I sprued up as follows: On the thorax of the insect, at the bottom, I put the main sprue, about 6-10mm thick. I make it as thick as possible, even as thick as the width of the abdomen. An Orb spider has very thin legs. This is a problem. You have to sprue each leg just above the foot, sometimes on the shin and also on the thigh.( sorry, I don't know the correct names) With an Orb I would probably use 4 sprues on each leg 1.2mm thick, only on the smooth parts. Also you have to thicken all the joints with a bit of wax applied with a hot needle. If you don't there will be porosity there, and they will break off after casting. Normally sprues will be about 1-2mm thick. >From the main sprue a 2.5mm sprue will go to under the pedipalps. Always sprue underneath the insect. Thicken the joint with wax between the head, thorax and abdomen if necessary A 3mm sprue will go to under the middle of the abdomen from the main sprue. Your Orb will have 35 sprues when you finished.. A spider abdomen becomes soft after thawing and deforms under its own weight as it rests on the sprue.I think an Orb won't ( I have never cast one) but I think they have a harder skin than the soft furry abdomen of say a Sun spider. Low temperature wax applied with a very hot needle permeates into the skin without loosing too much detail and solves the problem somewhat. I have learnt that over sprueing is always the best. It makes finishing of a real mission but it is better than having a miscast. I always cast in silver and I make up a fresh alloy at 95% fine with 5% deox .I don't use copper although I have been successful with a 97% solution. Fine silver does not work for me. I use higher purity alloys because when the insect is sprued up it is not in a natural position and therefore it makes bending it back easier after casting. 

I use Satincast 20 First I mix up some plaster and paint it on the insect. Then as it starts to gloss over I mix more and paint on more. I do this at least three times. If you let it harden too much between the layers they separate and you get major flashing. As the last layer glosses over I put the flask over and pour it full. Obviously vacuuming is not an option. I would use a 4 inch flask for that size spider and I would carve the button cavity about a third larger, so you can put a LOT of metal in. No less than 100- 150 grams. I burn out for 8 hours at least. Centipedes, as they burn out, for a long time smell like baking biscuits (no kidding) Most insects smell like Satan's own cesspool.. Be warned… I use a normal cycle, then I keep the temperature at 700 to 750 Celsius for at least two hours sometimes more. I cast at 650-700 Celsius. I don't get sulphur contamination. I wind my (cheepo) spin casting machine two and a half times normally, but for insects I add one more turn. As soon as the button is not red in neon light I quench. I had never had temperature tear yet…    Cleaning up is a real hassle. I use diamond and tungsten carbide burrs and steel brushes. I clean the oxide off with sulphuric acid, by heating the insect up and then quenching it in cold acid. Man, I hate doing that! But it works well. Sometimes a bit of chitin is stuck on top of the insect but it usually blends in with the texture of the body. I have found, trying to burn the Chitin (sp?) out of an insect is a waste of time (for me). I just blast it away with the highest temp and max power I can get away with. Then Liver of Sulphur and a light pass with a steel brush. I have often wondered if electro polishing would not be a better option, on account of all the fine little hairs get broken off with a steel brush., Maybe someone could tell...                                 

I have never been successful with winged insects, even if I thicken up the wings with wax. I have successfully cast the legs and claws of a road kill Pels  Fishing Owl. I know the bone did not burn out completely, being calcium, so I assume that the silver encased it. I have never been successful at casting sea shell or coral. I vulcanize it and cast instead. A bit macabre, I know, but all insects shown were either dead or dying.

Cheers, Hans Meevis