It depends on the level of stiffness: Kiel researchers investigate beetle penises

The penis of the thistle tortoise beetle is as long as the beetle itself. Something that initially sounds exciting, can be complicated during mating. This is because the females also have a hyper-elongated sexual organ, which is additionally twisted in the shape of a spiral. How beetle reproduction can nevertheless be successful has now been investigated by Dr. Yoko Matsumura, Dr. Alexander Kovalev and Professor Stanislav Gorb from the Zoological Institute of Kiel University. In the journal Scientific Reports, the researchers report on penis specific mechanical properties, which are most suitable for propulsion into the female, and come to new conclusions about sexual selection. (Mehr in: Pressemitteilungen – idw – Informationsdienst Wissenschaft)