Final answer:
The scorpion does not fit in with the wasp or millipede because it is an arachnid, not an insect, and has eight legs rather than six.
Step-by-step explanation:
The arthropod that does not fit in with the others among a scorpion, a wasp, a millipede, and other insects such as a house fly or dragonfly is the scorpion. This is because scorpions are not insects but belong to a different group of arthropods known as arachnids. Insects like wasps, dragonflies, and millipedes have six legs, and most undergo some form of metamorphosis during their life cycle. Arachnids like scorpions, however, have eight legs and do not undergo metamorphosis in the same way insects do. While all mentioned here are arthropods, which comprise a vast group including insects, arachnids, crustaceans, and others, their classification within the arthropod phylum differs.