As has been mentioned already, one or more extension tubes will get you closer than the macro lens by itself can. That will only get you so far, though.
Somewhat counter-intuitively, if you want really huge magnification while keeping the camera assembly to a manageable size, you need to go to a wider lens, not a longer lens. At sizes of 1:1 or smaller, a longer lens gives you additional working space at little cost, but as the magnification gets bigger, the amount of extra room required behind the lens becomes an extreme penalty. Canon's answer is a "reverse zoom" (the 1:1-5 65MP-E), which is essentially a zoom lens with the far end of the lens anchored in place (instead of the end at the flange, as with a normal zoom); reducing the focal length of the lens increases the magnification at the film/sensor plane. The cheap option for this strategy would be to reverse-mount a wide-angle lens on extension tubes or bellows.
If you want to get really big and don't mind fishing around in the used equipment and odds-and-sods end of the photo market, a really interesting option would be the Minolta micro lenses. They are actually microscope-type lenses, available at 12.5mm and 25mm focal lengths, IIRC. They're used on a bellows, and you can get adapters that will couple the Minolta MC/MD mount lenses/bellows to a Canon EF-mount camera. The 12.5mm lens will let you get close-ups of the eyes of the things that live on the eyes of the insects you're talking about. (You can probably find similar combinations of things that will let you use Zeiss microscope lenses on your camera as well.) Forget about depth of field, though -- there isn't any.