Microscopic Lens???

Is there a microscopic lens available that I can attach directly to my Canon EOS XTi without having to use an adapter and attach it to a digital microscope?

No, you would need a amount adaptor. Size of the lens mount vs a digital microscope are totally different sizes usually.

You get 5x magnificatoin with the 65mm/2.8 MP-E lens or
alternatively you can get 20x magnification using some old
FD lenses, extentin tubes and bellows at double the cost of
a 65mm lens. Higher magnifications are not possible because
of the high coma that the lenses generate. At 20x you will
be photographing at f64 which means you'll need lots & lots
of light and steady subjects because the depth of field is
only 0.05 mm thick and the viewfinder very dim.