sentences:  The free cheeks (or librigenae) narrow back into straight, slender genal spines that reach as far back as the third thorax segment.The articulating middle part of the skeleton (or thorax) consists of seven segments that end in needle-like spines.The semi-circular tail-piece (or pygidium) has six axial rings that decrease in size backwards and four or five pairs of rearward pointing marginal spines.