arduino mega + 2x SM-s4303R Servos problem

Hi i would like the servos to move the legs of the spider back and forth please see image link. In stationary mode the servos are at 90 degrees. When the spider moves one leg goes forward (180 from 90) and one leg goes backward (0 from 90) to create motion.

http://postimage.org/image/j7crzmjo/

Thanks