Hadogenes paucidens, Pocock, 1896

Farley, Roger D., 2011, Pectine development in scorpion embryos and first and second instars, Euscorpius 120 (120), pp. 1-47 : 5-7

Hadogenes paucidens ( Hemiscorpiidae )

An external view of a diverticulum (D) is shown in Figure 17. The diverticulum is an outgrowth of the ovariuterine tubules (OT). Some very small diverticulae (D’) are also evident as tubular outgrowths in this figure. Inside the large diverticulum is an embryo with its head oriented toward the distal end of the diverticulum and the proximal end of the appendix (A). Thus oriented, maternal nutrients can enter the mouth as a result of rhythmic contractions of the embryo pharyngeal muscles ( Mathew, 1948, 1956). Some information is provided about appendix development ( Mathew, 1948, 1956, 1959b) and structure ( Subburam & Gopalakrishna Reddy, 1989).

The embryo of Figure 18 is at about the same stage as the embryo inside the diverticulum of Figure 17. For Figure 18, the outer wall of the diverticulum was opened so the embryo could be seen. The embryo is covered with an embryonic membrane which was torn in some places during the dissection (asterisks). The mesosoma is very large and advanced in development as compared with the prosoma and metasoma. There are no prominent limb buds on the ventral mesosoma although small ones may not be seen because of the covering membrane. Limb buds for the pedipalps and legs 1 − 4 are present, and there are large dorso-lateral processes on the segments of the mesosoma. These processes are thin-walled and filled with fluid ( Mathew, 1956; Farley, 1999), and they readily collapse during preparation for microscopy. An appendix is present at the anterior end of the embryo.

Figure 19 is a higher magnification of a torn embryonic membrane as in Figure 18. The tear during dissection enables a view of the external (EM) and internal (EM’) surfaces of the membrane. This appears to be mainly a layer of cells held together by fibrous material. As the embryo enlarges, the embryonic membrane does not enlarge with it, so gradually the dorsal portions of the embryo grow beyond the membrane. There may be little or no membrane present in more advanced embryos.

A later stage of an embryo is shown in Figure 20. Again, the outer wall of the diverticulum was opened and removed so the embryo could be seen. The appendages and part of the prosoma are no longer covered by embryonic membrane. The pedipalps are divided distally, and segment demarcations can be seen in the legs. No prominent limb buds are evident in the ventral mesosoma although small ones may not be seen because of the covering of the embryonic membrane. The large dorsal processes have collapsed as usual during preparation for microscopy. The mesosoma is very large and disproportionate in comparison to the prosoma and slender metasoma.

Figure 21 shows the earliest example of pectines seen in this species. Although the mesosoma is very large in the embryos (Figs. 18, 20), the developing pectines are not also increased in size. In the species with apoikogenic development, the early pectines are a prominent part of the mesosoma (Figs. 1 − 7, 12, 13). The early pectines of H. paucidens are very small in comparison with the large size of the mesosoma, and the initial stages of pectine formation may be short in duration so they are missed with the procedures used herein.

In this species, the pectines apparently originate as two small flaps from opisthosomal segment 3 without the prolonged earlier stages that are seen in more basal species with apoikogenic development (Figs. 1 − 7, 12, 13). Those early stages may be temporarily displayed during pectine formation in this species, but those stages if present are not prominent. The pectines of Figure 21 are joined with the ventral surface of opisthosomal segment 4 as occurs in embryos with apoikogenic development (Figs. 5, 7). A groove is starting to form along the length of this early pectine of H. paucidens , also as seen in embryos with apoikogenic development (Figs. 4, 13). The groove divides the pectine into anterior and posterior regions (Fig. 21). The posterior region has the outline of teeth, but the teeth are not yet separated from each other. The groove is deeper in the more advanced embryo of Figure 22.

The bilateral lobes of the genital operculum are starting to appear in Figure 21, and there are sturdy setae (bristles, B) forming on the ventral surface of opisthosomal segment 4. These setae are shown in more detail in Figures 23 − 25. Such setae are also present in the embryos of another species with katoikogenic development ( Pandinus imperator , Scorpionidae ; Fig. 31), but they are not seen in any of the species with apoikogenic development examined so far.

The embryo of Figure 22 is slightly more advanced than the embryo of Figure 21. The pectines have separated from the ventral surface of opisthosomal segment 4, and there is a prominent groove (asterisks) along the length of each pectine. The teeth are partially separated from each other. The lobes of the genital operculum are larger than those in Figure 21.

The pectines in Figure 23 are more advanced than those in Figures 21 and 22. In Figure 23, the pectines are attached only at their proximal ends so the distal ends can move freely. Legs 3 and 4 were removed from this embryo, as is commonly done, so the structures in the anterior mesosoma can be seen. The bilateral flaps of the genital operculum are prominent, and the sternum can be seen at the ventral surface of the prosoma. Sturdy, transitory setae (bristles, B) are present on the ventral surface of the pectines and the ventral opisthosoma. As occurs in embryos with apoikogenic development ( Farley, 2005, 2008), spiracles (Sp) become evident on the ventral surface of opisthosomal segment 4 after the pectines are separated from the ventral surface of that segment. Also evident in Figure 23 is a faint crease (Ste) indicating the posterior edge of the developing sternite of opisthosomal segment 4.

The embryos of Figures 24 and 25 are at about the same stage as Figure 23. The pectines are attached to the ventral body wall only at their proximal ends. Spiracles and sternites are starting to appear on the ventral mesosoma. In embryos with apoikogenic development ( Farley, 2005, 2008), the pectines are also advanced and forming peg sensilla at the time when spiracles and sternites start to appear on the ventral opisthosoma.

In Figure 24, epithelial invaginations can be seen for spiracles and book lung tissue ( Farley, 2008, 2010) in opisthosomal segments 4 − 6. In this species and Pandinus imperator (Fig. 31), the transitory setae (bristles, B) appear on the ventral mesosoma before spiracles and sternites start to form. As evident in Figures 24 and 25, the setae are aligned in a pattern that outlines the location of the lateral and posterior margins of the future sternites. A faint crease indicating the pos- terior edge of the sternites (Ste) is evident in these figures.

The ventral surfaces of opisthosomal segments 5 and 6 are shown at higher magnification in Figure 25. The short, sturdy setae (bristles, B) are pointed at their tip, and would seem to be effective for defense and predation at the ventral surface of the individual, possibly an ancestor. The lateral edges of the sternites are not yet indicated by creases, but the setae show where the sternite lateral edges will eventually appear.

Figure 26 shows the ventral surface of the prosoma of an embryo at about the same stage as in Figures 23 − 25. The appendix has been removed, but the chelicerae are highly modified for grasping the proximal end (teat) of the appendix so that nutrients can be delivered to the mouth in response to rhythmic pharyngeal contractions ( Mathew, 1948, 1956). Rhythmic contractions (50 − 60/min.) of the pharyngeal muscles were seen in dissected embryos of this species.

In Figure 26, the preoral tube ( Farley, 1999, 2001d, 2005, 2008) is starting to form surrounding the mouth. The rostrum (labrum) forms the dorsal wall of the tube. The medial surfaces of the pedipalp coxae form the lateral walls of the preoral tube. The cuticle of these medial coxal surfaces have a specialized texture that may macerate food and help secure the proximal appendix. The coxapophyses of the first and second leg are enlarged lobes at this stage. They are starting to extend forward and will eventually form the ventral wall of the preoral tube. A more advanced stage of formation of the preoral tube is shown in the first instar of Figure 14 ( Superstitionia donensis ) and the second instars of Centruroides vittatus and C. gracilis ( Farley, 2005, 2008).

The tip of the pedipalps is bifurcated in the embryo of Figure 26; evident are the features of a claw that will have fixed and movable fingers. On the pedipalps and legs, there are small transitory setae (bristles, B) like those on the ventral surface of the mesosoma (Figs. 21, 23 − 25). These setae disappear, and many sensilla are present on the body surface after the first molt ( Farley, 2005, 2008).

The metasoma in Figure 27 was removed from an embryo at a later stage of development than in Figures 18 − 26. The metasoma has five segments as in the instars, but the telson is only a small lobe and not yet tapered. The fifth metasomal segment has two dorsal lobes that increase surface area of the segment and may facilitate gas exchange with maternal hemolymph as is hypothesized for the dorso-lateral processes on mesosomal segments ( Mathew, 1956). The metasoma of embryos with apoikogenic development is commonly flexed ventrally during development (Figs. 1, 2, 12, Farley, 1998, 1999, 2001a-d, 2005). The flexed metasoma was often removed in the present investigation so the ventral mesosoma could be examined with the SEM. In embryos in this investigation with katoikogenic development, the metasoma lengthens with little flexure relative to the mesosoma (Figs. 18, 20, 27 − 29).

The metasomal segments in Figure 27 have short, sturdy setae (bristles, B) on their ventral surface much like those on the embryo mesosoma (Figs. 21, 23 − 25) and appendages (Fig. 26). The cuticle of the metasomal segments in Figure 27 lacks specialization for the dorsal flexure of the metasoma that begins as newborn scorpions continue development on their mother’s back ( Farley, 2005). In the first molt, the metasomal segments are highly sculptured with lateral pivot points for dorsal flexure and a cuticle shape that prevents ventral flexure ( Bowerman, 1972a, b; Root, 1990; Farley, 2005).