FARLEY, R.D.: Striking Developmental Changes in the First Molt of Newborn Scorpions

The scanning electron microscope is used for the first time to compare structures in the first and second stadia. In the buthid Centruroides vittatus (Mexico, southern U.S.) and all other scorpions, the newborn climb up on their mother’s back and remain there without feeding for several days. At this location, they undergo their first molt and then disperse, fully capable of foraging in the terrestrial habitat. The first stadium is a continuation and extension of embryological development while the first molt results in the sudden appearance of some adult exoskeletal features necessary for terrestrial survival. The sting becomes sharp and functional. Metasomal segments are modified for dorsal rather than ventral curvature. Open spiracles and booklung lamellae appear. Tubular, digitigrade walking legs become plantigrade and oval in the transverse plane. Each walking leg in the first instar has a blunt, cup-shaped tip while distal claws (ungues, dactyl) are present in second instar and subsequent stages. In the ventral mesosoma, small flap-like plates with closed spiracles near the posterior margin become broad, flat sternites with open spiracles farther from the posterior margin, as in adults. There are some sharp bristles in the first stadium, but the second stadium has carinae (keels), granulations, serrations, hairs and sensory sensilla (including trichobothria). Small pores are the only pre-molt indication of the pectinal mechano/chemoreceptor pegs that appear after the first molt. Scorpion embryos and the first stadium have some structures like fossil scorpions thought to be aquatic; the first molt produces numerous exoskeletal features that appear to be terrestrial adaptations in extant adults.