If I remember correctly, a 100mm Macro will only let you be about 4" from the front of the lens to the critter to get a 1:1 magnification shot. Knowing what kind of body you have would be a huge plus. If it's a canon, you'll get 1.6:1 shots at this distance; 1.5:1 with Nikon; and, of course, 1:1 with any full-frame camera.If you don't want to be inside the camera while you take shots at this magnification, I'd opt for something like the Sigma 150 or 180mm. They give you something like 11-14" of working distance respectively. I own the 180mm and it's amazing. They just came out with a 150mm with OS, which is bound to be a spectical. If you don't mind being SUPER close, and you happen to shoot canon, they make a 65mm MP-E lens which allows you to get anywhere between 1:1 and 5:1 macro with a turn of a dial. I believe this lens loses infinity focus, though, so it is quite specialized. If you're going to be using it for other critters too, including bugs, I'd go for something more than 100mm. You'll lose alot less shots due to moving front elements, lack of silent AF motors, and having to be too close. Tamron makes a 180mm Macro as well, which is supposed to be superrrr sharp, but the AF isn't great.