In this tutorial we will rig a spider like robot, same settings will be used for rigging the insect legs.
Most of the times the leg setup for this type of character will include using spring IK but, spring IK will force you to forget FK like movement like ball Row in biped characters. In this tutorial well develop a rig that will give you the ability to have these type of attributes. So lets start.
Here is a spider like model made by one of my friends pit.possum@hotmail.de
This robot has six legs so we develop a rig for it like picture below.
and we simplify it for this tutorial to look at just one leg : the middle on

Now below is the bone hierarchy for this rig
For this rig we need to duplicate the leg right after the root bone and rename them with _IK suffix
Picture in left shows the result for this action. Now we need the spring IK. Normally you wont find this IK in Maya and you need to first activate the plugin in the window ---> preferences ----> plugin manager. After that you must write ikSpringSlover; in the command line. Now if you open the joint tools option box and look at the solvers you can see spring solver.


Create a spring IK from R_M_1_IK to R_M_IK_Tipp and name R_M_Spring_IK like the picture Below.
 Next job is to create 1 degree EP curve and put EP for each bone in hierarchy (hold V key for vertex snap). Then create cluster for each CV in the curve. These clusters will be used for later movement of the skinned joints with user defined attributes.
Next job is to create a spline IK for R_M_1 to R_M_Tipp using the settings below and using the curve you made in previous step.
Rename this new IK R_M_Spline_IK.
New how will the spring IK chain move the spline IK one?
Its simple just select the bones that has _IK suffix and then select the curve and use smooth skin with selected joins option.
Now if you move the spring IK handle both chains will move and if you try moving the clusters youll see that you can have FK like movement in joints of skin hierarchy. Now its time for makings our animation control handles (I call them Helpers). Create a NURBS curve primitive and move it to the place of tip bone and freeze transformations and rename it R_M_Tip_HLP. Duplicate it and move to the place of R_M_2 join and move it a little more up in just Y direction freeze transformations and rename it R_M_POL_HLP. The picture shows the result for this step.
Now parent constraint the Spring IK handle to the tip helper and pole vector constraint it to the pol helper.
Select tip helper and these attributes to it : tipRow, m4_Row, m3_Row as float and with minimum of -10 and maximum of 10, the default value will be 0. Now open the set driven key setup window and use this helper circle as driver and select the tip cluster as driven. Use the values you like the result for cluster translate in -10 and 10 values for the helper. And 0 translate in clusters for 0 value of helper.
The last job is to create other helpers for root and global controls.
Make two more circles , leave one of them in origin of the scene and call this one global_HLP. Move next one to the point of the root joint in hierarchy and name it root_HLP. Freeze transformations for this new helper. Now parent constraint the root joint to the root helper. Select the pol helperand group it by itself and name it R_M_POL_GRP. Parent constraint this group to root and tip circles. Now if you move the tip helper the system will move correctly.
For the global helper, parent everything to it except the clusters and the curve you used for spline IK. Leave them out of this group. You are done and now can do the setup for other legs.
Keep in mind that if you parent clusters or spline IKs curves under global helper and try to scale the global helper youll get double transformation.
Hope you enjoyed this setup. Keep in mind that this setup will have problems sometime when you move the tip helper and try the attributes for FK, you wont get the exact result you expect but, its the only way to have this movement on top of spring IK.
If you have any problem or question email me at : ali@cgfx3d.com
there are more tutorials in CGFX3D
have fun