The Spanish word 'pulga' meaning 'flea' comes from the Vulgar Latin word 'pulica', which itself developed from the Classical Latin word 'pulex'. This is a straightforward evolution where the Latin word was simplified in Vulgar Latin and then underwent further phonetic changes as it entered Spanish, with the medial '-ic-' becoming '-g-', a common sound change in Spanish development from Latin.

The word 'pulga' is relatively basic in Spanish and doesn't have many simpler related words in common use. However, there are derived terms like 'pulguiento' (flea-ridden) or 'pulguera' (a place infested with fleas), but these are actually more complex than the root word 'pulga'.

While English 'flea' is not related to Spanish 'pulga', there are some scientific terms in English that come from the same Latin root 'pulex'. For example, the order of insects that includes fleas is called 'Siphonaptera', but their family name 'Pulicidae' comes from this Latin root. You might encounter this in terms like 'pulicidal' (flea-killing) in veterinary contexts.