Prionopodella Verhoeff, 1925

Edgecombe, Gregory D., 2008, Gonopod segmentation and the Australian centipede Prionopodella (Chilopoda): testing a basal position in the Scutigeromorpha, Journal of Natural History 42 (17 - 18), pp. 1289-1301 : 1290-1300

publication ID	https://doi.org/ 10.1080/00222930801947912
persistent identifier	https://treatment.plazi.org/id/03AF879B-6F5A-997A-FE5C-FACC8D11FD10
treatment provided by	Felipe
scientific name	Prionopodella Verhoeff, 1925
status

Treatment

Prionopodella Verhoeff, 1925 View in CoL

Type species

Prionopodella pectinigera Verhoff, 1925 .

Prionopodella pectinigera Verhoeff, 1925 View in CoL

( Figures 2–5 View Figure 2 View Figure 3 View Figure 4 View Figure 5 )

Material studied

QUEENSLAND: Syntypes, 2 ♀♀ (one juvenile), „, Colosseum, housed at Naturhistoriska Riksmuseet, Stockholm; QM S75517, S75518 View Materials , 2 ♀♀, 2 km NNW

of Mt Robert, 21 ° 219S 148 ° 290E, 360 m, 19.xii.2000 - 25.iii.2001, pitfall, D. Cook and G.B. Monteith, vine scrub.

Description

The following supplement to Verhoeff’s (1925:32–33) description is based on the new specimens from Mt Robert.

Size. Length of body 14–15 mm.

Colour. Head and tergal plates pale yellow-brown; pale brown longitudinal bands along each side of lighter median band on TT1 and 2 in one specimen; legs yellow, without bands on prefemur, femur or tibia.

Head. Antenna 1.9–2.1 times length of body. 68/61 and 58/58 articles in flagellum I; 124–135 articles in flagellum II. Anterior projections of cephalic suture short, fairly divergent.

Epipharynx. Lateral bar of labral trapezoid with narrow longitudinal groove along whole length of bar ( Figure 3A View Figure 3 ). Labral bristles differentiated into narrow outer band of short, pectinate bristles and wider inner band of longer simple bristles ( Figure 3B View Figure 3 ). Two clusters of sensilla along midline of labral trapezoid: more distal unpaired, transverse group of about 20 bottle-shaped sensilla at termination of median ridge, and proximal cluster composed of three aggregations of sensilla at proximal sclerotized bulge ( Figure 3C View Figure 3 ); proximal aggregations comprise a lateral pair of groups of about 18 mostly bottle-shaped sensilla and a few button-shaped sensilla ( Figure 3D View Figure 3 ), and slightly distal to the lateral aggregations a median side-by-side pair of button-shaped sensilla. Chevron-shaped spine row of triangular and distallycurved denticles at border between labral and clypeal part of epipharynx; immediately proximal to spine row is dense median field of pectinate spines ( Figure 3E View Figure 3 ) composed of broadly rhomboid distal portion with pectinate spines and narrow, subparallel-sided proximal portion with multifurcating spines ( Figure 3G View Figure 3 ). Broadly ovate cluster of about 16 nipple- and bottle-shaped sensilla on medial part of clypeal triangle, a short distance behind dense field of branching spines ( Figure 3G View Figure 3 ); ovate groupings of about 10 nipple-shaped sensilla form pair of lateral clusters set in weak depression at a level of about midlength of dense median spine field. Pair of lateral spine fields ( Figure 3F View Figure 3 ) proximal to lateral clusters of sensilla within clypeal part of epipharynx; spines uniformly slender, elongate.

Hypopharynx. Elongate, projecting deeply into preoral chamber ( Figure 5A View Figure 5 ). Bars of lateral sclerotised fork with lateral bulges. Median excavation on proximal part of frontal surface bordered laterally by flattened bipectinate bristles ( Figure 5B View Figure 5 ); pectinate bristles additionally form a few rows on distal part of hypopharynx. Paramedial rows of button-like sensilla on tip of tongue. Cluster of sensilla-like structures in front of mouth between median excavation and converging flattened bars of proximal fork arranged in an ovate, transverse band.

Tergites. Shapes as shown in Figure 2A, B View Figure 2 . Spiracles short; stoma saddles gently inflated. Large unpaired spines associated with a bristle confined to TT4-5 or TT4-6: T 4 with three spines paired with a bristle on each side of midline in QM S75517, one spine paired with a bristle on each side of midline in QM S75518 View Materials . T 5 with one or two spines paired with a bristle on each side of midline in S75517, one or no spine(s) paired with a bristle on each side of midline in S75518 View Materials ; one spine paired with a bristle on each lateral side of S75517, one or no spine(s) paired with a bristle on each lateral side of S75518 View Materials . T 6 with one spine paired with a bristle on each side of midline ( Figure 2E View Figure 2 ) and one spine paired with a bristle on each lateral side of S75517; no spines on T 6 of S75518 View Materials . All tergal plates with mix of Stachelborsten and Tastborsten of similar length ( Figure 4D View Figure 4 ), all with short paired spines at their bases ( Figure 4C View Figure 4 ). Spinula conical ( Figure 4B View Figure 4 ), most separated by a few cuticular polygons that lack spinula, generally more densely arranged on stoma saddles ( Figure 4A, E View Figure 4 ) than on medial part of tergal plates ( Figure 4D View Figure 4 ). Large spines lacking on margins of tergal plates; small spine or pair of spines ( Figure 4F View Figure 4 ) developed at base of short Stachelborsten; marginal Stachelborsten alternating with Tastborsten ( Figure 4G View Figure 4 ).

Legs. Tibial end spines in 1/0 arrangement on legs 1–3, 1/1 arrangement on legs 4–12 or 4–7 and leg 15, 1/2 arrangement on leg 13 or commencing from leg 8 (with small posterior spine on legs 8–10). Tarsal papillae on legs 1–14 ( Figure 5G View Figure 5 ) as described by Verhoeff (1925), on successive tarsomeres; as few as three setae in cluster beside tarsal papillae. For number of tarsomeres in QM S75117 and QM S75118 View Materials , see Table 1.

Female gonopod. Maximum length 1.0–1.2 times maximum width (ratio A/B of Würmli 1973, fig. 1); longitudinal median suture in proarthron complete. Syntelopodite strongly divergent. Ovate depression (Grube of Verhoeff) on proarthron shallow, bearing a few short setae and similarly long slender hairs. Proarthron 1.2–1.3 times length of mesarthron (ratio C/D of Würmli 1973). Cluster of up to eight thickened setae at distomedial corner of mesarthron ( Figure 5D View Figure 5 ). Sinus between mesarthron broadly parabolic, rounded medially; width of mesarthron 26– 29% maximum width of sinus (ratio F/G of Würmli 1973). Proarthron+mesarthron 0.95–1.05 times length of metarthron (ratio C+D/E of Würmli 1973). Syntelopodite with incomplete joint between proarthron and mesarthron on one side of QM S75518 View Materials only, not reaching lateral or inner margins, completely indistinct with SEM; joint indistinct in light microscopy on QM S75517. Metarthron with a single row of about 12 sensilla along its ventral surface and another row of about eight sensilla along its outer surface ( Figure 5E View Figure 5 ); sensilla conical (apparently sensilla coeloconica; cf. Ernst and Rosenberg 2003), surrounded by circular rim ( Figure 5F View Figure 5 ). Metarthron terminating in a blunt point.

Discussion

The most outstanding character of Prionopodella cited by Verhoeff (1925, 1936) is an articulation between the proarthron and mesarthron of the female gonopod. Examination of the slide upon which Verhoeff based his drawing (1925, pl. 1, fig. 6) shows that an articulation is indeed well defined; the drawing is accurate in all fundamental respects. This is not the case in the smaller, alcohol-preserved syntype, which has no indication of an articulation, and in the new specimens an articulation is only partially developed on one side of one specimen. It is not continuous with either the inner or outer margins of the gonopod and, though visible in light microscopy, it has no expression whatsoever when the specimen is examined by scanning electron microscopy ( Figure 5C View Figure 5 ). Given its variable expression, the diagnostic value of the articulation is evidently less than was argued by Verhoeff. It may be that the absent or incomplete articulation in the three smaller specimens is due to their immaturity, i.e. that an extra articulation is acquired late in ontogeny, though such a late ontogenetic change might be rather unexpected given that species with a single articulation (between the mes- and metarthron) conserve that structure from the agentialis stage ( Thereuonema tuberculata: Murakami 1956 , figs. 2, 3; Thereuopoda clunifera: Murakami 1959 , fig. 4).

The variable presence of an articulation between the proarthron and mesarthron in Prionopodella pectinigera is identified as a pseudoarticulation. The slide-mounted type with maximal expression of an articulation shows that tractor muscles (see below) in the syntelopodite are confined to the proarthron, whereas the mesarthron lacks independent musculation that would signal a true segment. Analogous instances of variably developed articulations that do not correspond to segments in the Chilopoda are observed in the widespread subdivision of the tarsus into two articles in many Lithobiomorpha and Scolopendromorpha . Within those groups, secondary fusion of the tarsal articulation is observed in several lineages (e.g. Lithobius (Monotarsobius) and most species of the Cryptopidae and Scolopocryptopidae ), or an articulation may be faint or variably developed. Another instance of an apparent increase in segment numbers within appendicular structures is an extra articulation that delimits four (rather than the usual three) segments in the second maxillary telopodite in the geophilomorph genus Macronicophilus ( Pereira et al. 2000) .

Conspecificity between the new specimens from Mt Robert and the types from Colosseum is supported by the facts that a trace of an articulation on the syntelopodite was visible on one of the Mt Robert specimens (unique within the Scutigeromorpha ), the inner margins of the mesarthron share the very wide, shallow gape seen in the types and exceeding that in all other scutigeromorphs at such an advanced stage in ontogeny, the head and tergal plates are yellow-brown, without marked longitudinal bands of darker pigment, with a similar density of bristles (mostly Stachelborsten), the isolated bristles have short paired spines at their bases, and relatively few spine/Stachelborste pairs are developed.

The Mt Robert specimens are smaller than two of the syntypes, which likely accounts for the absence of two rows of spines paired with bristles that flank the midline seen from T 5 in the larger syntypes, as well as the dearth of spines on the stoma saddles ( Figure 2E View Figure 2 ). The specimens from Mt Robert are peculiar in the context of Scutigeromorpha as a whole in having their maximal development of large unpaired spines associated with bristles on TT4 and 5, versus the usual condition of spines most elaborated on TT6 and 7. Characters that remain useful for generic diagnosis are the very long antennae, short spiracles ( Figures 2A, B View Figure 2 ), and strongly divergent syntelopodite of the female gonopod ( Figure 2C View Figure 2 ). In the phylogenetic analysis described below, Prionopodella pectinigera does not nest within any other genus and its continued recognition as a monotypic genus is accepted. Though Verhoeff (1925, p. 32) emphasized a consistent comb-like shape of the spinula in the female (‘‘sehr feinen und immer kammartig in mehreren Spitzchen vereinigten Haarspitzen’’) as a character by which Prionopodella could be distinguished from the remaining Australian scutigerids, the new specimens have few multifurcating spinula and instead have simple, conical spinula or short spicula ( Figures 4A, B, E View Figure 4 ) on a much larger area of the tergal plates. The morphology of these conical spinula is the same as in Thereuopoda , Thereuopodina and Pilbarascutigera , and is hardly diagnostic.

Phylogenetic position of Prionopodella

Analysis

To test the veracity of Verhoeff’s hypothesis that Prionopodella pectinigera reveals a primitive state of gonopod composition, this species was incorporated into a character set for external morphology of scutigeromorphs ( Edgecombe and Barrow 2007). To include additional diversity from Australia that could potentially impact on the relationships of Prionopodella , the monotypic genus, Prothereua Verhoeff, 1925 , is also coded, based on examination of the type and newly-collected specimens of its type species, P. annulata Verhoeff, 1925 , from Western Australia. The character and state descriptions used in the present analysis are as in the previous work, available online at http://www.mapress.com/zootaxa/2007f/zt01409p050.pdf. (Appendix therein for character list, Table 4 for matrix.) Codings for the 22 species (including five pleurostigmophoran chilopod outgroups) apart from Prionopodella pectinigera and Prothereua annulata are unaltered. Prionopodella pectinigera codes as follows: 1010111102 1111110010 0000201011 0000221111 111100000- 0001000. Prothereua annulata codes as: 1010111102 111110- 010 0000201111 0000221111 111100000 - 0001000.

The most charitable interpretation of Verhoeff’s theory about gonopod composition would identify an articulation as present in the syntelopodite of Prionopodella pectinigera and absent in all other Scutigeromorpha . Coding the presence of additional articulations in the gonopod as plesiomorphic is compatible with evidence from Lithobiomorpha ( Figure 1 View Figure 1 ), though the precise boundary between parts of the syntelopodite of Scutigeromorpha have not been determined in Lithobiomorpha because of profound differences in gonopod structure. The cluster of setae at the distal end of the mesarthron, a landmark that serves to homologise the mesarthron-metarthron articulation across the Scutigeromorpha , is not developed in lithobiomorph gonopods. The only confidently identified point of homology between the two groups is the condyle where the basal article of the gonopod articulates to the first genital sternite. Musculature does not facilitate homology between articles ( Borucki 1996: figs. 99, 101, for Scutigera coleoptrata and Lithobius forficatus , respectively); the intrinsic muscles of the syntelopodite are tractor muscles with no obvious homologue in lithobiomorphs, and the metarthron is devoid of muscles ( Borucki 1996; see also Verhoeff 1902 – 1925, pl. 12, fig. 7 for musculature of S. coleoptrata ). Without precise homologies between the second article in the syntelopodite of Prionopodella and one of the four articles in the lithobiomorph gonopod, a character cannot be rendered any more exact than either ‘‘Articulations in gonopod: (0) two or more; (1) one’’ or an ordered multistate character scoring for four, three or two articles (as mapped in Figure 1 View Figure 1 ). The matrix is analysed with and without this character, using the first (binary) coding option described above when it is included.

The codings include nine characters for the details of the epi- and hypopharynx ( Koch and Edgecombe 2006). In all of these, Prionopodella pectinigera scores as a typical member of the Scutigeridae rather than in an isolated position as would be expected from Verhoeff’s hypothesis. As in all other scutigerids, a groove runs along the lateral bar of the labral trapezoid ( Figure 3A View Figure 3 ), the labral bristles consist of a narrow outer band of pectinate bristles and a wider inner band of longer simple bristles ( Figure 3B View Figure 3 ), a medial pair of side-by side sensilla on the proximal labral part of the epipharynx is situated entirely distal to the lateral cluster of sensilla ( Figure 3C View Figure 3 ), the cluster of sensilla on the medial part of the clypeal triangle is separated from the dense field of branching spines ( Figures 3E, G View Figure 3 ), and the bars of the lateral sclerotised fork on the hypopharynx have lateral bulges.

To compute the most parsimonious cladograms, the data were analysed with TNT version 1.0 ( Goloboff et al. 2003). Multistate characters 25 and 47 are ordered. The small number of terminals permits a rapid solution with implicit enumeration. Jackknife resampling was used as a measure of nodal support, with 1000 jackknife replicates using 36% deletion, each replicate using a traditional heuristic search (1000 random stepwise addition sequences followed by Tree Bisection-Reconnection branch swapping).

Results

Parsimony analysis of the 57 character matrix (i.e. excluding a character for a second articulation in the gonopod) with the parameters specified above yields four shortest cladograms of 81 steps (Consistency Index 0.8395; Retention Index 0.9378; Rescaled Consistency Index 0.7873), the strict consensus for Scutigeromorpha of which is depicted in Figure 6 View Figure 6 . In all of the shortest cladograms, Prionopodella , Prothereua , Thereuopodina and the sister taxa Pilbarascutigera + Thereuopoda form a clade united by the relatively wide sinus between the margins of the female gonopod mesarthron. Though the nodes supporting the deep nesting of Prionopodella within the Scutigeridae and Thereuoneminae are relatively weakly supported (jackknife frequencies,50%, and collapsed with one extra step added to tree length), in all shortest cladograms Prionopodella is several nodes from the base of the Scutigeridae , and scutigerid monophyly is well supported (jackknife frequency 90%).

Adding a binary character for gonopod articulations shared by Prionopodella and Lithobiomorpha but not other Scutigeromorpha , the 58-character matrix yields the same four shortest cladograms as does the 57-character version. Two extra steps to tree length are required to accommodate this character, which optimises as homoplastic in Prionopodella and Lithobiomorpha rather than homologous.

Accordingly, the present analyses retrieve a monophyletic group of Thereuoneminae that is endemic to Asia and Australia ( Thereuopodina : Sri Lanka, India, Australia; Prionopodella , Prothereua and Pilbarascutigera : Australia only; Thereuopoda : south, east and southeast Asia, Bismarck Archipelago, Australia). Prionopodella provides no compelling evidence for an Australian sister group to the remainder of the world’s Scutigeromorpha , but rather it belongs to a thereuonemine clade that is widespread throughout northern tropical ( Thereuopodina , Thereuopoda ), arid ( Pilbarascutigera , Thereuopoda ), and temperate ( Prothereua ) Australia.