Smeringurus mesaensis (Stahnke, 1957)

Farley, Roger D., 2011, Pectine development in scorpion embryos and first and second instars, Euscorpius 120 (120), pp. 1-47 : 3-4

Smeringurus mesaensis ( Vaejovidae )

More is known about the details of pectine development in this species than any other scorpion. The early stages of embryogenesis were described by ( Farley, 1996, 1998, 1999, 2001a-d), and changes in the pectines were sometimes included. The main features of pectine development in this species are provided here to show the sequence that appears to be typical for apoikogenic development. This also gives a basis for comparison with other species examined in this investigation.

Figure 1 shows a ventral view of a scorpion embryo at a stage when the metasoma has nearly all its five segments and has started a ventral flexure. Limb buds in the prosoma and mesoma are prominent, and as reported earlier ( Farley, 2001a) the limb buds on the third opisthosomal segment are larger than the others in the opisthosoma. The third-segment limb buds continue to increase in length and extend dorso-laterally as primordial pectines as the limb buds disappear on most of the other opisthosomal segments. The prosomal limb buds in Figure 1 are still partially covered with amnion, but the pedipalps have started to divide at their distal tip, and the chelicerae are widely separated from each other on either side of the stomodeum (mouth). The two chelicerae will eventually come together dorsal to the mouth (e.g., Fig. 12, 14; Farley, 1998, 1999, 2001b-d).

Figure 2 is at a more advanced stage with the pectine as an elongate structure that extends dorso-laterally from the ventral surface of the third opisthosomal segment. The pectine appears to be attached to the body wall for most of its length. It is fairly uniform in width except for a rounded tip, and it has a smooth surface without the transverse striations that eventually appear (Figs. 3, 4, 7, 13). At this stage, a rostrum (labrum) is evident just anterior to the mouth, and the chelicerae are still far apart on either side of the mouth. The pedipalps are divided distally; legs 1 − 4 are becoming segmented. The metasoma is sharply flexed ventrally and apparently has all five segments with a tapered telson at its tip. Other examples ( Smeringurus mesaensis ) of pectines at this stage are shown in Farley (1998, 2001a).

Especially to be noted in Figure 2, the segments and appendages in all three body regions (prosoma, mesosoma, metasoma) are undergoing substantial development. This is also evident in the other examples of apoikogenic development in this investigation (Figs. 5 − 7, 12, 14). This is in contrast to the species with katoikogenic development (Figs. 18, 20, 28, 29) where the mesosoma is especially large and prominent in comparison with the prosoma and metasoma.

The elongate pectines initially have no transverse striations (Fig. 2; Farley, 2001a), but striations eventually appear. The striations begin first at the distal end of the pectines so the pectines look like paddles at this stage. Figure 3 is a ventro-posterior view of an embryo (supine) with paddle-like pectines. In this view, opisthosomal segments 1 − 8 are clearly visible. The first opisthosomal segment is much reduced in size, and the third opisthosomal segment is clearly the source of the pectines. The pectines slightly overlap the ventral surface of opisthosomal segment 4 at this stage; the overlap becomes more substantial in later stages (Figs. 5 − 7, 21). The prosomal legs are segmented. Other examples ( Smeringurus mesaensis ) of pectines at this paddle-like stage are provided in Farley (1998, 2001a, d).

In a later stage, there are transverse striations for almost the entire length of the pectines. The pectines are narrow in width, ribbon-like and joined with the ventro-lateral body wall. An example of this stage is shown in Figure 7 ( Centruroides vittatus ) and in Farley (2001c, d) for Smeringurus mesaensis .

In Figure 4, a longitudinal groove (asterisks) divides the pectine into anterior and posterior regions. The anterior region is still joined with the lateral wall of the mesosoma while the posterior pectinal edge is starting to separate slightly from the body wall. In later instars, the anterior region will have numerous mechanoreceptors and will contain nerve fibers extending toward the central nervous system from the thousands of peg sensilla on the ventral surface of the pectinal teeth (Figs. 8, 9, 11, 16; Foelix & Müller-Vorholt, 1983; Brownell, 2001; Gaffin, 2001; Gaffin & Brownell, 1992, 1997a, b, 2001; Kladt et al., 2007; Wolf, 2008). The striations at the posterior margin of the pectine in Figure 4 continue indentation and separation of teeth (Figs. 5, 6). A longitudinal pectinal groove is also present in Figures 13, 21 and 22.

In Figure 5 the pectines are oval in shape and have distinct anterior and posterior regions. They are still attached to the ventral surface of opisthosomal segment 4. The transverse striations at the posterior margin have not yet formed separate teeth. At this stage, there are small lobes (coxapophyses 1, 2) medial to legs 1 and 2. These lobes gradually extend anteriorly and form the ventral wall of the preoral tube (Fig. 14; Farley, 1999, 2001d. 2005, 2008).

In Figure 5 the legs (L1-L4) are segmented, and there is a sternum on the ventral surface of the prosoma. Opisthosomal segment 1 is no longer evident externally. Segment 2 is evident but has not yet started to form the paired lobes of the genital operculum that can be seen in later stages (Figs. 14, 15, 21 − 23, 32). Other examples of this oval-shaped stage of pectinal development are shown in Farley (1999, 2001b-d, 2005).

In Figure 6, the thin and transparent amnion was not removed, but the surface features beneath it can be seen. The pectines now have teeth at their posterior edges, and spiracles are evident near the posterior margin of the developing sternites. At the stage of spiracle appearance ( Farley, 2005), the pectines are attached to the body wall only at their proximal end. In Figure 6 the distal pectine ends have probably separated from the ventral surface of opisthosomal segment 4 but are still held there by the amnion. Other examples of embryo pectines at this stage can be seen in Figures 22, 32, and Farley (2001b, c, 2005).

Farley (2001c) provides some information about the development of pectinal peg sensilla in embryos of this species. The sequence is like that shown in Figures 8, 9 and 16 for other species. Small, pale knobs appear on the ventral surface of the pectine teeth. An opening forms in the center of each knob as the wall around the opening increases in height. The pore becomes oval-shaped, apparently the beginning of the elongate slit that will appear later at the distal end of the fully developed sensory peg (Fig. 11).