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Mosquitoes Walk On Water Thanks To Its Leg Segment Known As Tarsus

A new study has provided more insight into the effortless manner in which insects like mosquitoes and water striders are able to walk on water. The secret seems to lie in the leg segment known for its great flexibility called the tarsus. The findings have been published in the journal AOP Advances this week.

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Water strider walking on water

No creature has been given a burden that it cannot bear – we all have been endowed with what is needed for us to live. Similarly, mosquitoes have been equipped with body ‘apparatuses’ that allow them to go by their daily activities, much of which rely on the anatomy of their legs. A mosquito leg is composed of three parts known as segments – the femur, the tibia and the tarsus – that are covered with microscopic, hydrophobic scales. The femur starts from the abdomen, connecting to the tibia, which in turn is linked to the tarsus.

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The 3 main segments of the mosquito leg – the femur, the tibia and the tarsus – together with other parts of the mosquito’s anatomy

As can be seen in the picture above, the tarsus is longer and thinner than the other two segments. Furthermore, it is also flexible. The femur and the tibia, on the other hand, are short, thick and stiff.

The tarsus is the only part of the leg that gets into contact with water when the insect is walking on the surface.

The new study, unlike past ones, focuses on the individual segments instead of correlating the insect’s ability to walk on water with respect to the whole leg.

The team of scientists led by Jianlin Liu from China University of Petroleum set out to analyse the forces generated by the leg segments upon a water surface.

The researchers carried out their experiments on a Culex mosquito species known as the Culex pipiens pallens.

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A Culex mosquito feeding on a human host (Culex pipiens pipiens)

The picture below shows a mosquito on water with its legs in contact with water, with the tarsus in a horizontal position, while the tibia and the femur are barely touching the water (a), and a microscopic image of the leg (b):-

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Photo credits: Jianlin Liu/China University of Petroleum.

The methodology of the scientists required for them to stick the mosquito leg to a steel needle. The angle and force between the leg and the water surface were adjusted by the researchers with the help of a camera and a microscope.

According to their observations, the tarsus is the only leg segment that is responsible for the insect to walk on the water surface: the tarsus’s buoyant horizontal contact with the water surface is key to it. As the ultra-flexible tarsus sticks to the surface, the six legs generate an upward force 20 times greater than the body weight of the insect.

Led author Liu said in a press release:

“This finding overthrows the classical viewpoint that the longer the mosquito leg, the more efficiently it produces buoyant force”.

Furthermore, it was found that the amount of leg in contact with the water being reduced to the minimum causes the water’s adhesive force upon the insect to be greatly decreased which helps with takeoff.

The following pictures illustrates the sequence of the bottom half of the tarsus walking onto the water surface progressively (a-c). Pictures d to f show the opposite, with the whole hind leg that is not able to support the body weight:-

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Photo credits: Jianlin Liu/China University of Petroleum.

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