My instant reply was.
SPIDERS AREN'T INSECTS!
Arachnids are a separate category in the arthropod phylum apart from insects. Genetically speaking, you'd sooner have crab-girl and lobster-girl than spider-girl. Arachnids branched off the evolutionary tree sooner!
Very glad MC pointed this out the next page.
Also weird trivia: Spider legs are a natural example of hydraulics.