Developmental changes in the embryo, pronymph, and first molt of the scorpion Centruroides vittatus (Scorpiones: Buthidae)

Abstract

For the first time the scanning electron microscope was used to compare developmental changes in scorpion embryos and the first and second stadia. In the buthid species of this study, Centruroides vittatus, 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 in a few days they disperse, fully capable of foraging in the terrestrial environment. The results here support earlier suggestions that the first stadium (pronymph) is a continuation and extension of embryological development. The first molt results in a nymph with exoskeletal features much like those in the adult. In the first molt the metasoma becomes relatively longer, and the sting (aculeus) becomes sharp and functional. The metasomal segments are modified for dorsal flexion and sting use. The embryos and the pronymphs have spiracles that open into an invagination near the posterior margin of flap-like abdominal plates in segments 4-7 of the ventral mesosoma. The second instars have spiracles that lead to book lungs farther anterior in sternites. Tubular legs with cylindrical segments in embryos and pronymphs become more sculptured and oval in the transverse plane. Each leg in the pronymph has a blunt, cup-shaped tip while distal claws (ungues, dactyl) are present in the second instar and subsequent stages. There are some sharp bristles and primordial sensilla in the pronymphs, but the second stadium has adult-like surface features: rows of knobs or granulations (carinae), serrations on the inner surfaces of cheliceral and pedipalpal claws, filtering hairs at the mouthparts, peg sensilla on the pectines, and mechano- and chemoreceptor sensilla on the body and appendages. Scorpion embryos and pronymphs have some structures like fossil scorpions thought to have been aquatic. There is a gradual development of features that appear to be terrestrial adaptations. Evidence is provided for the formation of the sternum from third and fourth leg coxal primordia and possibly from the first abdominal segment. This study is the first to provide evidence for a forward shift of the gonopore along with other structures in the anterior abdomen.

Development of respiratory structures in embryos and first and second instars of the bark scorpion, Centruroides gracilis (Scorpiones: Buthidae)

“…Comparison of the ultrastructure of first and second instars in this and an earlier study (Farley, 2005) shows that the first molt is a time of accelerated development with modification and formation of many structures. The second instars are nymphs, very much like the adult except smaller in size and not yet sexually mature (Fig.…”

“…As described earlier (Farley, 2005), the first instar on the mother's back is still somewhat embryonic, with many developing features that are a continuation of those in the embryo. The first instar mouth is directed forward (Fig.…”

“…be used to view the chamber (atrium) just inside the open spiracle (Farley, 2005). This procedure along with section of the spiracle and atrium may show no indication of book lung lamellae, or possibly some slender strands of cells and cuticle extend forward from the atrium (see Fig.…”

“…The second instars are nymphs, very much like the adult except smaller in size and not yet sexually mature (Fig. 17A,B; Farley, 2005). The coxapophyses of the first and second legs are broad and flat, forming the floor of the preoral chamber.…”

Posição filogenética dos ácaros Actinotrichida (Chelicerata) e espermatologia comparada dos Halacaridae (Actionotrichida)

“…Escorpiões e solifuges retêm precursores de apêndices no primeiro segmento opistossomal durante o desenvolvimento embrionário (Farley 2005: fig. 1, Yoshikura, 1975.…”