Most species eyes don't go black, unless they have been dead for awhile (they will often go "dark" for evening hunting, though!)
I would do as Yen says, for future reference. Put a spoonful of fly pupae in with him (of if you have BB spikes or pupae, I would put in quite a few pupae, and quite a few spikes, so that they hatch at different times.) You can also leave some pupae out a day or two before you leave and put those in there to hatch in the next day or so.
Another tip, gleaned from "low budget" chameleon keeping...get a small jug of water (or reuse a 1/2 gallon milk jug or iddy-biddy Tampico punch jug) well-washed & rinsed, fill 3/4 with water, leave the lid off & freeze. Replace the lid, punch a SMALL hole near the bottom with a large safety pin or slit with a box knife (very small cut). Place OVER the mantid's cage so that it will drip INTO the cage as it thaws. This should last a few days, and the mantids can go a couple of days without water, as well. (They will also get some moisture from their food.) If using a net cube...highly recommended...you may want to put a large cake pan or something underneath the cage to catch drips.
You will probably want to try this a week before you leave, so that you can adjust the hole size, if necessary, and to test & see how long it lasts, whether too much water builds up in the cage, etc. (If using something solid, like a Kritter Keeper, you may want to heat up a needle & poke a few holes in the bottom, to allow water to drain.) Warning: Do NOT poke another hole in the top to increase water flow, as this will just make it POUR out! Adjust the hole size at the bottom only, so that vacuum keeps it from all coming out at once!
I hope that helps, and if someone DOES try this, I'd be curious to know the result! (I've never had to leave my babies for so long!)
