Complex sentence: He cites reports from others who believe that they use either sounds or commune through gestures with their tentacles, but as he sees Martians working together without either of the aforementioned means, it is his belief that the Martians use telepathy.
Split into simpler sentences: He cites reports from others who believe that they use either sounds or commune through gestures with their tentacles. The narrator sees a good deal of Martians working together without either of the aforementioned means, and it is his belief that the Martians use telepathy.