Micronanostructures of the scales on a mosquito’s legs and their role in weight support

We show here that the mosquito cannot only give rise to a higher water-supporting force than the water strider if the ratio of the water-supporting force to the body weight of the insect itself is compared, but also can safely take off or land on the water surface, and also can attach on any solid surface like the fly. We found that the mosquito’s legs are covered by numerous scales consisting of the uniform microscale longitudinal ridges (nanoscale thickness and microscale spacing between) and nanoscale cross ribs (nanoscale thickness and spacing between). Such special delicate microstructure and/or nanostructure on the leg surface give a water contact angle of $\sim 153°$ and give a surprising high water-supporting ability. It was found that the water-supporting force of a single leg of the mosquito is about 23 times the body weight of the mosquito, compared with a water strider’s leg giving a water-supporting force of about 15 times the body weight of the insect.

Figure 1

(Color online) The hierarchical microstructures and nanostructures of the scales on a mosquito’s leg (the fifth section of the hind leg). (a) The numerous scales on the leg surface oriented along the leg; (b) the elaborate arrangement of the longitudinal ridges and the cross ribs. Note the scale’s theca as the (red) arrow points.

Figure 2

(Color online) (a) The test system used to measure the water-supporting force of a single leg of the mosquito. The test machine together with the steel bar and the needle can move vertically with a controlled speed of $1\mathrm{mm}∕\min$. The first section of the mosquito leg was glued to the tip of the needle. The initial angle of the leg with the water surface can be adjusted by changing the angle of the needle with the bar. The digital balance has a high precision of $0.1\mu \mathrm{N}$. (b) A typical curve measured for the water-supporting force versus the moving displacement of the machine after the leg just touches the water surface. The contact angle of a water drop on the surface of the mosquito leg is $\sim 153°$ (see the inset). The temperature in the laboratory is $20°\mathrm{C}$ and the relative humidity is about 50%.

Figure 3

The measured water-supporting force versus the moving displacement when the leg was vertically penetrating the water surface.

Figure 4

(Color online) The measured water-supporting force versus the moving displacement (dimple depth) when the cut leg was parallelly pushed down the water surface.

Figure 5

The micostructures and nanostructures of a mosquito’s foot: (a) the foot consists of two claws and two first level composite pads formed by hundreds of setae, together with some long tiny hair and scales covering the foot back; (b) the sideview of the setae. On the tip of the seta there is the second level of a tiny pad, the shape of which looks like a microman’s foot with length about $2–3\mu \mathrm{m}$, width about $1\mu \mathrm{m}$, and thickness varying from $\sim 200\mathrm{nm}\text{to}\sim 500\mathrm{nm}$. The diameter of the seta ranges from $200\mathrm{nm}\text{to}250\mathrm{nm}$.

Figure 6

(Color online) Microscope image of the hind leg of a fly. The setae on the fly leg surface are much larger than those on the mosquito’s leg, giving a space between about $50–120\mathrm{mm}$. The contact angle of a water drop on the fly leg surface ranges from $\sim 45°$ to $\sim 75°$, giving a hydrophilic property.