She does... it's just not an Earth-accurate spider. More Cybertronian I guess.
Right on the 8 minute mark of the 12th Episode (Predatory), she jumps up and does a transformation sequence into that image linked above. And you can see that her robot legs are gone, and she has a spider abdomen hanging at the back... plus the spider legs are extended and spaced out. The confusing thing about that mode is seeing the robot head and arms (which would be in place of the 7th & 8th legs) - but would make for an easy toy to produce (as the robot legs would just combine to form the spider abdomen, and the spider legs would just be moved around).
And even if she didn't transform on the cartoon before taking on the helicopter mode, I think it would be assumed that her pre-Prime mode was a spider, based on her name, her spider aesthetic (legs and fangs), and her ability to fire spider webs.
Either way, it would warrant two different Airacnid toys, if Hasbro pads out the line with more toys of the main characters... as opposed to mere redecos that don't even reflect colour schemes on the cartoon (like we had with Animated toys).