Place the chocolate in a small microwave-safe bowl and microwave in 30-second increments, stirring every 30 seconds until melted and smooth. Add the corn syrup and mix it in—the mixture will become thicker. Scrape it out onto a piece of cling wrap and wrap it well. Let sit overnight, until firm.
Knead the chocolate to soften it and smooth it out. Pinch off walnut-size pieces and roll them into ovals between your palms. Use a toothpick to poke four holes in each of the long sides, where the legs will go. Work the toothpick up and down a little, to create a large hole—you will need some wiggle room!
Melt the candy coating in the microwave, and pour some of it into a paper cone or zip-top plastic bag with a tiny hole cut in the corner. Line a baking sheet with waxed paper or parchment, and pipe out lots of small spider legs. Use your spider bodies as a guide to help you know how large to make them. They need to be sturdy enough to support the body, so go over the legs once or twice until they’re a good thickness. Make more than you need, because some will inevitably break during assembly! Once your legs are piped, refrigerate the tray to set the legs.
To assemble the spiders, dip the tip of a leg in melted candy coating, then gently insert it into one of the holes in the body. Repeat until the spider has 8 legs. I found it was easiest to do one side, then flip the body upside down and insert the legs on the other side, and let the spider dry upside down. If the legs start breaking or melting, refrigerate them until they are firm again. Once the chocolate has set, your spiders are ready to be used!