a purely viscous liquid, of high viscosity (about 100 times that of water). This suggests that no specific complex fluid behaviour is needed by this adhesive system during beetle locomotion. We describe a scenario for the contact formation between the spatula at the setal tip and a smooth substrate, during the insect walk. We show that the attachment dynamics of the insect pad computed from the high measured viscosity is in good agreement with the observed insect pace. We finally discuss the consequences of the viscosity of the secretion on the insect adhesion.
甲虫的垫状物上覆盖着长而可变形的刚毛，每个刚毛的末端都有一个涂有分泌液的微尺度末端板。最近的研究表明，覆盖末端板的垫状分泌物层负责产生强大的吸引力。然而，对于这种分泌物本身，人们知之甚少，因为它是在极其微量的情况下产生的。本文首次对鞘翅目马铃薯甲虫 Leptinotarsa decemlineata 的垫状分泌物的流变特性进行了实验研究。由于分泌物是在末端板水平上极少量产生的，我们首先开发了一种基于毛细作用的程序来收集分泌物进行流变实验。为了通过被动微流变学研究收集到的流体（小于 1 nl），我们设法将最初呈干粉形式的微尺度探针（三聚氰胺珠）纳入其中。最后，通过光学观察和记录来观察珠粒的热运动，以确定周围介质的机械性能。我们通过收集的体积实现了这种定量测量，该体积比该技术通常需要的 1 µl 样品体积小得多。令人惊讶的是，发现甲虫分泌物表现为一种纯粘性液体，具有高粘度（约为水的 100 倍）。这表明在甲虫运动过程中，这种粘性系统不需要特定的复杂流体行为。我们描述了在昆虫行走过程中，在刚毛末端的匙形结构与光滑基底之间形成接触的情况。我们表明，从高测量粘度计算出的昆虫垫的附着动力学与观察到的昆虫步速非常吻合。最后，我们讨论了分泌物粘度对昆虫附着的影响。
