Quirks and Quarks

How an oddly large beetle penis could inspire new medical technology

The beetle's penis is soft as 'pudding' at tip, but stiff at the base.
Thistle Tortoise Beetle (Cassida rubiginosa) (Pavel Kirillov/Wikimedia Commons)

A penis longer than its own body

The thistle tortoise beetle is a tiny green insect about seven to eight millimetres long with a penis longer than its own body. The penis stays coiled up inside the body until it's time to mate with a female. The female's reproductive duct is a series of twists and turns, like a telephone cable, which the male needs to penetrate with its penis. 

Confocal laser scanning microscopy image of the flagellum tip of C. rubiginosa. (Science Advances)

Stanislav Gorb, a professor of zoology from the Zoological Institute at the University of Kiel in Germany, is the senior author of the study that looked into the mechanical properties of the penis. "The penis has to go from the left to the right, and then at some point, right to the left, and change the direction several times," says Gorb. "A female makes life for this male penis pretty difficult. And that's why the nature found a solution for that in the form of mechanical gradient of properties." 

How they tested it

Gorb says because the beetle and the penis are so small, it was very difficult to study under the microscope. In preparing the penis for testing, they had to first learn to separate it from the surrounding tissue while keeping it fresh. Then hung the penis over two microscopic needles. And with a mechanical sensor they developed in conjunction with a micromanipulator, they pushed the sensor along different segments of the entire penis to measure its mechanical properties.

They discovered the penis had a gradient of mechanical properties to allow it to snake into the female's complicated reproductive duct. Then Gorb says they analyzed what type of material along the penis produced this gradient. "We find out that at the tip fo the penis, there is a very special protein," says Gorb. "If you wish, this is a kind of natural rubber. It is a rubber like material, which is in fully hydrated condition, can be as soft as pudding, in fact. And the more we go to the base of the penis, the less this material is present there."

(Yoko Matsumura/Science Advances)

That makes the base of the penis more stiff to give the beetle pushing power when penetrating the female. 

Potential medical applications

In their scientific paper, Gorb says the results could provide hints on how to improve catheter design. If catheters or even endoscopes are too flexible, they can bend and make it difficult to penetrate with any accuracy. Too stiff, and they can break or do damage. 

"We searched some literature and we found out there is still some problems to make a very long catheters," says Gorb. "We think that these kind of properties [of the beetle's penis] would be really desirable. And nowadays, it is not a big to make polymers with a gradient of mechanical properties."