Insertion of Legs

In hexapods, myriapods and crustaceans the coxae of the legs articulate with the body (rarely the hind coxae may fuse to the thorax of some insects), typically on either side of a sternite, i.e. a ventral segmental shield. 

In arachnids, things are a bit different.  In spiders and whipscorpions, a single median sternal plate is more or less well developed and the leg bases (coxae) are inserted around the sternal plate.  Some mites (Parasitiformes) also have a more or less well developed sternal plate.

In acariform mites, pseudoscorpions, opilionids, ricinuleids, and solfugids, however, the leg coxae are fused to the body wall and there is no sign of a sternal plate.  This condition is taken one step further in some mites, when the coxae become indistinguishable from the venter and remain only as internal thickening to which muscles attach (apodemes).