At this point we're going to leave the prosoma, as having looked at the eyes, slit sensillae and trichobothria i'd like to stick with sensory structures of a scorpion and that brings us onto a structure unique to the group... the pectines
Sternum, genital operculi and pectines of the extant Gertschius crassicorpus. Image modified from Graham and Soleglad (2007).
Although pectine are sensory structures, their function is still a little unclear, however, they are larger in males and are thought to be used in selecting sites for spermatophores (see Stockwell, 1986). The pectines are made up of a basal plate onto which 2 articulated combs are joined. Just like normal combs, these have rows of teeth along a main branch (called a rachis), which is usually jointed. Each of the teeth have regions of special sensory structures called peg sensillae. There are literally thousands of microscopic peg sensillae on the pectens. These are often arranged into rows and work like antennae, picking up odorants and tastants on the substrate.
In front of the pectines are paired genital plates (OPERCULI); in scorpions they lack respiratory structures.
The overall anatomy of pectens has remained the same since the origin of scorpions. The earliest preserved pectens are found on the aquatic Palaeoscorpius and Allopalaeophonus and it is possible that the earliest scorpions laid their eggs in shallow nests in a similar manner to eurypterids and xiphosurans. If this were the case, than rather being used to position a spermatophore, they may have been used for digging (see Kjellesvig-Waering's monograph on fossil scorpions).
It is speculative to discuss how pectines arose, but it has been suggested that they may have been part of the genital segment and are homologous to (have the same evolutionary origin as) the median appendage of eurypterids, in particular the paired furcae at the end of the eurypterid genital appendage. If this is true, then a protoscorpion/ eurypterid ancestor would have lost the close association between the median appendage and the genital operculi, and later evolved true pectines.
Due to the microscopic structure of peg sensillae, little can be said regarding their evolution on the pectines, but many arthropods (both aquatic and terrestrial) possess peg sensillae in regions of the cuticle. Finally, there are no real trends regarding the number of teeth or divisions of the main branch (rachis). Teeth are very variable in both extant and extinct taxa, and a fully fused rachis can be seen in fossils such as Proscorpius and Branchioscorpio and recent species such as Megacormus.
As for the genital operculi- one morphological feature hinting at the ancestory of scorpions are the eurypterid deltoidal plates and like scorpion opercular plates, these are paired structures. In early scorpions the genital operculi were large and subquadrate (see Branchioscorpio) but circular (Anthracoscorpio), elongate (Kronoscorpio) and small subquadrate (Gigantoscorpio) varieties are present in Palaeozoic scorpions. When scorpion Infraorders were based upon sternal plates, the shape of opercular plates were important in understanding scorpion evolution, however recent systematic evidence shows that there is no real trend in their shape. Confusingly, the genital plates were originally fused, before separating completely. Today the genital operculi remain separate although in most bothriurids they have become loosely fused again.