FARLEY , R. D.; University of California, Riverside: Scorpion respiratory organogenesis: spiracles, sac-like invaginations and book lung lamellae

Scorpions are thought to be monophyletic, but comparative studies (SEM) show variation in the developmental stage when certain structures appear (heterochrony). Also, transitory structures in embryos of some species may be absent or only slightly expressed in others. Fossils provide evidence of terrestrialization (spiracles, book lung lamellae) in the Devonian/Carboniferous. The initial spiracles in embryos of the basal scorpion families (e.g., Buthidae) lead to sac-like invaginations or slowly developing book lung lamellae. After birth, the first instars continue development on the mother’s back until the first molt to independent, foraging nymphs. In embryos of some species (e.g., Vaejovis spinigerus, Vaejovidae), spiracle formation is preceded by deep, bilateral lung-like invaginations in the ventral surface of mesosomal segments. In these and other species, small spiracles appear early (before or concurrent with flap-like, primordial sternites) near the ventral, posterior margin of 4 mesosomal segments. Book lung lamellae develop slowly; their formation continues after birth into the first instar and first molt. The spiracles appear to migrate forward as the mesosomal sternites develop. The forward position in nymphs and adults gives space for the elongate poststigmaticus muscle. The spiracles can thus be opened as needed, conserving water. This muscle was not seen in the initial marginal spiracles of embryos. In contrast to later molts, the developing book lungs are not shed with the first molt exuvium. The number of lamellae increases in subsequent molts as the juvenile grows. The pace of book lung development may be a factor for reported species differences in mobility and vitality of first instars. They are weak, clinging, still somewhat embryonic in form and non-fluorescent with UV.