Carefully, twist off the top of each Oreo cookie sandwich. You want to the filling to stay on one side of the cookie sandwich.
Draw a circle with green gel frosting slightly larger than the M&M candy on half of the cookie halves that have the filling. This will be the "glue" that holds the candy in place.
Draw a circle with blue gel frosting slightly larger than the M&M candy on the remaining cookie halves that have the filling. This will be the "glue" that holds the candy in place.
Place M&M candies in the center of each gel frosting circle. Press lightly to ensure that the candy holds.
Use the red gel frosting to draw on "blood vessels".
Notes
Add Spiders: Use a dark icing and make a web and decorate with a candy spider.