As I understand it, there is a species of caterpillar that has the ability to fire its feces at predators at distances of up to one meter.
How could the ability to do this evolve? In order for this behavior to be of any reproductive advantage to them, they would have to be able to actually use this to keep predators at bay.
But in order for a caterpillar to use this tactic successfully, everything would need to be in place already. They would have to have anuses that could launch fecal pellets at velocities high enough to cause physical pain to predators who get hit with them, they would have to be able to aim their butts at birds or other ‘moving targets’, and they would have to have a stockpile of stored ‘ammunition’ ready in case of emergencies.
How could all of these things have come together simultaneously by random mutations? It seems hard to believe.
IANAE, but I can imagine how this might work.
First step: A catepillar develops some sort of noxious substance in its feces. This, by itself, could be advantageous at deterring predators.
Second step: A catepillar with aforementioned noxious feces develops the capacity to store it up for when needed. Given the first, this is also advantageous.
Third step: The relevant muscles in such a catepillar gradually become stronger and stronger, so as to propel the feces further. Any such gradual, incremental increase in range would be an advantage. Simultaneously, their aim would also be improving, in a similar gradual process.
Some insect larvae merely cover themselves in their own excreta in order to discourage predators, perhaps it was an enhnacement of something like that.
However, there is also the fact that in order for a mutation to confer a heritable advantage, an individual organism displaying it doesn’t have to survive a predation attempt if it has many siblings that share the mutation and if the predator is dimly intelligent enough to learn that these things aren’t good to eat.
So, suppose there was some mutation that resulted in the larva merely crapping itself with fright - the excreta might not actually be toxic (so if it had remained inside the larva swallowed whole, the predator would not have noticed), just unpleasant tasting - the predator might well be inclined to spit it out and not bother eating the many siblings, who stand a good chance of carrying the same mutated gene.
So there’s a selective pressure for fecal incompetence under attack, perhaps the next step is a mutation that results in the same action, but when the larva is merely threatened - it excretes a big dollop of sticky poop - the predator might see this and decide to shop elsewhere, but it also might just carefully pick up the larva and leave the poop, so there’s a selective pressure to be able to deliver the poop a little more judiciously. Sooner or later, a mutation that causes just such a skill might arise and be selected.
And so on.
You’ll notice a lot of ‘might’ in there - I’ll be the first to admit that this is pure speculation, but not unreasonably so, I feel.
A lot of what may seem to hard to believe about evolution stems from a lack of appreciation of the time taken for these things to happen.
Imagine the longest time you can possibly imagine. Now multiple it by 10. When you are dealing with time-scales that we can barely grasp, with our tiny, tiny, miniscule lifetimes , pretty much anything becomes a whole lot likelier.
SO all those mights have a whole lot of time to occur.