Given what little we know, I suggest two possibilities, neither of which are all that nice to think about.
1) Ares has somehow discovered a way to bring the bugs into our world without needing vessels.
2) Given that the 'form' a spirit takes is based on the magician (i.e. a spirit of fire may look like a fiery tornado, or an angel with a flaming sword, or a brightly colored pony with a tattoo on its flank) at the time of summoning, the magician chose to make the spirit appear as an insect.