Re: Errr....
" I don't see a dust filter on any one of them!"
Yes, that's because through clever design those are integrated into the case. Essentially meshed surfaces are fairly decent dust filters. The idea is that they catch all the big particles while the rest will simply be blown out by the fans. I've first seen an eary concept of those in an expensive measurement device. There was a hive-like structure perhaps 2-4 cm deep just mounted on the air inlet. This structure cought all the relevant dust and kept the rest of the device virtually dust free.