Taxonomic revision of the Afrotropical Agabus raffrayi species group with the description of four new species (Coleoptera, Dytiscidae)

Abstract We revise the Afrotropical Agabus raffrayi species group, motivated by the discovery of new diversity in Kenya and South Africa. Whilst Agabus is mainly a holarctic genus, the Agabus raffrayi group is restricted to high altitude regions of eastern Africa and temperate parts of South Africa, from where we describe the southernmost Agabus in the world. The following new species are introduced: Agabus anguluverpussp. nov. from Mount Kenya in central Kenya, Agabus austellussp. nov. a widespread species in South Africa, Agabus riberaesp. nov. from the Kamiesberg and northeastern Cederberg ranges in the Northern and Western Cape Provinces of South Africa and Agabus agulhassp. nov. from the Agulhas Plain, Western Cape Province, South Africa. We provide a distribution map, a determination key for males, quantitative measurements of diagnostic characters, habitus photos and detailed photos of male genitalia for all described species in the group, as well as images of diagnostic characters and habitats. The presence or absence of an elongated section between the subapical broadening and the base of the apical and subapical teeth of the male aedeagus is a useful novel character, first revealed by our study. In contrast with the most recent revision of Afrotropical Agabus, we show that Agabus ruwenzoricus Guignot, 1936 is restricted to eastern Africa; South African records of this species having been based on misidentifications, no species of the group being common to southern and eastern Africa. We speculate that the raffrayi group may display phylogenetic niche conservatism, being restricted, as an originally temperate taxon, to higher elevations in tropical eastern Africa, but occurring at lower altitudes in temperate South Africa.


WC
Width of metacoxal plate WS Width of metasternal wing (correct term is lateral extension of the metaventrite but we use here the term "metasternal wing" to adhere to previous literature on the group, e.g., Nilsson and Persson 1990).

Measurements
Characters were measured using a WILD 445111 10x/21B ocular on a LEICA M125 microscope. Individual measurements were taken as follows: Metatarsomeres 2 and 5 were measured from a lateral perspective, using the maximum length and width.
The ratio of the width of metacoxal plate to the width of metasternal wing (WC/ WS) was measured as in Nilsson and Persson (1990): WS was measured at the shortest distance between the mesocoxa and the metacoxal plate, with WC continuing along the line of WS (see Fig. 2).
Protarsal claw/protarsomere 4. The length of the protarsal claw was measured from a lateral perspective, using the maximum distance between the base and apex of the claw (see Fig. 3); maximum length of protarsomere 4 was also measured from a lateral perspective.
Pronotum/Interocular distance was measured in dorsal view, using the maximum distance for pronotum width and minimum interocular distance (see Fig. 4).
Body length was measured in dorsal view, from the anterior margin of the head to the tip of the elytra.

Preparation of genitalia
Male genitalia were extracted from the tip of the abdomen using fine forceps. The aedeagus and parameres were then carefully separated from the last divided sternite (VIII) and glued onto a mounting card on the same pin as the specimen. The removed sternites of the abdomen as well as parts removed during extraction of genitalia were also mounted on the same card. Dry specimens were submerged in hot water for 15 minutes prior to preparation in order to soften the body to facilitate extraction.

Photographs, figures and tables
Photographs were taken using a Canon EOS 5D Mark II DSLR camera with a Canon MP-E 65 mm 1-5× macro lens mounted on a motorized rail (Cognisys Stackshot). Elytral microreticulation was imaged using a Canon EOS 600D camera attached to a Leica Z6 APO macroscope with a 2× objective lens. Aedeagal apices of South African species were imaged with the same system, as temporary mounts in hand sanitizer gel. Images were stacked using the PMax algorithm in Zerene Stacker and manually edited in Photoshop. Boxplots were made using R version 3.4.3.

Taxonomic results
Agabus raffrayi group Nilsson (1992a) gives a full diagnosis of the Agabus raffrayi group. It is noteworthy that Nilsson was unable to find a single synapomorphic character for the group, but states that the very similar appearance of the aedeagus amongst the species of the group might suggest a common evolutionary history. The aedeagus shape of the four new species described here does differ somewhat from the five species known to Nilsson, but follows the same basic design. Nilsson (1992b) described larval morphology of some species belonging to the raffrayi, ambulator and ragazzi groups. He concluded that A. raffrayi (and probably also A. ruwenzoricus) larvae can be distinguished from the two other groups by its short urogomphus as well as terga with long spiniform setae (among other characters).
The nine species of the group recognised in this revision are all endemic to the Afrotropical region (see Fig. 1). Three species are endemic to the Republic of South Africa, one of which is widespread there. Two species are endemic to Ethiopia, two species endemic to Tanzania, and one endemic to Kenya. Based on current understanding, only one of the nine species has a geographical range spanning over several countries, namely A. ruwenzoricus, collected in Kenya, Rwanda, Uganda and the Democratic Republic of the Congo. Aedeagus not prolonged between subapical broadening and base of apical and subapical teeth (as in Figs 5D, 6)  Pronotal bead broad, especially anteriorly. Aedeagus in ventral view with apex straight (Fig. 7A), in lateral view evenly thickened and not distinctly broadened subapically; subapical tooth slightly angled both at base and apex (see Fig. 8F). Females with coarse microreticulation on pronotum and elytra, much coarser than in males.  Diagnosis. With a prolonged preapical section of male aedeagus and a pronotal hypomeron which is not visible in lateral view, this species is most similar to A. pallidus and A. ruwenzoricus. From the former it is separated by its broader metasternal wing (Figs 2, 12) and from the latter by the lack of interocular spots (compare Fig. 4D, B).
Description. Habitus as in Fig. 11D, I. Colour: Head black, most specimens with a small rufous anterior area, interocular spots not present. Pronotum black with rufous margins. Elytra rufotestaceous to brown. Ventral surface black, hypomeron and epipleuron testaceous. Legs rufous to rufopiceous. Antennae and palpi testaceous to rufotestaceous.
Microreticulation: Medium impressed on head, pronotum and elytra, similar in both sexes. Composed of a mixture of small and somewhat larger, uneven meshes.
Female: Externally similar to males but colour of the elytra tends to be slightly lighter. Distribution. Ethiopia (see Fig. 1). Rocchi (1975) listed the distribution of A. raffrayi to also include the Democratic Republic of the Congo, Uganda, Rwanda, Tanzania, Zimbabwe, and South Africa but these specimens are likely to belong to other species.
Habitat. Found in small, often temporary, streams and pools in streambeds at elevations between 2100 to 3200 m Persson 1990, 1993;Nilsson 1992a).
Etymology. The name refers to the collector of the type specimens, Achille Raffray. The name of the synonym A. limbicollis refers to the well-defined lateral bead of the pronotum (Latin: limbus = border, collum = neck).
Comments. The fact that A. raffrayi and A. pallidus are distinguishable only on the width of the metasternal wing led some previous authors to suggest the occurrence of a single species which was dimorphic with regard to this character (Jackson 1956). Nilsson and Persson (1990) provided a detailed account of this argument, analysed a large series of specimens and concluded that the variation should rather be interpreted as two separate species. We agree with this assessment and concur that male genitalia are not diagnostic for these two species, only the width of the metasternal wing being reliable. In our measurements, the pronotum is marginally broader in A. pallidus but the small sample size forbids any strong conclusions at present (Fig. 13). Nilsson (1992b) described the larval morphology of Agabus raffrayi along with some representatives of two other Afrotropical Agabus groups. Description. Habitus as in Fig. 11E, J. Colour: Head black, most specimens with a small rufous anterior area, interocular spots not present. Pronotum black with rufous margins. Elytra rufotestaceous to brown. Ventral surface black, hypomeron rufous, epipleuron testaceous to rufotestaceous. Legs rufous to rufopiceous. Antennae and palpi testaceous to rufotestaceous.

Agabus pallidus
Microreticulation: Medium impressed on head and pronotum, similar in both sexes. Composed of a mixture of small and somewhat larger, uneven meshes. Elytral microreticulation similar, but less strongly impressed and more uneven, with some relatively elongate meshes, especially close to suture. One female examined (Ethiopia, Arsi, 13 km E Bekoji) has more strongly impressed elytral reticulation, with distinctly wider grooves between meshes.
Male genitalia: Subapically broadened, and prolonged between the subapical broadening and the apical and subapical teeth ( Fig 8E). Subapical tooth with varying appearance (similar to magnitude in variation seen in Fig. 5A-C). Female: Externally similar to males. Distribution. Ethiopia (see Fig. 1). Habitat. Found in small streams and often temporary waterbodies at elevations of 2250 to 4000 m (Nilsson and Persson 1990;Nilsson 1992a;Nilsson and Persson 1993).
Etymology. The name refers to the pale nature of the type specimens (Latin: pallidus = pale).
Comments. See comments for A. raffrayi. Diagnosis. Most similar to A. pallidus and A. raffrayi but separated from these taxa by the presence of distinct interocular spots on head (compare Fig. 4B and D). The  metasternal wing is rather narrow; the WC/WS frequency distribution being intermediate between A. pallidus and A. raffrayi, but most similar to A. pallidus (see Table 1, Fig. 12). The aedeagus has an extended portion between the subapical broadening and the apical teeth, and the pronotal hypomeron is not visible in lateral view.
Microreticulation: Medium impressed on head, pronotum and elytra, and rather similar in both sexes. Composed of a mixture of small and somewhat larger, uneven meshes.
Structural features: Body length: 7.36-8.08 mm (see Table 1). Hypomeron not visible in strict lateral view (as in Fig. 10C, D, compare with 10A, B), lateral bead of pronotum narrow and well defined. Metasternal wing narrow, WC/WS 3.0 or more in most specimens (see Table 1, Fig. 12). Pronotum broad, more than twice as broad as interocular distance (see Table 1, Fig. 13).
Male genitalia: Subapically broadened and prolonged between the subapical broadening and the apical and subapical teeth (Fig. 8C). Subapical tooth with varying appearance (similar to magnitude in variation seen in Fig. 5A-C).
Female: Externally similar to males. Distribution. Kenya, Rwanda, Uganda and the Democratic Republic of the Congo. Nilsson (1992a) and Omer-Cooper (1965) also give South Africa and Zimbabwe but these records are likely to belong to other species (see below).
Habitat. Most records are from small mountain streams and rivers at elevations of 1900 to 3100 m, but it has also been found in stagnant waterbodies (Nilsson 1992a). We found the species in a very small cold-water forest stream at an elevation of 1900 m in the Taita Hills, Kenya (Fig. 15).
Description. Habitus as in Fig. 11A, F. Colour: Head black to rufous with testaceous to rufous interocular spots. Pronotum rufopiceous to black and rufous to testaceous at margins; some specimens with two diffuse rufous to testaceous spots in the middle of the pronotum. Elytra ferruginous to rufopiceous. Ventral surface rufopiceous to black, hypomeron and epipleuron testaceous. Legs rufous to rufopiceous. Antennae and palpi testaceous to rufous.
Microreticulation: Males with medium impressed reticulation on head and pronotum and slightly finer reticulation on elytra giving a shiny appearance, all meshes being a mix of smaller and somewhat larger meshes.
The two females studied varied greatly in microreticulation, but shared having mostly isodiametric meshes on elytra and the same variable meshes on head and pronotum as males. One female (Mt. Meru) had very coarse meshes; giving head, pronotum and elytra a matte appearance while the other female (Kilimanjaro Bismarck hut) had the same shiny appearance as males.
Male genitalia: Subapically broadened, and prolonged between the subapical broadening and the apical and subapical teeth. Subapical tooth robust, with distinct curvature (see Fig. 8A).
Female: Elytral and pronotal microreticulation much coarser than in males. Distribution. Known from Meru and Kilimanjaro mountains in northern Tanzania (see Fig. 1).
Habitat. Régimbart (1908) reports that the type specimens (from Mt. Meru) were found in very cold water, at an altitude of 3500 to 4300 m. On Mt. Kilimanjaro it has been found at lower altitudes between 2200 and 3100 m (Nilsson 1992a).
Description. Habitus as in Fig. 11B, G. Colour: Head rufopiceous to black with testaceous to rufous interocular spots. Pronotum rufopiceous to black with testaceous margins; some specimens with two diffuse rufous to testaceous spots in the middle of the pronotum. Elytra ferruginous to brown. Ventral surface rufopiceous to black, hypomeron and epipleuron testaceous. Legs rufous. Antennae and palpi testaceous.
Microreticulation: Males with medium impressed reticulation on head and pronotum. Females with much coarser meshes than males, giving pronotum and elytra a matte appearance. Females also with mostly elongate meshes on pronotum. Males with a mixture of small and somewhat larger meshes on both pronotum and elytra, while female elytra tends to have more uniform small meshes. Both sexes with overall larger meshes on pronotum than elytra.

Male genitalia:
Subapically broadened, and prolonged between the subapical broadening and the apical and subapical teeth. Subapical tooth with curvature as in Fig. 8B, less robust than in A. sjostedti.
Female: Elytral and pronotal microreticulation much coarser than in males. Distribution. Known from Kilimanjaro and the Loolmalasin mountains in northern Tanzania (see Fig. 1).
Habitat. Régimbart (1908) reports that the type specimens were found in cold runoff water from a glacier at 3000 to 3500 m.
Etymology. The name literally translates to "Dytiscus-like". In his original description Régimbart (1908) explains that "J'ai donné à l'A. dytiscoides ce nom à cause de la grande similitude de forme et de couleur des males dans les deux especes. [I gave it the name A. dytiscoides because of the great similarity in form and colour between the males in the two species.]". Despite great differences in size and many other characters, the pale pronotal margins and the distinct shoulder between the pronotum and elytra are somewhat reminiscent of a Dytiscus. That said, Régimbart also mentioned similarities with A. raffrayi, making it difficult to be sure exactly what he was referring to in choosing this name. Diagnosis. This species is in some respects similar to A. sjostedti and A. dytiscoides in that females are matte due to a coarse dorsal microsculpture, and although the pronotal hypomeron is not or barely visible in strict lateral view, the pronotal bead is broader anteriorly (see Fig. 10C). The subapical portion of aedeagus is not prolonged, which is similar to South African species of the group, but the apex is straight in ventral view which is unique in the group. The aedeagus is evenly thickened, and essentially lacks the subapical broadening seen in most raffrayi group taxa (see Fig. 8).

Agabus anguluverpus
Description. Habitus as in Fig. 11K, O. Colour: Head rufopiceous with testaceous interocular spots and an anterior testaceous area. Pronotum brown to rufopiceous with testaceous margins; some specimens with a diffuse rufotestaceous area in the middle of the pronotum. Elytron brown to testaceous brown. Ventral surface rufous, hypomeron testaceous, epipleuron testaceous brown. Legs rufous to testaceous. Antennae and palpi testaceous. The three specimens collected were all teneral individuals, especially the two paratypes, and as a result there is a probability that non-teneral individuals of this species will be somewhat darker than described here. In particular the pronotum may be darker medially in non-teneral individuals.

Microreticulation:
Males with medium impressed reticulation on head and pronotum. Females with much coarser and larger meshes than males, giving pronotum and elytra a matte appearance. Both sexes with a mixture of small and somewhat larger meshes.
Structural features: Body length: 7.36-7.52 mm (see Table 1). Hypomeron not or barely visible in strict lateral view (see Fig. 10C), lateral bead of pronotum broad and well defined, broader anteriorly (see Fig. 10C). Metasternal wing narrow, WC/ WS 3.0 or more in both males and females (see Table 1 and Fig. 12). Pronotum more than twice as broad as interocular distance (see Table 1 and Fig. 13), lateral margins straighter anteriorly and more curved posteriorly.
Female: Elytral and pronotal microreticulation much coarser than in males. Distribution. Only known from Mount Kenya in central Kenya (see Fig. 1).
Habitat. Lake Ellis is situated at an altitude of about 3500 m on Mount Kenya's eastern slope (Figs 20, 21).
Etymology. The species name refers to the angled subapical tooth of the male genitalia (Latin: angulus = angle, verpus = penis).   Diagnosis. Most similar to A. riberae sp. nov. and A. agulhas sp. nov., but distinguishable by a combination of having a scutellum darker than or as dark as elytra, base  of aedeagal subapical tooth lacking a distinct incurvation (compare Fig. 8G, I) and a relatively narrow metasternal wing (see Table 1). The pronotal hypomeron is not visible in strict lateral view, the aedeagus does not have a prolonged subapical portion and in ventral view its apex is asymmetrically curved. The discal elytral microreticulation of most specimens is dominated by relatively small, isodiametric meshes.
Microreticulation: Relatively fine on both pronotum and elytra, and rather similarly impressed in both sexes. The microreticulation of the elytral disc is dominated by relatively small, somewhat isodiametric meshes in most specimens examined (e.g. Fig. 9B, C), although this character does vary somewhat between populations in this relatively widespread species (see Fig. 9). In particular, the male from Harkerville Forest (see Fig. 9D) has a reticulation composed of much larger meshes than seen in other material of this species. This specimen conforms to A. austellus sp. nov. on other morphological characters, and COI sequence data (I. Ribera, pers. comm.).
Structural features: Body length: 6.80-8.40 mm (see Table 1). Hypomeron marginally visible in strict lateral view, lateral bead of pronotum narrow and well defined. Metasternal wing narrow, WC/WS 3.1 or more in both males and females (see Table  1 and Fig. 12). Pronotum broad, more than twice as broad as interocular distance (see Table 1 and Fig. 13).   The ratio between width of the metacoxal plate and metasternal wing (WC/WS) in the A. raffrayi group (incl. specimens of both sexes). Thick black line inside boxes represents medians, left and right box borders 25 th (Q1) and 75 th (Q3) percentiles respectively. Whiskers were calculated with the boxplot. stats function in R using the default coefficient value of 1.5 (drawn to the highest and lowest value within 1.5*IQD (Inter Quartile Distance = Q3-Q1) away from the 75 th and 25 th percentiles respectively). Note that this character fully separates A. raffrayi from A. pallidus. Figure 13. The ratio between pronotal width and interocular distance in males of the Agabus raffrayi group. Symbols as in Fig. 12. Note the very narrow pronotum of A. dytiscoides, a distinguishing feature of this species. Legs: Protarsal claws long, > 1.6× as long as protarsomere 4 in most specimens (see Table 2 & Fig. 14). Metatarsomeres usually short and broad; metatarsomere 2 < 1.8× as long as broad in most specimens (see Table 2), metatarsomere 5 < 3.3× as long as broad in most specimens (see Table 2).
Male genitalia: Aedeagus lack the prolonged section between the subapical broadening and the apical and subapical teeth present in some species in the group (see Fig. 8G). In ventral view the apex is asymmetrically curved (Fig. 7B). There is some variation in the shape and size of the subapical tooth of the aedeagus (Fig. 6A-D), this being relatively small in most populations ( Fig. 6A-C).
Female: Externally similar to males. Some specimens with dorsal microreticulation slightly more strongly impressed.
Distribution. Republic of South Africa, where the species is relatively widespread, from the Bokkeveld Plateau in the south of the Northern Cape Province, most mountain systems of the Western Cape Province and east along the Great Escarpment to the Drakensberg (see Fig. 1). This wide geographical range encompasses winter, summer and bimodal rainfall regimes.
Ecology. Found in streams, pools beside streams and remnant pools in seasonal running watercourses. Most localities are situated in Fynbos or alpine grassland (e.g., Fig. 17), but also recorded from densely forested streams. Sites span a wide range of altitudes, from the type locality at 250 m (Fig. 16) to almost 3000 m in the Kwazulu-Natal, Drakensberg (Fig. 17), most being at intermediate elevations.
Etymology. The name refers to the fact that the species is widespread in South Africa and therefore truly an Agabus of the south (Latin: austellus = southern parts).
Comments. Nilsson (1992a) and Omer-Cooper (1965) assigned specimens from South Africa and S. Rhodesia [=Zimbabwe] to A. ruwenzoricus. We have studied one series of specimens cited by both authors from the Swartberg Pass (Swartbergpas) W. Cape Province, housed in Lund (MZLU), which correspond to A. austellus sp. nov. Males of this species do not have a prolonged preapical section of the aedeagus, a distinguishing character not previously noted in the raffrayi group. It seems likely that all previous records of A. raffrayi, A. pallidus or A. ruwenzoricus from South Africa, and also possibly those from Zimbabwe, are misidentified and mostly refer to A. austellus sp. nov. This species is somewhat variable in body shape, elytral microreticulation (see Fig 9A-D) and the shape and size of the subapical tooth of the aedeagus (see Fig. 6A-D). We interpret this variation as comprising a single species, however, particularly given the very similar mtDNA COI sequences observed amongst populations spanning the entire known range in South Africa (i.e., Bokkeveld Plateau to Drakensberg, I. Ribera, pers. comm.).
Description. Habitus as in Fig. 11M, Q. Colour: Head black with rufous interocular spots and an anterior rufous area. Pronotum black with rufous borders. Elytra rufopiceous to black. Ventral surface black, testaceous lines on abdominal segments rarely present, hypomeron and epipleuron rufotestaceous to rufous. Legs rufous to rufopiceous. Antennae and palpi testaceous.
Microreticulation: Relatively fine on both pronotum and elytra, and rather similarly impressed in both sexes. The microreticulation of the elytral disc is typically dominated by relatively large, somewhat irregular meshes (Fig. 9E).
Structural features: Body length: 7.21-8.24 mm (see Table 1). Hypomeron marginally visible in strict lateral view, lateral bead of pronotum narrow and well defined. Metasternal wing broad, WC/WS 3.0 or less in most specimens (see Table 1 and Fig. 12). Pronotum broad, more than twice as broad as interocular distance (see Table 1 and Fig. 13).   Legs: Male protarsal claws long, > 1.6× as long as protarsomere 4 (see Table 2 and Fig. 14). Metatarsomere 2 short and broad, < 1.8× as long as broad (see Table 2) in most specimens. Metatarsomere 5 long and slender, > 3.3× as long as broad in most males (see Table 2).
Male genitalia: Tip of aedeagus short, lacking the prolongation of the area located between the subapical broadening and the apical and subapical tooth present in some species in the group (see Fig. 8H). There is some variation in the shape and size of the subapical tooth (Fig. 6E, F), this being relatively long and narrow in most specimens examined (Fig. 6F), but with both narrow and broader teeth being observed within the same population.
Female: Externally similar to males. Some specimens with dorsal microreticulation slightly more strongly impressed.
Distribution. To date known only from Kamiesberg Range in the Northern Cape Province, and the eastern fringes of the Cederberg, Western Cape Province, Republic of South Africa (see Fig. 1), material from both areas being confirmed from COI sequences. The Kamiesberg represents a northerly outlier of Fynbos and Renosterveld vegetation in predominantly arid Namaqualand, and consequently have a diverse flora with a number of localised endemics (Helme and Desmet 2006). The mountains support the bulk of the global population of the endemic dytiscid Andex insignis Sharp, 1882 and a number of new, apparently endemic, water beetles have been described from the area in recent years (Bilton 2013(Bilton , 2015(Bilton , 2016. Agabus riberae sp. nov. appears to be the only Agabus present in Kamiesberg, where it is abundant. In the Cederberg the species has been found close to Wupperthal, in the relatively dry northeastern fringes of the range. A. austellus sp. nov. is the only species so far recorded from the wetter central areas of the Cederberg. All sites known to date experience predominantly winter rainfall. Ecology. Found in streams and associated pools in the Kamiesberg and northeastern Cederberg ranges (Figs 18,19), in either Fynbos or Renosterveld vegetation between 480 and 1000 m elevation. Typically netted from marginal vegetation, including at the base of tussocks. Also found amongst grasses in a spring pool with cold water. Ecological differences between this species and A. austellus sp. nov. are unclear, but may relate, at least in part, to rainfall.
Etymology. Named after our late friend and colleague Ignacio Ribera, who will be sorely missed.
Comments. Despite the relatively minor morphological differences between this species and A. austellus sp. nov. we consider these two taxa distinct. COI sequences for four specimens of A. riberae sp. nov. investigated differed by 4.5-4.7% from A. austellus sp. nov., more than that observed between many well-established species in the subgenus Acatodes Thomson, 1859 (I. Ribera pers. comm.). This is supportive of the recognition as a distinct taxon, particularly given the relative uniformity in COI sequence observed across the wide geographical range of A. austellus sp. nov.  Diagnosis. Very similar to A. austellus sp. nov. and A. riberae sp. nov., but distinguishable by the distinctly curved base of the aedeagal subapical tooth (compare Fig. 8I with Fig. 8G, H and see Fig. 6G), the scutellum being lighter than the elytra and its relatively narrow metasternal wing (see Table 1 and Fig. 12).
Description. Habitus as in Fig. 11N, R. Colour: Head black with weak rufous interocular spots and an anterior rufous area. Pronotum black with slightly rufous margins. Elytra blackish brown to black, with a lighter scutellum. Ventral surface black, testaceous lines on abdominal segments reduced or absent, hypomeron and epipleuron rufotestaceous to rufous. Legs rufopiceous to rufous. Antennae and palpi testacous.
Microreticulation: Relatively fine on both pronotum and elytra, and slightly more impressed in females. The microreticulation of the elytral disc is composed of a mix of small and larger, somewhat irregular meshes (Fig. 9F).
Structural features: Body length: 7.60-8.00 mm (see Table 1). Hypomeron marginally visible in strict lateral view, lateral bead of pronotum narrow and well defined. Metasternal wing very narrow, WC/WS > 3.6 in all specimens (see Table 1 and Fig. 12). Pronotum broad, more than twice as broad as interocular distance (see Table 1 and Fig. 13).
Male genitalia: Aedeagus without the prolonged section between subapical broadening and the apical and subapical teeth which is present in some species in the group. In ventral view the apex is asymmetrically curved. Base of subapical tooth distinctly curved basally (see Figs 8I, 6G).
Female: Externally similar to males. Dorsal microreticulation slightly more impressed than in males.
Distribution. Only known from the type locality, a lowland valley wetland at 31 m on the Agulhas Plain, Western Cape Province, Republic of South Africa (see Fig.  1). The most southerly distributed Agabus species in the world.
Ecology. Collected from the base of large tussocks in a valley wetland. Despite having largely lentic conditions, this is likely to experience some seepage flow, particularly following periods of high rainfall in winter and spring.
Etymology. Named after the Agulhas Plain, on which the type locality is situated. The Agulhas Plain is itself named in reference to nearby Cape Agulhas (Portuguese -Cabo das Agulhas = Cape of Needles), the most southerly point on the African continent. As with other members of the species group, A. agulhas sp. nov. has sharp, needle-like, teeth at the aedeagal apex.

Discussion
Agabus is one of a number of largely temperate northern hemisphere water beetle lineages which have colonised the Afrotropical region. Other examples include Nebrioporus Régimbart, 1906 and Ilybiosoma Crotch, 1873 within the Dytiscidae and Helophorus Fabricius, 1775 (Hydrophiloidea, Helophoridae). In all cases, these genera are restricted to relatively high elevations in East Africa, from Ethiopia southwards, but occur across a much greater range of altitudes in temperate regions of South Africa, particularly the Cape. South African Agabus have to date been considered to belong to Agabus ruwenzoricus (Nilsson 1992a), or prior to the recognition of A. ruwenzoricus, Agabus pallidus (Omer-Cooper 1965). Instead of forming part of a widespread species, distributed from East Africa to the Cape, we demonstrate that South African Agabus are all endemic to the region, and comprise a group of three semi-cryptic species, one of which is relatively widespread. These species constitute the southernmost records in the world for this otherwise largely Holarctic genus which is most diverse in the northern hemisphere and absent from South America and Australia (Miller and Bergsten 2016). The discovery of Agabus anguluverpus sp. nov. on Mount Kenya, shows that our knowledge of the Agabus fauna of high altitude areas in East Africa is also incomplete. Many of the mountain systems associated with the Rift remain poorly investigated for aquatic insects, and we suspect that additional, new, species of the genus remain undiscovered.
In his revision of the raffrayi species group, Nilsson (1992a) considered that the shape of male genitalia, although universally used for species-level identification in the genus elsewhere, was largely uninformative and consequently this character was not used in the determination key to the group. Here we show instead that the three distinct species found in South Africa differ from other raffrayi group species by having a preapically shorter male aedeagus. Coincidentally, this genitalic feature is also characteristic of the new species we describe from high elevations on Mount Kenya. We hypothesize, however, that a preapically prolonged aedeagus may be a synapomorphy of A. ruwenzoricus and its relatives, and that the shorter plesiomorphic state may not necessarily indicate a close relationship, at least between South African and Kenyan beetles.