Working on a spiders heart now... I think I will eventually overcome my strange anxiety for spiders. Beautiful creatures. Their heartbeat is the same as with us, humans. (30 to 70 beats per minute) and when they'reexcited, their heart beats as fast as 200 times per minute... 
They carry their heart on their back. The blood is called hemolymph, it's color is bluish because of the copper-containing respiratory pigment hemocyanin. It has the same function of hemoglobin, but instead of iron, it contains two atoms of copper in the center of the molecule, which bind to oxygen. ('Biology of spiders', by Rainer F.  Foelix) You can find the book online... very very interesting stuff! On the sketch you can see the heart and vessels. It is an open circulatory. 
I'll make the heart not in wax, as I first thought, but in sculpey. It will probably give me the opportunity to work faster and smaller...