Recipe credit: Not Martha
And these don't need to be just for Halloween - maybe you are having a dinner party for that entomologist or arachnologist in your life, or a child's creepy crawly party, or just because - because they are too darn cute.
Don't fill your cavities more than half full; you don't want a dome to form. If you happen to get a large dome, just take some dental floss and cut them off so your spider will sit level.

Make sure to grease and flour your pan well. Some bakers swear by the baking spray with flour. I have not used this product; but as you can see in the upper right-hand photo, I had problems with two of the six sticking to the pan.
If you don't want to glaze your spiders, and don't want to deal with filling them with cream and fruit as Megan at Not Martha did, you can always pipe some of the frosting into the cake itself by inserting a small decorating tip into the bottom of the cake in two or three places.
The cake recipe I used is from the back of the Hershey's cocoa powder container. It is so soft and moist, it really doesn't need much icing. For the head and legs, I used Betty Crocker tub frosting (gasp!) So unlike me to use a packaged frosting. Normally I would make a ganache for this - but laziness won out.
For this recipe I used a Wilton mini-ball pan, and made only half recipe of the Hershey's cake recipe - this yielded six spider bodies, with a little left over - maybe enough for one or two more bodies.
Staying with the spider theme - here is a picture of a tarantula we saw on our hike this past weekend. :)