KIEL, Germany, July 7 (UPI) -- Thanks to laser scanning microscopes and mathematical models, researchers at Kiel University now have a better understanding of beetle sex.

For male thistle tortoise beetles, the key to successful reproduction, it turns out, is having just the right amount of penile stiffness.


The latest findings, detailed in the journal Scientific Reports, mark the first time scientists have studied the biomechanics of beetle genitalia. Previously, researchers have focused solely on genital appearance.

Male thistle tortoise beetles, Cassida rubiginosa, have a penis nearly as long as their body. Not to be outdone, females have an unusually long genital tract, or spermathecal duct. Complicating matters, the sexual duct is spiral shaped.

"Like most insects female beetles have a spermatheca," researcher Yoko Matsumura said in a press release. "The sperm is preserved there for a while before the eggs are fertilized."

Given the length and shape of the spermatheca, researchers set out to understand how male beetles rise to the challenge impregnating their mates.

"The first stage of our investigation was to see how the shape of the spermathecal duct influences sexual intercourse," said Alexander Kovalev. "The more turns there were in the opposite direction, the more energy was required for the penis to penetrate."

Researchers examined the male penis using a laser scanning microscope and determined that it is differently stiff. In other words, some parts are stiffer than others. Scientists designed a computer model to better determine which combinations of stiff and soft would enable males to quickly and efficiently penetrate the spermatheca.

"We now know: It is important how stiff the penis is," Matsumura explained. "The fastest was the penis model with a soft tip and hard base."

Researchers aren't sure why females have a sexual organ that makes penetration more difficult, but it may have something to with the average length of thistle tortoise beetle sex. Intercourse can last anywhere from 40 minutes to several hours. Scientists suggest females may prefer males who can penetrate the quickest because the two insects are more at risk of predation while engaged in reproduction.