Mortonagrion

Dow, Rory A., 2011, Mortonagrion indraneil spec. nov. from Borneo, and a redescription of M. arthuri Fraser (Odonata: Zygoptera: Coenagrionidae), Zootaxa 3093, pp. 35-46 : 44-45

Relationship of Mortonagrion with Agriocnemis and Argiocnemis

Mortonagrion has had a confused history since it was erected. A number of species have at various times been placed in Mortonagrion , and at other times placed in the allied genera Agriocnemis and Argiocnemis . The issue of the relationships among these genera has never been satisfactorily resolved, for instance see Pinhey (1974) and Hämäläinen (1989). Until now the sole character that has been considered to reliably distinguish Mortonagrion from Agriocnemis and Argiocnemis is a detail of wing venation: “ ab continued on as 1a in the same straight line, no angulated medio-anal link intervening at the junction ” ( Fraser 1933: 409). The species currently placed in Mortonagrion , excluding M. aborense and M. alcyone and the African M. stygia Fraser, 1954 , share a number of other characteristics, for instance: complicated postocular markings in the males, mesostigmal plates strongly raised up along one or more edges in females (e.g. Figs 8–10 ) and triangular paraprocts (e.g. Fig. 32 ) in females (rather than the rather formless paraprocts typical in the Coenagrionidae ). These species certainly appear to be more closely related to each other than to any other species, but none of the characters mentioned is unique to this group of species, all being found in some species typically placed in Agriocnemis .

M. aborense and M. alcyone both share some characters with Argiocnemis rubescens Selys, 1877 , the genotype of Argiocnemis , that are not shared with the other Mortonagrion species: all have simple postocular markings, relatively simple broad triangular mesostigmal plates and formless paraprocts in the female, and very similar male anal appendages; they also share a typical scheme of colour changes from immaturity to maturity. See Hämäläinen (1989) for a summary of the complicated history of A. aborense ; however it differs significantly in genital ligula structure from all other species examined or for which illustrations were available from all three genera (see below). A. alcyone appears to have become associated with Mortanagrion almost by accident, as it does not even consistently share the venational character, for instance the lectotype male in the BMNH has an angulated medioanal link joining ab with 1A.

M. stygia has mature colouration more typical of many Agriocnemis species, but the female variably has triangular paraprocts. The genital ligula is of Agriocnemis / Mortonagrion form (see below). It is difficult to come to any firm conclusion on the placement of this species at the moment.

Probably the most promising character to distinguish Argiocnemis from Agriocnemis and Mortonagrion , and possibly the latter two genera from each other, is the structure of the genital ligula. The genital ligula of Agriocnemis femina (Brauer, 1868) ( Fig. 25 ), Argiocnemis rubescens rubeola ( Fig. 26 ), Argiocnemis sp. ( Fig. 27 ; a problematic form distinct from, but possibly with a name buried in the synonymy of, A. rubescens ), Mortonagrion aborense ( Fig. 28 ), M. alcyone ( Fig. 29 ), M. arthuri ( Fig. 30 ) and M. indraneil ( Fig. 31 ) were examined by Scanning Electron Microscopy (SEM). With the exception of M. aborense , the genital ligula of all the species examined is broadly similar; the terminal segment has simple-shaped broad apical lateral lobes, at rest directed back on either side of the shaft. In Mortonagrion and Argiocnemis there are rows of short setae or spines dorsolaterally on either side of the shaft; in Agriocnemis femina the spines are absent from the shaft, but there is a small clump of spines on each side of the segment that lies between the apical lobes of the terminal segment. In Argiocnemis rubescens , Argiocnemis sp. and M. alcyone the terminal part of the lateral lobes has a fringe of narrow teeth along its margin, but these teeth are absent in the other species examined. Pinhey (1974) illustrated the genital ligula of all African species known to him, including M. stygia , and Agriocnemis pygmaea (Rambur, 1842) which is also widely distributed in Asia; the genital ligula of all of these species is of the general Agriocnemis / Mortonagrion form. However with the equipment available to him Pinhey may not have been able to see certain features of the genital ligula, such as spines or setae, but it seems likely that a fringe of teeth such as occurs in M. alcyone , Argiocnemis rubescens and Argiocnemis sp. would have been visible to him; presumably no such fringe occurs in any of the species examined by Pinhey. As noted by Pinhey (1974: 271 as M. simile Ris, 1930 ) and illustrated by Asahina (1982: 457, Figs 9–10 as M. binocellata Fraser, 1922 ) the genital ligula of M. aborense ( Fig. 28 ) is of a different form from all the other species considered, with, as Pinhey puts it “flagella united into a single short, broad scoop over the stem.”

I consider the placement of aborense and alcyone in Mortonagrion to be extremely doubtful, with alcyone belonging with the Argiocnemis species which it closely resembles and aborense possibly deserving its own genus. However in order to prevent further confusion it appears best to leave the status of these species unchanged at this time. Mortonagrion may itself prove to be merely a specialised group within Agriocnemis , but this should be considered as part of a full review of the relationships between Mortonagrion , Agriocnemis and Argiocnemis ; such a review is a major undertaking that must include the African as well as Asian species, SEM studies of the genital ligula and molecular studies. A final resolution of this issue will not be based on characters of wing venation.

Figures

FIGURES 7 – 10. Prothorax and mesostigmal region of female Mortonagrion (scale bar 100 μm unless otherwise noted): (7) M. alcyone mesostigmal plates in dorsal view; (8) M. amoena posterior pronotal lobe and mesostigmal plates, lateral; (9) M. arthuri posterior pronotal lobe and mesostigmal plates (scale bar 200 μm), lateral; (10) M. indraneil paratype (SAR 06 _ COE 353) posterior pronotal lobe and mesostigmal plates, lateral.

FIGURES 25 – 32. Genital ligula of Agriocnemis, Argiocnemis and Mortonagrion species (scale bar 100 μm): (25) Agriocnemis femina; (26) Argiocnemis rubescens rubeola; (27) Argiocnemis species; (28) Mortonagrion aborense; (29) Mortonagrion alcyone; (30) Mortonagrion arthuri; (31) Mortonagrion indraneil paratype (SAR 05 _ COE 152). Mortonagrion female anal appendages (32) Mortonagrion indraneil female paratype (SAR 06 _ COE 353), scale bar 200 μm.