Abstract
Stick insects (Carausius morosus) have two distinct types of attachment pad per leg, tarsal “heel” pads (euplantulae) and a pre-tarsal “toe” pad (arolium). Here we show that these two pad types are specialised for fundamentally different functions. When standing upright, stick insects rested on their proximal euplantulae, while arolia were the only pads in surface contact when hanging upside down. Single-pad force measurements showed that the adhesion of euplantulae was extremely small, but friction forces strongly increased with normal load and coefficients of friction were 1. The pre-tarsal arolium, in contrast, generated adhesion that strongly increased with pulling forces, allowing adhesion to be activated and deactivated by shear forces, which can be produced actively, or passively as a result of the insects' sprawled posture. The shear-sensitivity of the arolium was present even when corrected for contact area, and was independent of normal preloads covering nearly an order of magnitude. Attachment of both heel and toe pads is thus activated partly by the forces that arise passively in the situations in which they are used by the insects, ensuring safe attachment. Our results suggest that stick insect euplantulae are specialised “friction pads” that produce traction when pressed against the substrate, while arolia are “true” adhesive pads that stick to the substrate when activated by pulling forces.
Highlights
Many insects are fast runners and skilful climbers [1,2,3,4]
The arolium surface appears smooth under low magnification, but scanning electron microscopy (SEM) revealed the presence of fine folds running mainly along the proximal-distal axis of the pad (Fig. 3B and [21,32])
Our results show that the two attachment pad types of stick insects, arolia and euplantulae, are specialised to serve fundamentally different functions: First, arolia are ‘‘true’’ adhesive pads, whereas euplantulae are ‘‘friction pads’’ that mainly produce friction, but negligible adhesion
Summary
In order to allow climbing insects to forage efficiently and to escape rapidly from predators, conflicting demands must be met: attachment forces must be firm and reliable, but voluntary detachment should be fast and require little energy. Variation of attachment structures within insect orders might represent adaptations for climbing in different natural environments [1,8,17]). Only cockroach species with prominent ‘‘toe’’ (arolia) and ‘‘heel’’ pads (euplantulae) were able to climb on smooth surfaces [1,8]. Attachment forces of arolia were maximal when pulled towards the body, while euplantulae generated maximum forces when pushed away from it [15,18]. Legs above the centre of mass of vertically climbing cockroaches pulled using only the arolia whereas legs below the centre of mass pushed with the euplantulae in contact [15]
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