If you think an erection lasting more than 4 hours is a problem, try one lasting more than 99 million years.

That's how long the penis of a newly discovered arachnid fossil has been standing at attention. The harvestman, a spider relative also known as a daddy longlegs, was encased in amber during the Cretaceous in what is now Myanmar. Its distinctive penis, with a heart-shaped tip and a bit of a twist at the end, was erect at the time.

"It was very surprising to see the genitals, as they are usually tucked away inside the harvestman's body," said Jason Dunlop, the curator of the arachnid, millipede and centipede collections at the Museum für Naturkunde in Berlin, who reported the discovery online Jan. 28 in the journal The Science of Nature. [See Images of the Preserved Harvestman Arachnid with Erect Penis]

Amorous arachnids

Arachnid genitals are varied. Some spiders, for example, have grasperlike pedipalps that they use to pass a sperm bundle to females. Male orb-web spiders can detach their pedipalps and leave them behind inside a mate in order to escape their cannibalistic female sex partners.

A close look at the erect harvestman penis found trapped in amber in Myanmar. (Image credit: Jason Dunlop, The Science of Nature (DOI 10.1007/s00114-016-1337-4))

Harvestmen, on the other hand, have extendable penises that are similar to mammal penises. When not in use, these organs are stashed inside the body. [7 Amazing Bug Ninja Skills]

The new harvestman specimen belongs to an ancient species called Halitherses grimaldii. A private collector sent it to Dunlop and his colleagues. Harvestman fossils are rare — only 38 have ever been found, the researchers wrote in their new paper — but harvestman genitals are even more elusive. This is the first amber specimen visibly preserving the structure of the penis, Dunlop told Live Science in an email.

"These penis details (shape, form of the tip, etc.) are very important for saying where this amber species fits in the harvestman family tree," he said. "In fact, we couldn't find an exact match in terms of penis shape with any living species."

As a result, the researchers propose that the spider belonged to a previously unknown (and now extinct) family of harvestmen. The team is investigating several other new species found in Burmese amber, but none of those are preserved with visible genitals, Dunlop said.

An undignified death

The spider penis extends more than half a millimeter from the lower abdomen of the harvestman. The arachnid's round body is about 2 millimeters long and sits atop spindly legs. The glans of the penis is heart-shaped. From it protrudes an additional structure called the stylus, which curves and twists to the right.

There was no female trapped alongside the erect harvestman. It's possible that the arachnid was mating when the amber enveloped him, but that the female was nearby and escaped, or wasn't preserved. Or maybe the erection was no fun at all.

"In harvestmen, the penis is sometimes pushed out by increasing blood pressure," Dunlop said. "Maybe as the animal struggled when it got caught in the sticky tree resin, its blood pressure rose and the penis was pushed out accidentally?"

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