How Beetles Operate Penises Longer Than Their Bodies


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

Beetles bonking
Tortoise beetles (Cassida rubiginosa) putting their extraordinary genitalia to good use. Yoko Matsumura

If you have a voyeuristic interest in the extreme forms of animal sex, you, like the creator, should have an “inordinate fondness for beetles”.  Some beetles have penises several times longer than their bodies, which raises many uncomfortable questions, some of which have now been answered by scientists anxious to tackle the big issues.

Beetle penises have been compared to Swiss army knives, but that understates their complexity. Below is a photograph of the appendage of the cowpea weevil (Callosobruchus maculatus) under an electron microscope.


Many beetle species are only distinguished from each other by the exact shape of their genitalia – a few extra bumps here and there and representatives from closely related species become unable to do the deed.


Johanna Rönn, Department of Animal Ecology, Uppsala University/National Geographic CC BY-SA 1.0 via wikimedia commons

The scale of these appendages raises obvious questions, such as “why?” “how?" and “is it fun?” The last one might prove trickier to answer, but Dr Yoko Matsumura of Kiel University, Germany, has provided some answers on the first two at the Society for Experimental Biology Annual Meeting in Brighton.

"The female 'duct' may be very long and so ensures that only the longest penises get to fertilize her eggs," Matsumura said in a statement.  "Another benefit of having a long penis found in some other insects is that a male is able to efficiently 'scrape' the female's duct free of other males' sperm, which would compete with his own."


However, there is a price to pay for developing such an enormous tool. By having beetles mate in tiny versions of CAT machines, Matsumura observed that the penises don't stay hard all the way to the end, instead having muscles around the base that force the stiff bottom end in. "The long ducts in females slow down penetration, so the time-pressured males have co-evolved to counteract this obstacle,” Matsumura said. “After running computer simulations, we think that this composition is the fastest at penetrating the female duct, so it appears softer tips are better."

For those who don't believe that science is worth funding for understanding or wonder alone, Matsumura thinks there may be medical applications for her work. “I think that knowing how to precisely control a narrow tube in a duct could help develop harmless catheters or injections in the medical world,” she said

The study of beetle genitalia has been a challenging one for scientists, since the appendages in question are not only small, but hard to see. The penises are held internally when not in use, which includes after the beetle dies. To make examination possible, the CSIRO invented an instrument called the phalloblaster, which injects a mixture of alcohol and compressed air, to give dead beetles erections.

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