How are fleas primarily characterized in terms of body shape?

Fleas are primarily characterized by their laterally compressed, or flat side-to-side, body shape. This unique body structure allows them to navigate through the fur of their hosts more easily, providing an advantage in their parasitic lifestyle. The flattened shape prevents them from being easily brushed off and helps them move quickly and efficiently through hair or feathers while they are feeding.

Additionally, this anatomical feature is essential for their survival, as it helps them conceal themselves within the host's coat and escape predation when necessary. The other options describe body shapes that do not accurately represent fleas; for example, being round and hairy or bulky and spiny does not align with their actual physical characteristics.