I'm not sure you're right about this. You're not allowed to be cruel to animals, which I think would apply equally well to a dog and a cow. Likewise there's no prohibition against killing animals per se, it is OK for you to shoot your dog if you do it in a humane fashion. At least in the US, our animal laws seem to be derived from property rights, which is natural enough given the existence of livestock.

Your example of the mouse fed to a snake is a good one; I find that very ethically troubling. It's also different depending on whether it's being done in a scientific context (for study of snakes as predators) or in a domestic one. On the whole I'm against it, but shouldn't I thus also be against the use of feeder fish or insects to lizards and so on? I find it increasingly hard to draw a bright-line distinction.

One day a couple of years ago I found one of my cats had trapped a dragonfly, more of curiosity than malice (so the cat didn't mind when I moved his paw out of the way). The dragonfly was a bit stunned and one of its eyes was bashed inward like a piece of plastic, so I thought it was a goner. But I picked up to take a look because I'd never seen such a large specimen before. Now, dragonflies are interesting, as a photographer friend of mine who specialize din insects explained many years ago. They are the lions of the insect world, ie large and predatory, and cannibalistic. They live for up to 2-3 years as flightless nymphs, hunting in both water and on land, and for about 6 months as adults. They are somewhat territorial, and there are also far more males than females so mating is highly competitive (for the males) and usually involuntary (for the females). By the end of the summer it's very common to see dragonflies going around with missing limbs, damaged eyes, and so on. I learned about them in England, where you see ones that are 2 or 3 inches long, but here in Northern California where I live now it's common to see ones that measure 4-5 inches - which bring me back to the one my cat caught. I picked the thing up and it sat on my hand, and fortunately I had a camera with me so I got a bunch of photographs. I assumed it was just going to die, but to my surprise, after a few minutes of just sitting there, it perked up, checked itself over with its front legs, stretched itself out in a few different directions, and took off. Then it spent a minute or two flying around badly - remember on its eyes had been bent quite out of shape. Again, I assumed it was going to wobble along until it hit something or got grabbed by a bird, but it kept hovering and then darting over short distances with increasingly good control, until eventually it seemed satisfied and zipped off with confidence. It hung around the garden for a few minutes longer (I presume to feed on some tiny insects) and if I hadn't had it in my hand I wouldn't have known it had suffered any damage.

Now I certainly don't think dragonflies are sentient or even intelligent - I think of most insects as being a bundle of hardwired reflexes. But watching the thing 'repair' its flight model was startling, and while it's not evidence of intelligence it suggests some sort of rudimentary self-concept. The very long lifespan of some dragonfly species presumably confers some sort of evolutionary advantage, so it seems reasonable to imagine they have some learning ability - fruit flies can be taught to navigate a maze [1] and dragonflies are known to have a capacity for selective attention [2], and insect brains are surprisngly complex [3] so I don't think I'm anthropomorphizing when I suggest the possibility that they have some sort of pre-conscious but nonetheless subjective life experience.

1. http://www.dnalc.org/view/1720-Training-Flies.html

2. http://ac.els-cdn.com/S0960982212013929/1-s2.0-S096098221201...

3. http://bioteaching.com/insect-brains-and-animal-intelligence...

BTW here's a picture if you're curious: https://plus.google.com/117583585506971591172/posts/8J7ZLgKk...