One reason for this could be that the mental image we form of any three-dimensional object takes the viewpoint which presents the maximum amount of information. To recall any other view requires a special and conscious effort. Birds, quadrupeds, and fishes reveal most of their salient characteristics from the side. Insects and other arthropods reveal most of theirs when seen from above. Molluscs or their shells may be viewed in various ways because they have such varied and often asymmetrical shapes. The most evocative image of any species is the one which provides the most information.

-The Art of Natural History -Animal Illustrators and Their Work- by S. Peter Dance