Mammals generally have evolved the same basic, boring way for males to fertilize an egg: just send a bunch of tiny individual sperm into the female reproductive organs. But in insects like the diving beetle, it gets way more insane.
The reproductive tracts of female diving beetles are like labyrinths, huge maze-like structures in which the male's sperm can easily get lost. That's why all the individual sperms come together as one giant conglomeration that can then navigate as a single entity, which increases the chances that they will navigate the reproductive organ successfully. Of course, in the case of diving beetles and other invertebrates, "success" doesn't mean fertilization of the egg — typically, the female will simply take on the sperm and store it, sometimes for years, until she is ready to reproduce.
A female will sometimes store an entire, enormous joined sperm entity for years in her spermatheca, something University of Arizona researcher Dawn Higginson found out firsthand when she squashed one of these structures on a microsope slide, and then "out came a single, enormous mass of sperm, all connected to each other and swirling and wiggling around." It's a charming image, and more importantly it was a completely unprecedented discovery, as Higginson explains:
"I could see the tails beating and moving the whole conjugate around. As far as sperm goes, this is clearly unlike anything we have ever seen before."
Higginson and her fellow researchers studied the sperm of 42 different species of diving beetle, a common insect found all over the world, in over 4,000 total species. They found all sorts of crazy forms of sperm, including what their statement describes as "thousands of sperm cells stacked up like badminton birdies, forming a long, fairly rigid rod of sperm resembling a fuzzy worm", as well as another example where sperm "are connected by their heads with some kind of glue."
So what's driving all this sperm diversity? Higginson says the most likely explanation is that female reproductive tracts have evolved to be fiendishly complicated, which is forcing the sperm to play evolutionary catch-up:
"We can reconstruct what the ancestors of these diving beetles looked like. We find they had conjugated sperm and rather compact female reproductive tracts. The female morphology undergoes evolutionary change, and then sperm compensates for that. We can't say from this study that the males are catching up but it's suggesting there could be an arms race.
In most cases of sexual selection, we expect co-evolution between a female preference for a male trait, like flashy tails in peacocks," Higginson said. "Females prefer males with flashy tails, and then females want bigger and flashier tails after that, setting up a co-evolutionary cycle. We are not positive that this is happening here, but is possible that all this diversity we see in the sperm is the equivalent of flashy peacock feathers, and females have evolved reproductive tracts that favor one kind of sperm over another."
"Perhaps there is something else that is influencing the evolution of the female reproductive tract. Perhaps it has to be big enough to handle an egg properly or something like that. The point is that the females are driving the sexual evolution. The sperm cells just have to keep up."
You can see a video above that shows a single conglomeration of sperm in the diving beetle species Hygrotus sayi... assuming that's the sort of thing you're into, of course. The researchers suggest the evolution of labyrinthine reproductive tracts in these beetles serves as a way for females to have more control over which male they reproduce with, and it also could be driving the faster emergence of new species — with such intense reproductive demands, it won't take long before some males' sperm becomes completely incompatable, and that means the dawn of two new species.