Lethe

Jong, Rienk De, 2017, Fossil butterflies, calibration points and the molecular clock (Lepidoptera: Papilionoidea), Zootaxa 4270 (1), pp. 1-63 : 22-23

corbieri. Lethe (?) corbieri Nel, Nel & Balme, 1993.

Nymphalidae : Satyrinae (subdivisions not yet generally agreed upon).

France, Dép. de Vaucluse, Céreste; Rupelian, early Oligocene.

Depository: holotype, Musée du Parc Naturel Régional du Lubéron, Apt, Vaucluse, France. Published figures: Nel, Nel & Balme (1993: Figs. 1–3 );? Pfretzschner (1998: Figs. 1–3 ).

A nicely preserved specimen, probably female. Antennae tricarinate (autapomorphy of Nymphalidae , cf. Ackery et al. 1999), club weakly developed. Eyes hairy. Forelegs reduced (as in all Nymphalidae and males of most Lycaenidae ). Forewing with vein 2A missing; radial formula 1, 2, 3+(4+5); R-branches and M1 originating very closely together from upper corner of cell; cell closed, about half as long as costa; Sc and 2A are weakly but distinctly swollen at their base; outer margin indented between M3 and Cu1 (or apically produced); prominent white-centred eye-spot between M1 and M2, and a small dark dot directly beneath it. Hindwing venation without special characters; cell closed; veins M3 and Cu1 originating from the same point (as in many Nymphalidae ); outer margin indented between M2 and M3; in the original description it is interpreted as a slight extension at the end of vein M3 reminiscent of a rudimentary tail. Rather well preserved pattern of eye-spots and a submarginal line on forewing and hindwing.

A swollen base of Sc and 2A in the forewing is an apomorphy of Satyrinae . It is, however, not universal, and absent in genera like Melanitis and Lethe ( Ackery et al. 1999; see also below). Moreover, a swollen base of Sc is also found in various other nymphalid genera, as mentioned below, under Satyrites reynesii , but in that case the forewing cell is open, or practically so. The eyespot between M1 and M2 also is widespread but not universal in Satyrinae .

The authors discussed the fossil at length and compared it with the information in the monograph of Miller (1968). They arrived at the conclusion that it most probably belongs to the “groupe de Lethe ”, at present in the tribe Satyrini . Since they could not identify the fossil more precisely, "nous nous contenterons de le placer dans le genre Lethe sensu lato " ("we must be satisfied with placing it in the genus …"), as an indication that the fossil belongs to the group of genera considered closely related to Lethe by Miller (1968). In their nomenclature they tried to express this by naming the fossil " Lethe (?) corbieri n.sp." However, the "groupe de Lethe ", apparently the same as what was called " Lethe -series, sensu stricto " by Miller (1968), contains 26 nominal genera that nowadays are divided among at least two tribes ( Satyrini and Elymniini ) in the classification followed by the Natural History Museum, London, in their catalogue of butterflies and moths of the world (http://www.nhm.ac.uk/our-science/data/butmoth/), which is based on the spreadsheet of Lamas_Genera_ 041108.xls. Moreover, the use of the genus name, even with a question mark, suggests a closer relationship than may be real. In extant Lethe there are no basally swollen veins in the forewing ( Ackery et al. 1999; Schatz & Röber 1892: Pl. 33); at most, Sc is thicker in its proximal part than the radius, or fused with the proximal part of the radius to a thicker vein (see, e.g., Bascombe et al. 1999, Fig. 9.10). It is uncertain if such a subtle difference is clearly visible in a fossil.

Peña & Wahlberg (2008) used the fossil for calibrating the molecular clock of their phylogenetic tree of Satyrinae , placing it at the point of divergence of Lethe and Neope , the only two genera of Miller's (1968) " Lethe - series, sensu stricto " included in their analysis. Wahlberg et al. (2009) included more genera of this group in their analysis of the Nymphalidae . Lethe ended up as sister to the genera Satyrodes and Enodia combined; together they were sister to the genus Rhaphicera , which in Miller's (1968) study was closer to the " Pararge -series", and the four genera together were sister to Neope . In this study the fossil was used to constrain the split between Lethe and its sister group ( Satyrodes + Enodia ) to the age of the fossil (supposed to be 25 Ma). This is not the same calibration point as in Peña & Wahlberg (2008). Supposing the relationship as found in Wahlberg et al. (2009) is correct, the calibration point in Peña & Wahlberg (2008) is too low down in the tree. Since the number of substitutions since the split between Neope and its sister group must have been greater than since the split of Lethe and its sister group, the substitution rate was higher, consisting of more substitutions over the same period), favoring the calibration point of Peña & Wahlberg (2008) rather than with the calibration point of Wahlberg et al. (2009). A higher substitution rate leads to lower divergence time estimates. If all 10 genera of the subtribe Lethina ( Wahlberg & Peña 2015) had been included in the analysis, the outcome may have again been different. Whether in this case the difference would be significant, remains to be seen. I just draw attention to a) the importance of finding the correct position of the fossil on the tree, and b) the large impact of the tree itself, for making reliable estimates of divergence times. Since there is still so much uncertainty about the phylogeny of the subfamily and the exact position of the fossil on the tree is difficult to ascertain, this fossil can, for the time being, only be used as minimum age calibration point at the root of the Satyrinae , based on similarities rather apomorphies.

Pfretzschner (1998) recorded a partly damaged forewing from Céreste as belonging to the same taxon. He again described (and figured) the basally swollen Sc and 2A, and the eyespot between M1 and M 2 in the forewing. Since the length of the cell (as far as can be deduced from what is visible of the veins, and extrapolating the front margin and termen to the probable position of the apex) is ca. 40 % of the length of the wing), whereas it is about half the length of the wing in Lethe (?) corbieri, it could well be the remnant of a different taxon. According to Pfretzschner (1998) the wing design and the uniform pigmentation of the wing surface are identical to Lethe (?) corbieri; but since the fossil is so fragmentary, and the external similarity of many extant satyrine taxa can be confusing, identification of this fossil with Lethe (?) corbieri seems premature. See also the discussion on the plasticity of wing pattern under Neorinopsis sepulta (Boisduval) . Moreover, in the satyrine tribes Brassolini , Morphini and Amathusiini , the eyespot of the forewing may be present on the underside only, and since the thin chitinous wing surface may be completely lost in the fossil and only the melanin may be preserved (see Pfretzschner 1998 for chemical background; see also Labandeira et al. 2016 for detection of melanin-bearing eyespots in much older insects), it cannot be decided whether the eyespot in the living insect was on the upperside or underside of the wing.

Figures

FIGURE 1. Localities of fossil butterflies plotted ion a modern map. See Table 1 for further, complementary information.

FIGURE 2. Apomorphies, their placement on a tree, and the assignment of calibration points. See text for explanation.

FIGURES 3 – 4. Wing venation of (A) Pareronia Bingham, 1907 (Pieridae), and (B) Hasora Moore, 1881 (Hesperiidae). See text for explanation.