It was a Miracle! Or unexpected, anyway...
Despite your best efforts to keep caterpillars out of the container of water in which the fennel plants reside, it will likely happen that, sooner or later, one will end up in the drink, having crawled thru an unused hole in the plastic cover, or, if you do not employ a plastic cover (for shame!), having fallen from the plant directly into his/her watery grave.
Recently, I discovered a smallish (3/4 inch) cat had managed to get thru the plastic cover and was submerged and motionless at about half the depth of the water in the container. I don't know how long he had been there, but I carefully removed him and placed him on a paper towel so I could get a look at him. I used my trusty hand-held 5x lens, and saw, in some detail, one very dead-looking caterpillar---no motion anywhere. Recalling that Kathy had told me she had had a cat come back to life after apparent drowning, I decided to not toss this guy into the fennel trash bin. I dabbed him dry with a bit of paper towel, then left him. An hour or so later, no sign of life so I went to bed.
Next morning, I came to check the cats and glanced over at the dead one...but he wasn't where I had left him! In fact, he was actively exploring one edge of the piece of paper towel! He was alive and kicking! He was baaaaaack!!!
My advice, therefore: never toss out a seemingly drowned cat! Dab him/her with kleenex or paper towel to get most of the water off and then put the cat on some fennel or paper towel in a container that can be closed and go away for a few hours, or even 10-15 hours. You may be pleasantly surprised when you return.
The Big Question is, how the heck do they do this trick? How can a cat be submerged in water for long enough to look and behave as dead as a doornail, then recover after a few hours and be apparently fine? My guy was fine, as it turned out, and went on to become a green monster and then pupate normally. Some guesses: Presumably a caterpillar is not a major user of oxygen, and maybe tiny bubbles of air are trapped by the various and sundry spiny projections and that air is available for use while submerged, and maybe, like other animals, submersion in water causes a reduction in oxygen use and a sort of shutdown of all but the "core" bodily functions in order to conserve oxygen. In other words, maybe the little squirmers know how to go on metabolic "hold". Lots of maybes here....if anyone knows the real answer, please send a note.
BTW, I think we think it would be considered tacky by butterfly and caterpillar lovers everywhere to perform experiments regarding this phenomenon...right?