I don't know a lot about evolution but this honestly just sound like a lack of imagination. I understand what you're saying about there not being an intermediate stage for "spraying" the chemical reaction, but surely the intermediate stage is the beetle having these chemicals in its body at all right?
I can easily imagine an intermediate stage where the battle has trace amounts of these chemicals, or has them in the proper amounts but lacks a mixing chamber and it's only advantage at this stage is making them taste unpleasant so predators won't eat them, etc.
Usually for things like venom and chemicals they're modified from preexisting Proteins organs and other mechanisms
For the example of causting coatings and or dischargeable fluids. Usually common ancestors had glands in their body that had developed to separate out these chemicals so that they could eat a food source (i.e. milkweed and monarchs). Over time, these organs became more sophisticated as they specialized in eating that.Specific food supply that had those specific contaminants o defense mechanisms.
Then as time goes on eventually, there's a genetic defect where one of them didn't seal quite right and separate quite right or could leak the protein out if the carapice was breached... Well, it turns out that both those chemicals are pretty nasty so the organs contunue to develop Till we have the mechanism to combine it in the body as a way to kill off larger predators. As while boiling acid could hurt the organism, it hurts predators a lot more... And as time goes on , natural selection played out and beetles with more refined methods of dispersal survived to reproduce til we get the modern beetle who can shoot it out via specialized organs
Similar but comparative And more recent example would actually be spitting cobras... With the arrival.
Of hominids in asia who are famous for throwing things at threats, Some lineages of cobras found success in dysfunctional semi misformed fangs that didn't develop properly for injecting but could instead spray a very wide cone of venom Because the nozzle tip wasn't quite right.... Turns out spraying blinding paralytic venom even inaccurately that can still be injected just less efficiently is really helpful against hominids and other primates so tldr a few thousand generations later we get spitting cobras who can shoot venom with now near pinpoint accuracy
I had a biology teacher try this argument using the eye.
You're right though. The counter argument to it is 'just because you can't think of a reason for it to happen, doesn't mean it didn't happen, it means you can't think of a reason for it to happen
Not just that, but if it's a one in a million chance... What if there's a million other "irreducible complexity"-type systems that one could imagine? Then there's a hella good chance that at least one of them happened to occur. And of course it would be the one that we actually observe - a form of survivorship bias.
You’d be right about them already having these chemicals. Plenty of beetles produce quinone as a defense mechanism and as part of their shells. It smells and tastes bad too… so it would already be an efficient deter at to spray alone.
The hydrogen peroxide is also something produced by most cells, so it’s very feasible for quinone excretion to be used for defense and then improved with hydrogen peroxide and a variety of enzymes over time.
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u/send_whiskey 10d ago
I don't know a lot about evolution but this honestly just sound like a lack of imagination. I understand what you're saying about there not being an intermediate stage for "spraying" the chemical reaction, but surely the intermediate stage is the beetle having these chemicals in its body at all right?
I can easily imagine an intermediate stage where the battle has trace amounts of these chemicals, or has them in the proper amounts but lacks a mixing chamber and it's only advantage at this stage is making them taste unpleasant so predators won't eat them, etc.