The most common and most likely answer to this crossword clue is the 10 letter word MILLIPEDES
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Buddy explains!
The straight part
We know that "They’re actually shorter than 500 feet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they’re actually shorter than 500 feet = millipedes.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MILLIPEDES mean?
MILLIPEDES noun- Any of many elongated arthropods, of the class Diplopoda, with cylindrical bodies that have two pairs of legs for each one of their 20 to 100 or more body segments.