[robotresearcher]

Right. The big spider shape is a hack that exploits the spider-detecting system of the predator. The predator has a very powerful response to spidery things. It's like a flare to an older/cheaper heat seeking missile.

If the decoy becomes too successful, the predator will evolve to have more sophisticated detector, find something else to eat, or become extinct.

Modern heat seeking missiles have fancy multi-frequency heat detectors or even IR CCD cameras with image processing to ignore the flares.