FNZ 46 - Nesameletidae (Insecta : Ephemeroptera) - Introduction
Hitchings, TR; Staniczek, AH 2003. Nesameletidae (Insecta: Ephemeroptera). Fauna of New Zealand 46, 72 pages.
(
ISSN 0111-5383 (print),
;
no.
46.
ISBN 0-478-09349-7 (print),
).
Published 14 May 2003
ZooBank: http://zoobank.org/References/BAB59AE7-4C0F-4901-A54C-76CA14D8B3A2
Introduction
Taxonomic history
The Nesameletidae are a family of amphinotic distribution comprising Ameletoides Tillyard, 1933 of Australia with one described species, Metamonius (Eaton, 1885) of South America with three described species, and Nesameletus Tillyard, 1933 of New Zealand with two described species and three more added here. The taxonomic status of each of these genera is in need of review.
In the past, genera of the Nesameletidae have been commonly placed within the subfamily Siphlonurinae, or as a separate subfamily among the Siphlonuridae sensu lato. The latter were recognised as a paraphyletic assemblage by McCafferty (1991) and Kluge et al. (1995). This consequently led to a split of the Siphlonuridae s.l. ("Siphlonuroidea") into several monophyletic taxa that are now generally classified as families, among them the Nesameletidae (Kluge et al., 1995).
The first imago of the Nesameletidae was described as Siphlurus anceps by Eaton (1883), who shortly thereafter established for it the new genus Metamonius (Eaton, 1885). Eaton (1883) described another species, later to be known as Nesameletus ornatus, under the name Chirotonetes (?) ornatus. He gave complementary details in successive addenda (Eaton, 1885, 1888). As the larvae were unknown at that time, he did not appreciate its similarity to Metamonius, but finally transferred Chirotonetes (?) ornatus to the holarctic genus Ameletus (Eaton 1899). The larva of Metamonius was first described as "Nymphe des Baetis-Typus" by Ulmer (1904), who did not associate it with its corresponding adult forms.
When Tillyard (1923) described the imago of a second species of Nesameletus, N. flavitinctus, he also placed it in the genus Ameletus, but at the same time he noted the considerable difference from its type species, A. subnotatus. Phillips (1930) in his revision of New Zealand mayflies gave a detailed account of Nesameletus ornatus and for the first time described its larva, but still referred to it as "genus Ameletus (New Zealand Type)". The genus Nesameletus was formally established by Tillyard, who also described for the first time the Australian genus and single species Ameletoides lacusalbinae in this contribution (Tillyard, 1933). He was also the first to provide diagnoses separating the two genera. Navas (1935) described the imago of another South American species, Metamonius hollermayeri. Lestage (1935a) was the first to appreciate the similarity of Ulmer's (1904) "Nymphe des Baetis-Typus" with the larva described by Phillips (1930). Lestage (1935a) consequently named Ulmer's South American larva Ameletus fuegiensis, but after notice of Tillyard's new genera he transferred it to Ameletoides (Lestage 1935b). Ulmer (1938) discussed Lestage's (1935b) placement and concurred. Demoulin (1955a) was the first to suspect a synonymy of the imago of Metamonius with the larvae previously described as "Nymphe des Baetis-Typus" by Ulmer (1904) and as Ameletoides fuegiensis by Lestage (1935a). The close relationship of the three genera was fully revealed when Demoulin (1955b) managed to associate the adult stages of Metamonius anceps with its characteristic larva. Demoulin (1955b) also gave a new diagnosis of the genus Metamonius and transferred Ameletoides fuegiensis to Metamonius. Riek (1955) revised Ameletoides.
Kluge et al. (1995) discussed some family characters as well as the possible phylogenetic relationship of the family. Kluge (1998b) enumerated autapomorphies of the family and rejected the discriminating characters for the different genera of Nesameletidae as stated by Tillyard (1933), leaving the taxonomy of the family unresolved.
As a first step to a better understanding of the Nesameletidae, we revise the New Zealand genus Nesameletus in this contribution. We list the hitherto known autapomorphic characters of the Nesameletidae and give a discriminating diagnosis of the genus Nesameletus. Based on museum collections and our own collections undertaken over the past six years, we redescribe the winged stages of the two previously known species, N. ornatus and N. flavitinctus, and give for the first time descriptions of their respective larvae and eggs. The descriptions of three new species are added.
PHYLOGENETIC RELATIONSHIPS WITHIN THE NESAMELETIDAE
Within the Nesameletidae, the different species are phenetically very similar to each other and previous authors have had difficulty in providing separate diagnoses for the three genera. Kluge (1998b) states that "separate diagnoses of these taxa are absent". Indeed, the diagnostic generic characters of Nesameletus listed by previous authors also apply to the Australian and South American species of the family.
Most of the numerous characters that were set out by Tillyard (1933), particularly by contrast with the Australian genus Ameletoides, do not withstand a closer examination. We cannot confirm any of the listed differences regarding the anatomy of the larval mandible, maxilla, tibia, imaginal claws, and hind wings. A strongly reduced paracercus and the basal attachment of MP2 also apply to Metamonius. We also cannot confirm any of the differences noted by Tillyard (1933), Lestage (1935b), and Demoulin (1955a) concerning the serration of the larval claws. Ulmer (1938) repeated Tillyard's assumption of a weak tibiopatellar suture in Nesameletus, a difference we also did not observe (Fig. 4). The additional characters Demoulin (1955b) lists for Metamonius also apply to Nesameletus.
For our cladistic analysis we have chosen 16 characters that were investigated in Ameletoides lacusalbinae, Metamonius anceps, Metamonius sp., and all presently known species of Nesameletus (Fig. 7). Rallidens mcfarlanei (Rallidentidae) and Siphluriscus chinensis (Siphluriscidae) were chosen for outgroup comparison.
The results indicate that each genus of the Nesameletidae is monophyletic. The monophyly of Nesameletus (Fig. 7 char. 5-7) is supported by the presence of terminal fibre-clusters in the eggs (Fig. 63, 65, 67, 69, 71, 73), small spines on the aboral side of the larval prementum basal to insertion of labial palps (Fig. 52-56), and the basal narrowing of the penes in the male adults (Fig. 79-83). We did not have the opportunity to investigate eggs of Ameletoides and Metamonius ourselves, so we base our conclusions on the observations of Koss & Edmunds (1974). However these authors did not observe terminal fibre clusters in Nesameletus either, so it may be appropriate to reinvestigate the eggs of Ameletoides and Metamonius in this respect. The male genitalia of Nesameletus (Fig. 79-83), Ameletoides (Fig. 84), and Metamonius (Fig. 85-86) are similar to each other and have a common medial indention. The monophyly of Metamonius (Fig. 7 char. 9) is also supported by the penis morphology (Fig. 85-86).
Autapomorphies of Ameletoides (Fig. 7 char. 8, 16) are the basal connection of MP2 to CuA in the fore wing (Fig. 18, 102), and possibly the long row of setae on the postmentum in the larva (Fig. 57). Demoulin (1955b) mentioned that the wing character was not consistent, but all the specimens in the BMNH show this character. The postmentum character could not be checked in Metamonius (Fig. 58). Its absence in Metamonius would establish the postmentum setae as further autapomorphic character of Ameletoides. Its presence in Metamonius would point to a sister-group relationship of both groups.
Zoogeographic extrapolations of other amphinotic mayfly taxa would lead us to assume a sister-group relationship between Ameletoides and Metamonius (see also Edmunds 1981), but so far we have been unable to determine an unique derived character that would convincingly establish a sister-group relationship between these two of the three genera. The reduced paracercus of Nesameletus and Metamonius (Fig. 7, char. 15) is in our view also not sufficient to establish a sister-group relationship between these groups, because this character is known to be highly variable within different mayfly taxa. However, as all three genera seem to be separate monophyletic entities, we refrain from synonymising the genera as suggested by Kluge (1998b).
The phylogenetic analysis within Nesameletus clearly points to a closer relationship of N. ornatus + (N. flavitinctus + N. murihiku). This clade (Fig. 7 char. 10-12) is very well supported by the presence of clustered cross veins in the apical half of cells Sc and R1 of the fore wing, the disconnected veins R3A and R3B (Fig. 2, 8, 10, 12), and possibly the two transverse bands of pigments in the subimaginal fore wing (Fig. 92, 94, 96).
A sister-group relationship between N. flavitinctus and N. murihiku (Fig. 7 char.13-14) is supported by the elongated penes (Fig. 80-81) and the thickening of the cross veins between R1 and R2 (Fig. 10, 12, 94, 96). On the other hand, N. austrinus and N. vulcanus may be sister-groups as well (Fig. 7 char. 12). However, this is only supported by the significantly weaker pigmentation of the subimaginal wings (Fig. 98-101) and the character polarisation of the wing pigmentation is somewhat uncertain with respect to the various character states in the different taxa. While the subimago of Ameletoides has numerous irregular blotches (Fig. 102), Metamonius has no pigmentation at all in the subimaginal wing. Outgroup comparison with Rallidens (no pigmentation in the subimaginal wing) would indicate an unpigmented subimaginal wing in the groundplan (see terminology, p. 14) of the Nesameletidae as well, but Nesameletus may have developed a pigmented wing in its groundplan, and the weak or lacking pigmentation in N. austrinus + N. vulcanus thus may be a secondary reduction within the genus. We are aware of the fact that these are not very strong characters to establish a sister-group relationship, but on the other hand we have not found any indication for a closer relationship of N. vulcanus or N. austrinus with any other species of Nesameletus. So at present a sister-group relationship between N. austrinus and N. vulcanus remains the best supported hypothesis.
Each species of Nesameletus has at least one unique autapomorphy. The strong development of chorionic fibre-coils (Fig. 63,73), the spines on the postmentum shoulders (Fig. 52), and the dark medial band across the larval cerci and paracercus (Fig. 23) are autapomorphies of N. ornatus. The wings tinged with greenish yellow and the irregular distribution of the basal prementum spines (Fig. 53) are autapomorphies of N. flavitinctus. The suprachorionic tubercles (Fig. 67) in the egg of N. murihiku, the reddish tinge of the pterostigma in the adult, and the medial notch of the male subgenital plate (Fig. 81) are autapomorphies of this species. The autapomorphic characters of N. austrinus are the elongated female subgenital plate (Fig. 90), the rounded apices of the penes (Fig. 82), and the extension of the pigmentation of abdominal ganglia to several segments. The reduction of the posterior spines on the anterior abdominal terga (Fig.104-107) and the reduction of the lateral abdominal spines are autapomorphies of N. vulcanus.
PHYLOGENETIC RELATIONSHIPS WITHIN THE EPHEMEROPTERA
The phylogenetic relationships of the Nesameletidae to other families have been unresolved and under discussion for a long time (McCafferty 1991; Kluge et al. 1995), but with the recent discovery of the larva of Siphluriscus chinensis (Siphluriscidae) a sister-group relationship between Nesameletidae and Siphluriscidae seems to be possible (Zhou & Peters in press). Both families share the unique anatomy of the larval mandible (Staniczek 2000; Zhou & Peters in press): its incisors are fused and form an elongated gouge-like scraping tool. The prostheca is reduced in both taxa. In Nesameletidae the prostheca is bipartite. Its ventral part is reduced to two bristles in the right mandible and a single bristle in the left one. In both left and right mandible of Siphluriscus the dorsal part is entirely lost and the ventral part is reduced to two bristles (Zhou & Peters in press). The middle articulation of the mandible in both taxa is elevated and forms a plate-like knob, the corresponding part of the anterior tentorial arm is kidney-shaped (own observation, AHS). Similar mandibular arrangements with fused incisors as, for example, found in the afrotropical baetid genus Xyrodromeus (Lugo-Ortiz & McCafferty 1997) are considered herein to have evolved independently. Further synapomorphic characters of Nesameletidae + Siphluriscidae could be the opisthognathous orientation of the larval head and its mouthparts. Correlated with this character is the extremely elongated larval postmentum in both taxa (own observation, AHS). The posterior projection of the vertex is another synapomorphy of the imaginal life stage of both groups (see also Kluge, 1998b).
Prior to the discovery of the larva of Siphluriscus chinensis, all these characters mentioned above were regarded as autapomorphies of the Nesameletidae. However, there still remain some uniquely derived characters that define the Nesameletidae as a monophylum (Fig. 7, char. 1-4). These characters include most likely the arrangement of the quadripartite apex of the maxilla and its spination pattern (Fig. 46-47). The medial part of the lacinia is extended to a membranous lamina (Fig. 46). In the left mandible, the ventral part of the prostheca is reduced to a single bristle (Fig. 39). Finally the tips of glossae, paraglossae, and labial palps are covered with stout setae (Fig. 48-50).
Some characters found in other families and that may be autapomorphic characters of higher taxa were thus not considered. For example, the elongated glossae and paraglossae are also present in the Siphluriscidae and Baetidae and might be also present in the groundplan of Baetoidea (Staniczek 1997). A tubular maxillar gill tuft as in the Nesameletidae is also present in the Rallidentidae (Penniket 1966), whereas similar maxillar and also coxal gill tufts as in the Siphluriscidae are also present within the Setisura. Maxillar gills were even suggested as synapomorphic character of Nesameletidae + Rallidentidae + Setisura by Demoulin (1969). Kluge et al. (1995) rejected the homology of the maxillar gills in these groups, but a closer study of the different maxillar gill tufts in these taxa may shed new light on the phylogeny of these taxa.