Insects are champions of climbing even on the smoothest of surfaces thanks to their sticky footpads. The pads, found on different parts of their legs, can be smooth or hairy but both types allow the insects to attach equally well via a thin film of a liquid they secrete. Looking closely at the hairy pads, scientists have identified four distinct designs of hair tip, resembling spatulas, discs, lances or needles. But how do these tool-shaped hairs develop their different forms? Having previously investigated the hairs on the feet of fruit flies (Drosophila) and found that fibres of the protein actin form a scaffolding that contributes to their spatula-shaped hair tips, Ken-ichi Kimura and Naoe Hosoda, extended their research to the sticky structures on ladybird (Harmonia axyridis) limbs.
After collecting samples of legs from adult ladybirds and youngsters at specific life stages after moulting, the researchers examined them under a microscope. The resulting images revealed all four hair tip designs in the sticky attachment pads of each male's leg, while the female ladybird footpads contained only three, missing the disc-shaped hairs. The duo also pinpointed the location of each type of hair tip on the male footpads; finding spatula and lance-shaped hairs arranged around the perimeter of the pad, disc-shaped hairs located at the back of front-leg pads and the front of middle-leg pads, and needle-shaped hairs populating the remaining surface of the pads. Further imaging of the hairs also revealed that the hair shaft is hollow and surrounded by a socket located on the surface of the insects’ skin.
Next, the Japanese researchers focused on male ladybirds, setting out to identify how actin contributes to the development of each disc and needle-shaped hair. Using a dye that specifically targets actin, Kimura and Hosoda observed that 12 hours after moulting the footpads initially appeared flat, like terraces, with the hair sockets, hair shafts and finally hair tips progressively forming over the next 30 hours. The team was also curious to investigate the structure of the hairs beneath the insects’ skin, suspecting that they might be linked to neurons, as previously reported for some fruit fly hairs. This time using a dye that reveals neurons, the scientists found that there are in fact two types of hair: one associated with a neuron, which is thought to act as a sensor and give feedback to the insects as they move, and another without a neuron.
Knowing that actin is a key player in the formation of the tip structures of Drosophila sticky hairs, Kimura and Hosoda wondered whether the protein was also significant in the growth and development of the ladybird's attachment hairs. Comparing the growth of normal hairs with that of hairs that had been injected with a drug that prevents actin molecules from assembling and forming long chains, the duo revealed that in both cases there were bundles of the protein in the hair shaft. However, while actin bundles branched out to form a scaffold at the hair tip in untreated ladybirds, in the insects that had been injected with the interfering drug, the scaffolding bundles failed to assemble, leading to significant malformations of the hair tips.
Fruit flies and ladybirds both depend on the same mechanisms to produce the different tip designs that help insects hold on tight. Now that the authors have verified the importance of the actin scaffolding, they are investigating new avenues, fusing developmental biology and biomimetics in search of the ultimate ladybird-inspired adhesive.