I once was a guest at a lab where they were doing genetic research on Drosophila Melanogaster. Apparently they were in the middle of a big multi-part, long term experiment, and one of the researchers invited me to take a look at one of her test subjects through a binocular, stereo microscope. For some reason, I noticed something she had never seen herself...the fly had parasites, clusters of little 8-legged mites that seemed to congregate around the sockets where the wings attached to the thorax. The little buggers were alive, and quite active, but I had been very lucky to see them, you had to focus precisely on them or they vanished against all the minute structures on the fly's body.

I had her take a look while I recited the old doggerel

"Big bugs have little bugs
Upon their backs to bite them.
And little bugs have littler bugs,
And so on ad infinitum."

She was not amused.