For rattlesnakes, unless they are an individual with really cool markings, there are basically three points of interest--Head, rattle, and tongue. It's fairly easy to get the head and tongue in one pic at a good size, as long as you are lucky and the snake is cooperative. Since the rattle is at the opposite end of the other two, getting that in at a reasonable size is tougher! But I did it here.
I don't often shoot at f/32, but I did here. There was a price to be paid as a result, the original isn't super-sharp at 100% view. But it sharpened up pretty well. Viewing at 100%, I also get an intimate view of every bit of dust on my 1Ds sensor, and brother, there are a lot of them!
I'm going back to the zoo this afternoon. I may be the only adult male in America not watching the Super Bowl!
speckled rattlesnake
180mm macro lens
controlled situation
What is that thing near his pit? Hope it is not a tick 
have a great day at the zoo. The game is not till later this evening.