Fluids reveal stick insect’s movement secret

The fluid released by stick insects has been found to help them control their movements rather than help them adhere to inverted surfaces, new study finds.  

A research team at the University of Cambridge studied the Indian stick insects species Carausius morosus and found that the fluid found on the insects’ feet does not help them adhere to vertical and inverted surfaces, as previously thought, but in fact helps them unstick their feet more easily allowing greater control over their sticking power.

“It has generally been assumed that the fluid on their feet must be involved in helping insects like stick insects adhere to a surface by capillary and viscous forces – in the same way that a beer glass will stick to a glass table if it's wet on the bottom but our research shows that the fluid is likely used for something else entirely - it may even help insects unstick their feet,” said PhD student David Labonte.

In the study, published in the journal Soft Matter, the team measured how much force was required to detach the foot of a stick insect from a glass plate at different speeds. Then they compared the sticking performance of wet and dry adhesive pads and revealed that there is a striking lack of differences between the two.

Labonte said: “Both wet and dry adhesive pads behave in a similar way to soft, rubbery materials in that, when they are pressed against another surface, there is a large area of contact between the two surfaces. Both pad types then rely on shear forces to control their stickiness: insect and gecko feet are much stickier when they are pulled towards the body.

“The fluid that insects have on their adhesive pads doesn't seem to increase the pads' stickiness by means of capillary or viscous forces, and the same may hold for the fluid on the feet of spiders and tree frogs.”

The scientists suggest the fluid released by stick insects acts as a lubricant to make detachment easier, giving insects greater control over adhesion at very short timescales.

“If you think of commercial adhesives, like Scotch tape, there are often bits of tape or residue left behind when you remove it quickly. But a stick insect needs to be able to unstick its feet without expending a lot of energy or leaving bits of its foot still stuck to a leaf,” said research associate Dr Walter Federle from the University of Cambridge's Department of Zoology.

The team hopes their findings will help the production of small electronic devices, where scientists need to pick up and place down tiny parts with ease and accuracy.

Paper: http://pubs.rsc.org/en/content/articlepdf/2015/sm/c5sm01496d