Development of segments and appendages in embryos of the Desert Scorpion Pariroctonus mesaensis (Scorpiones : Vaejovidae)

The scanning electron microscope was used to study the changing features of scorpion embryos from the blastula through early stages in the development of appendages. The earliest scorpion fossils (Silurian period) have struct ures more advanced than the embryos herein, so the possibility is considere d that these embryos still retain and display some features indicative of e volutionary patterns in adult pre-Silurian ancestors. The blastodisc stage is followed by a knob-like germinal center that gives rise to most of the e mbryo body. The germinal center elongates on the ventral surface of the sph erical yolk mass. The broad cephalic lobe is first delineated from the foll owing pedipalpal segment. The limbbuds for the pedipalps and anterior walki ng legs appear, as additional segments are added at a growth zone at the re ar of the embryo body. Initially, in the cephalic lobe there are no limbbud s; then the cheliceral buds emerge from the posterior part of the lobe. The stomodeum appears first in the anterior half of the cephalic lobe, but an oral groove forms and the mouth is displaced posteriorly within the groove. This repositioning allows space anteriorly for invagination (semilunar gro oves) of epithelium for the brain and medial eyes. The mouth is directed ve ntrally in all stages of this study. The widespread chelicerae are initiall y posterior to the mouth, but later move anterior and dorsal to it. Small l imbbud bulges on mesosomal segments disappear later and never become protru ding appendages. Metasomal segments are produced free from the yolk surface in a ventral flexure beneath the embryo body. The telson starts as two sph erical lobes, but later elongates and tapers distally, not yet developing t he sharp sting (aculeus) seen in Silurian and all subsequent scorpions. The walking legs are digitigrade, as in most fossil aquatic scorpions. Segment s are delineated in the appendages; the chelicerae and pedipalps are divide d distally for chela (claw) formation. Bilateral swellings (limbbuds) on th e third abdominal segment become larger than the others, indicating the sit e of pectine formation. The early fin-like pectines are somewhat posterior in the mesosoma, suggesting ancestral swimming, maneuvering, and balancing for the elongate abdomen. The pectinal surface is initially smooth but late r transverse striations increase the surface area as a possible respiratory adaptation. Pectinal teeth (present in Silurian and all subsequent scorpio ns) and foward movement and merging of anterior abdominal segments are not yet evident in embryos of this study. (C) 2001 Wiley-Liss, Inc.