pookie is correct, if they do not have good soil aeration they will literally "choke to death." Sphagnum moss is another possible media to use but some chams will eat this, too (Luna did!). I think Carlton mentioned in an earlier post to put screen over the top of the pots to keep crickets out, but lets in air and water. I used plastic canvas (the stuff used in crafts) and love it! it will not rust, has no sharp edges and is easily cut into any shape to fit pots. I then hold it in place with bent paper clips. it comes in lots of colors so you can color-coordinate (LOL!) and can buy it at any place that sells craft supplies. It costs about 25 cents per 12x12 sheet. Get the one with the largest opening (I think 7 mesh).
Here's a pic of how I have one of Luna's plant's set up with it. Also, here's a link with LOTS of orchid info

Growing Orchids
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0.1 veiled - Luna
0.2 house geckos - Gaia & Tia (both MIA 
0.2 felines - Kyndra and Líta
1.0 African Clawed Frog (for summer)
0.5 Mad. Hissers (for summer - all girls, no little ones, whew!)