Centruroides vittatus (Farley, 2005)

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

Centruroides vittatus ( Buthidae )

Figure 7 is a ventral view of an embryo mesosoma of this species. Evident are ribbon-like pectines with transverse striations. The pectines are elongate and fairly uniform in width except for tapering at the proximal and distal ends. The pectines are an integral part of the body wall, forming part of the ventral surface of opisthosomal segment 4. A slight indication of opisthosomal segment 1 can still be seen. Opisthosomal segment 2 has two small ridges, not yet starting to form the bilateral flaps of the genital operculum (Figs. 14, 15, 21 − 23, 32).

In Figure 7, the earlier limb buds have disappeared from opisthosoma segments 4 − 8. There may be an initial indication of spiracles (Sp) on some segments; the spiracles appear to be medial to the more lateral location of the earlier limb buds. The distal ends of the pectines will eventually separate from the ventral surface of opisthosomal segment 4, and spiracles will subsequently appear on this segment ( Farley, 2005).

The early stages in the formation of pectinal peg sensilla are shown in Figure 8. On the ventral surface of the pectinal teeth, these primordial chemoreceptors begin as small, pale knobs on the cuticle surface. A small opening develops in the center of the knob. These receptors are developing well before any others on the scorpion body. The presence of a central pore raises the possibility that even here in the embryo, these receptors may be sensitive to chemical stimuli. The cuticular walls of these sensilla increase in height, and the central pore apparently lengthens to become an elongate slit (Fig. 11; Swoveland, 1978; Foelix & Müller-Vorholt, 1983; Gaffin & Brownell, 2001; Wolf, 2008; Gaffin, 2010).

Figure 9 provides further support for the hypothesis that these early knob-like sensilla on the pectinal teeth may be functional in advanced embryos and first instars, well before formation of the final structure of the peg sensilla in the first molt (Fig. 11). In Figure 9, the central core of the embryo pectine appears to be mainly yolk, but the ventral part of each tooth has a narrow band with nerve fibers that extend from the peg sensilla toward the central nervous system. Some sensilla at this stage are only associated internally with a small cellular growth (Fig. 10), presumably the nerve and supporting cells that eventually give rise to long nerve fibers to the central nervous system. The cuticle of the pectine is underlain by a layer of hypodermal cells, which presumably is the source of the sensory nerves and supporting cells.

The peg sensilla appear to be fully formed and functional in the second instar (Fig. 11). They are very dense on the ventral surface of each pectinal tooth, separated only by 2 − 5 µm in the example of Figure 11. The pectines are 2 − 3 µm in length with what appears to be a flexible base and a distal slit that allows chemical stimuli to reach the sensory dendrites inside the sensilla (Foelix & Müller-Vorholt, 1983; Gaffin & Brownell, 2001; Wolf, 2008; Gaffin, 2010). In the embryo sensilla of Figure 9, it appears that a single nerve fiber extends inward from each sensillum with its central pore. In Figure 11, the pegs have a faint striated appearance as though there are numerous dendrites exposed to the elongate distal opening, as in the adult peg sensilla (Foelix & Müller-Vorholt, 1983; Gaffin & Brownell, 2001; Wolf, 2008; Gaffin, 2010).