I grew up watching the processes used by character and set designers. Even if my understanding of perspective and anatomy doesn’t approach their accomplishments, I still employ many of the assumptions made by the masters drawing aliens and fantasy-scapes:

Given a similar atmospheric and gravitational makeup, the development of creatures on a theoretical world would still be governed by the same earthly constraints. If the creature is a carbon based life form with something resembling DNA, it should share many of the broad characteristics found (past or present) among the fauna of earth simply because it must overcome some of the same gravitational challenges.

Even if assumptions for alien life are correct, there are still additional caveats set by a prospective audience with a particular set of aesthetic expectations. There just wouldn’t be too many people who would hang a picture of a giant bug in their house. I would like to create a 3d render of this concept, but the subject matter would mean printing this image smaller than 16″ x 20″. Not worth a return on the time invested. Only challenges make images like this possible, however temporary and unfinished they may be, because creating feeds the creative process.