I figured this weirdness needed some explaining.Start off by reading Bug Girl's excellent bit on the Phalloblaster. It's either not what you think, or exactly what you think.
First, a practical reason. If you go outside and collect a bug, and you want to find out what kind of bug it is, chances are it's going to be small. 99%  of all animals are smaller than a bumble bee. Second chance is that it's going to be rather nondescript. Entomologists call these LBBs (little brown bugs), and there are /tons/ of them. Third chance is that it's going to be of a rather large group, one with hundreds or more species in North America (or whatever other godforsaken place you live).
When entomologists (that's people who study insects) first started doing in depth microscope work, they discovered something really different. You could have two different insects that looked outwardly identical, but when you look closely you find the genitalia strikingly different. And not just different, but /consistently/ different. Consistently different enough that you can tell these very closely related species apart with a glance. And it turns out these differences and similarities have a lot of phylogenetic signal. That is, they can tell us a great deal about the evolutionary history of these related species. Questions like: which are most closely related? Are these groups we had previously designated good ones? Are there any correlations with ecology or behavior?
And, they're kinda weird looking. I've recently characterized insect genitalia as being "a costume party on Lovecraft's birthday". Spines and hairs and membranous sacs and clasping arms with hooks and all sorts of non-euclidean geometry. And that's just the males! The females have all sorts of other weird, internal structures, sometimes with large, piercing external structures like ovipositors.Here's just one example: a recently new to science forcepfly with massive clasping arms on the male genitalia
Male sure to check out the damselfly genitalia near the bottom.
So, they're great for identification, weird and different among species, but why?
This isn't just an insect question, by the way. This is something that researchers on many groups of animals have been asking, about duck penises, for example
, which are tightly wound helixes (the female orifice winds the /opposite/ direction, if you were wondering). Or Hyenas
. Yeah, I'll just leave that one there.
The answer is sexual selection. Or, really, it's three answers, three hypotheses, and not necessarily competing hypotheses. All of which lead to a rapid and divergent evolution of insect genitalia.
The first is called sexual antagonistic selection. The "interest" of the male is to get his spermatozoa as close to the ova as possible, so he evolves all sorts of structures to make that happen. The problem is, these structures often are sharp spines and hairs, hooks, and in extreme cases, a piercing structure that goes straight through the body wall in an act called traumatic insemination
. This is what bedbugs do. There are also often fluids included, ones that sometimes alter the behavior of the female, make her less prone to further mating, for example.
As you might imagine, the female doesn't benefit much from these insertions. They can lower the overall ability of the female to produce offspring, shorten their lifespans, and depress their immune systems. So she is heavily selected upon for defenses to combat the male structures. Over time this becomes an arms race between males and females, the females evolving better defenses, the males evolving spinier phalluses. In bed bugs, the females have an elaborate internal system of sacs to reduce the male's impact upon her immune system when he puts a big hole in her abdomen. Some even have a groove that directs the piercing phallus into a particular place of weakness, reducing the overall damage. Summary: sexual antagonistic selection is an arms race between males and females with the males initiating.
The second hypothesis is sperm competition. Sex in most animals goes a particular way. The males are promiscuous, and attempt to mate with multiple females. The females are passive and choosy. The main driver for this is that males generally have a low energy investment for reproduction. Sperm, in most cases, are cheap to make. Females on the other hand have a high energy investment, as eggs (or pregnancy in mammals) are expensive. In pretty much all cases the female investment is higher, and so there is reason for her to be more choosy in her mating. (When the opposite happens, as it does in some species, it's called a sex role reversal. Think seahorses). In cases where both parents take near equal energy investment, including post-natal parental care, both tend to do the choosing. Humans, for example.
So, if multiple males are mating with a single female, as is often the case, there is going to be competition between male sperm to reach the ova. The male that gets the sperm closest is the male that wins. Thus, elaborate genitalic structures, or sperm plugs, or genitalia that actually can't get out of the female once they're put in (the male just leaves them behind; think about that for a few minutes.), or just better sperm. Anything that will maximize the chance that that one male's sperm will get to the eggs first. Summary: Sperm competition is where males are competing with each other.
The third is female choice. Since females have the most energy invested in reproduction, there is reason for selection to act on their systems to maximize their ability to choose between spermatozoa, both before and after mating. The best example of pre-mating choice is the tail of male peacocks. Or courtship rituals. Or wing spots. Or nuptual offerings (that's a cool one; check out these balloon flies
.) Basically, anything the female can sense about potential mates which would allow her to pick ones that have characters indicative of health and genetic fitness. Post-mating choice is a bit...stranger. Somehow, females are often able to choose between sperm that she has received. This can be a matter of chemical constituents of the reproductive tract, or also how they are stored in a sac called the spermatheca. In any case, the female chooses and therefore the males are forced into an arms race with the female to try to catch up, finding ways to bypass these barriers. It's the inverse of sexual antagonistic selection. Summary: Female choice is an arms race between males and females where females are the initiators.
All of the above happen in combination, in most cases. It's seldom just sperm competition, or just antagonistic selection. Since there's all these arms races and competition, there's selection for insect genitalia to be flexible in construction. Or, more precisely, the fundamental flexibility of insect genitalia may be the greatest cause of their diversity, behind wings and the general segmented ground plan.
Two recent articles:www.livescience.com/1595-spiked-genitals-spur-beetle-evolution.html
- A story about spiky beetle phalluses.http://www.livescience.com/6015-spikes-genitals-flies-hook.html
- A story about flies with hooks, and how one entomologist is shaving them off with a laser.
Hopefully now you have a better understanding of my obsession. Or you're disgusted, or terrified.