Watch a spider or lizard scuttle across a ceiling, and you can't fail to be amazed by their gravity defying antics. But how insects and lizards remain attached is a hot topic in the world of adhesives. Somehow they anchor themselves to surfaces, yet are able to detach with ease to move freely:engineers and physicists would love to know how they do it. Walter Federle from the University of Cambridge explains that many insects' tarsal pads (pad like structures that insects stand on) are covered in microscopic hairs. When an insect lands splay legged on a wall or ceiling, they pull their legs inwards, towards their body, in such a way that the tips of the microscopic hairs lock on to the surface. When it starts walking, the insect simply stops pulling the leg inwards, allowing the hairs to detach before lifting the leg. But what happens when a beetle wants to clamber up a vertical surface? Instead of pulling towards the insect's body, its back legs appear to be pushing away from its body. They should detach, but they don't. Curious to find out how climbing insects scale walls without losing their footing, James Bullock and Walter Federle put climbing leaf beetles through their paces(p. 1876).
First the team took a close look at the three hairy tarsal pads at the ends of males' and females' legs. They found three hair types on the males' tarsal pads (pointed, disc shaped tips and spatula shaped tips) and two on the females' tarsal pads, which lacked the disc tipped hairs.
Next the duo filmed the insects' footwork through a microscope as they climbed up and down a smooth vertical surface. They found that the beetles were using the three tarsal pads on their front and rear legs completely differently depending on whether their feet were above or below their bodies as they climbed. The beetles mainly contacted the wall with the third tarsal pads on the front two legs (above the body) when ascending, as if walking on their toes. Looking at the insects' rear legs (beneath the body), Bullock and Federle could see that the climbers were mainly walking on their `heels',contacting the wall with the first tarsal pad as they pushed themselves upwards. And when the beetles turned around and descended head first, they used the same foot contact pattern: the forelegs (beneath the body) contacted the wall via the heel pad, while the feet above the body (rear) hung on to the surface by the toe pads.
Next the team investigated the stiffness of the microscopic attachment hairs on each pad. Knowing that the best adhesives are very soft to allow them to mould to surfaces, Bullock and Federle measured the rigidity of the attachment hairs on each tarsal pad with a force transducer, and were surprised to find that the material they are made from is incredibly stiff,probably to protect them from wear and tear. However, each hair is extremely flexible because they are very thin, and the hairs on the third (toe) pads are softer than the first (heel) pad hairs, which makes the third pads good at sticking to rough surfaces, while the stiffer first pad hairs are good for pushing.
Finally, the duo measured the attachment forces generated by each pad on rough and smooth surfaces and found that the soft toe pads locked on tighter to rough surfaces. However the stiffer heel pads grip on more tightly than the toe pads when being pushed away from the body, which is exactly what they need to do when beetles climb a wall.