The pectines are complex sensory organs that extend from the ventral surface of the anterior opisthosoma (mesosoma) in all extant scorpions and nearly all the fossil scorpions that have been examined. The pectines are synapomorphic for the Order Scorpiones. In this investigation, the scanning electron microscope (SEM) was used to study the development of the pectines in representatives from five scorpion families. In the more basal families (e.g., Vaejovidae) with apoikogenic development, the pectines start to develop early with enlargement of the limb buds on the third opisthosomal segment. The primordial pectines become elongate lobes attached to the ventro-lateral surface of the mesosoma. A series of striations develop on the pectinal surface, and these striations eventually become a row of teeth at the posterior margin of the pectines. As they develop, the pectines temporarily form the ventral surface of the fourth opisthosomal segment. The distal ends of the pectines separate from the fourth segment and become mobile, attached only at their proximal ends. This is the state of pectinal development as spiracles (indicators of terrestrialization) and sternites become evident on the ventral surface of mesosomal segments. The pectinal peg sensilla start development much earlier than the other sensory structures on the scorpion. The peg sensilla first appear in the embryos as small knobs on the ventral surface of the pectinal teeth. A tiny opening develops in the center of each knob, and nerve fibers can be seen extending toward the central nervous system. The early development of the sensilla and some first-instar choice experiments suggest the pectines may be functional in advanced embryos and first instars. The walls of the primordial knob-like sensilla gradually elongate so the second instar has many peg sensilla on the ventral surface of each pectinal tooth. In the more derived scorpion species with katoikogenic development (Hadogenes paucidens, Hemiscorpiidae; Pandinus imperator, Scorpionidae), the pectines start development in the embryos, but the early steps were not observed as in the more basal families. Pectinal formation appears to be accelerated with some early steps reduced or eliminated. In embryos of H. paucidens and P. imperator, the pectines are separated from the ventral surface of opisthosomal segment 4 and attached only at their proximal ends as the spiracles and sternites appear on the ventral surface of mesosomal segments. Spiracle and sternite development is preceded by the appearance of sturdy, transitory setae (bristles) on the ventral body surface. These setae have no apparent function in the embryos, but would seem to be advantageous in an aquatic environment with prey and predators beneath the scorpion.
Farley, R. D. 2011 . Pectine development in scorpion embryos and first and second instars. Euscorpius, No. 120: 1-47.