I can try to shoot a video of it sometime today, but they're a bit annoying to find in general.
Only in Extreme Hills is a bit weird. When I was orignally looking for them, and couldn't find them, I had presumed that they were going to be regional in likely wet areas, so I had checked classes for those like swampland and those first, but found nothing. I didn't even think to search directly in Extreme Hills, I only found it when doing a search on the entire source for the silverfish block.
I think the silverfish should drop whatever block they were hiding in. (example: break a stone silverfish block kill the silverfish, drops a stone block)
Edit (after reading entire thread): they should spawn everywhere it would make mining more fun and exciting , whoever came up with the idea of cave spiders spawning at y 20 and below (I think GerbilCrab475) that's something I actually would see Jeb putting in for Hardcore mode for a silly excise like "I made them spawn there because I thought Hardcore was too easy" and by the way GerbilCrab475 nice profile pic.
For me (and many others) they only spawn during the time when you first find the fortress. As soon as you click the Save & Quit button, they no longer spawn and you must find a new fortress or use McEdit to delete the chunks.
Before trying to be board police, you should perhaps ACTUALLY figure out where things are supposed to go
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