Even though Hemipterror would be the correct spelling, Hempiterror is easier to say. Basically because it flows better, whereas when saying Hemipterror, you almost have to take a pause between the 'p' and the 't'; there's an awkwardness about it.

Suppose it depends on how accurate you want to be, but nobody will really pick up on the mis-spelled version. I called one of my robots Carcinus as it was Greek for crab (and the robot looked like a crab) but whenever anyone hears the name for the first time, they just look at me blankly for a moment, then ask why I didn't just call it 'Crab'. So a different spelling probably won't matter much