Ranatra Fabricius, 1790

Tran, A. D. & Zettel, H., 2021, Taxonomic review of the Ranatra gracilis group sensu Lansbury, 1972 (Nepomorpha: Nepidae), with descriptions of four new species, Raffles Bulletin of Zoology 69, pp. 45-70 : 46-47

Ranatra Fabricius, 1790

Ranatra Fabricius, 1790: 227 (type species: Nepa linearis Linnaeus, 1758 , subsequently designated by Latreille, 1810: 434).

Remarks. Lansbury (1972) arranged the Oriental species of Ranatra in six groups: R. gracilis group, R. elongata group, R. varipes group, R. malayana group, R. filiformis group, and R. biroi group. Among these, the R. malayana group contained only a single species. Lansbury (1972: 294) stated that the descriptions for species groups were mostly for avoiding repetition in describing the same characters in each species of a group. Besides the key to species, Lansbury (1972) did not provide any discussion on comparative morphology across different groups.

Our comparative morphological study of most known species of Ranatra from India to Australia has shown that some characters used in the species group descriptions by Lansbury (1972) are useful for group discrimination, but more often they must be used in combination. These characters include the modification of vertex, the structure of fore femur, the shape of the posterior margin of metasternum, and the relative lengths of the operculum and connexivum of females. The relative lengths of the respiratory siphon and body, and the general form of paramere, are useful additional characters for recognising some species groups (i.e., R. gracilis , R. elongata , and R. malayana groups), but should be used in combination with the other characters listed above. Some additional characters in Lansbury’s (1972) group descriptions are not really helpful in defining these groups, e.g., relative heights of lorum and clypeus, relative widths of eye and interocular space, relative length of prothorax and fore coxa, relative length of prothorax and fore femur, the process of second antennal segment, and the relative distances between middle and hind coxae. These characters have turned out to be variable among species of the same group and to overlap across different groups.

Among the six species groups defined by Lansbury (1972), the R. gracilis , R. elongata , and R. malayana groups can be easily recognised as stated in the diagnoses below. In contrast, the distinctions among the remaining groups are not obvious, as there are just few characters separating them. Species of the R. varipes group, however, can be separated from R. filiformis and R. biroi groups by having a relatively shorter anterior lobe of the pronotum, ca. 1.1–1.4× the length of the posterior lobe (in R. filiformis group: 1.3–1.7×; in R. biroi group 1.3–1.8×), in combination with a broader fore femur with its larger tooth situated equidistant between its two ends (in R. filiformis and R. biroi groups, fore femur is more slender and its larger ventral tooth is nearer to distal end). Between the R. filiformis and R. biroi groups, according to descriptions by Lansbury (1972), the only difference is the relative widths of the eye and interocular space. In the R. filiformis group, the eye width is clearly less than the interocular width, while in the R. biroi group, the eye width is about equal to or greater than the interocular width (with the exception of R. nieseri , where the ratio of eye width: interocular width is variable, ranging between 0.9–1.1). It is not very convincing to separate two groups based on such a single character. However, until further evidence, especially molecular data, become available, it is still premature to decide whether taxa of both R. filiformis and R. biroi groups belong to the same monophyletic group.

We also propose a new group, namely the R. bilobata group, to include three species described after Lansbury’s (1972) revision: R. sulawesii Nieser & Chen, 1991 , R. sterea Chen, Nieser & Ho, 2004 , and R. bilobata Tran & Nguyen, 2016 . They share some similarities, as stated in the diagnosis below, and do not fall into any of Lansbury’s (1972) groups.

Pending further phylogenetic analysis, using both morphological and molecular character systems, to confirm the monophyly of each group, the group classification is still provisional, and it should only be used for the practical purpose of facilitating identification.

Below are diagnoses of the species groups, lists of species held in each group, and a key to the species groups.