Thistle tortoise beetles (Cassida rubiginosa) are herbivorous beetles of Eurasian origin. Because they feed on plants, they have been used as biocontrol agents to eliminate weed pests in various agricultural countries.

According to ResearchGate, these beetles have been released in New Zealand in 2007 as biocontrol agents against the Californian thistle (Cirsium arvense).

But more than that, a thistle tortoise beetle has many fascinating features. For instance, the penis of the thistle tortoise beetle is as long as its own body.

While humans won't probably complain about this, a recent study showed that in thistle tortoise beetles, a long penis means a complicated sex life, because female thistle tortoise beetles also have a hyper-elongated sexual organ.

As per the evolutionary contest, the female's organ became elongated so the male's organ adapted to it. Not only that, this beetle also has a very long and twisted spermatheca, where sperm is preserved before being fertilized.

Only those with the longest and flexible penises can successfully impregnate a female beetle. But how can sex be successful with such complex structure?

Instead of focusing on the size of the thistle tortoise beetle's penis, Dr. Yoko Matsumura, Dr. Alexander Kovalev and Professor Stanislav Gorb from the Zoological Institute of Kiel University recently investigated its biomechanical function. They found out that a successful mating for male thistle tortoise beetles means having just the right amount of penile stiffness.

To achieve that, the researchers found out that a soft penile tip is required.

"Using a laser scanning microscope, we found indications that the penis is differently stiff. Therefore, the tip of the penis is probably softer than the rest of the organ", Matsumura saud in a press release.

According to Matsumura, a soft tip allows the penis to be flexible enough to adjust to the narrow turns. With hard tip, the penis would find it difficult to bend to the corners.

The soft penis tip is the male strategy to penetrate the complicated reproductive system of a female beetle.

The study is published in journal Scientific Reports.