Actually, the 300/4 IS (on 7D) makes a pretty good macro lens, too...
For spiders and such, the 100/2.8 non-IS is fine. Get a tripod ring for it.... The Canon is expensive, but there are far less pricey clones on eBay all the time. That's one of the nicest things about the two Canon 100mm lenses. There are a number of very good 50mm, 60mm, 70mm, 90mm, 100mm, 105mm macro lenses... but most can't accomodate a tripod ring. The two Canon 100s can. Macro often needs a monopod or tripod... Though there are times handheld is possible (especiall if using a flash).
Other nice features...
Both have USM that makes them relatively fast focusing... for a macro lens. That might make them a bit more dual purpose, more useful for non-macro work. They also have focus limiters, where you can set the lens to only operate within a certain range. That also helps it focus faster in diffiernet situations.
They also are both IF or Internal Focusing designs. As a result, they start out larger than some other lenses, but don't extend during focusing. Some 1:1 macro lenses double or triple in length when set to their maximum magnification. This cuts into your working space. But, IF lenses actually act a little like a zoom... The 100mm is more like a 70mm by the time it's at its maximum 1:1 magnification.
Personally I don't have much need for IS for macro... It's not all that effective anyway at higher magnifications. Might be nice to have if using the lens a lot for dual purpose, for non-macro shots. But I'd rather put the money saved buying a non-IS lens into a flash and an off-camera shoe cord to use for macro shots (a regular flash can do, just use a rubber band to put a layer or two of white gauze bandage over the head to tone it down a bit).
EF 100/2.8 USM Macro lens. 550EX flash on off-camera shoe cord. EOS 30D. Handheld... well sort of (used an upside down 5 gal. paint bucket as a support for the camera and lens, handheld the flash).