Morphology of adults

General morphology of adult Orphninae

The diagnostic and taxonomic characters of the adult Orphninae described herein are extracted from Frolov et al. (2016) and based mostly on Triodontus itremoi Paulian. They are specifically suited for the Madagascan taxa although the main characters are common for the whole subfamily.


The head is wider than long, with the clypeus, frons, and vertex not separated from each other by sutures. For the descriptive purposes, we defined the following parts on the head dorsally (Fig. 1): anterior margin, anterior border, disc of frontoclypeus, canthus, eye, eye tubercle, and frontoclypeal process (horn or carina). In the majority of taxa of the Madagascan Orphninae, the head shape is subject to sexual dimorphism and allometric variability in males.

Triodontus itremoi male in dorsal view

Figure 1. Triodontus itremoi, male in dorsal view (left legs removed): aa. prn—anterior angle of pronotum, acly—anterior margin of clypeus, al. prn. proc—anterolateral pronotal process, an. prn—anterior pronotal margin, bs. elyt—elytron base, bs. prn—base of pronotum, canth—canthus, d. prn.—disc of pronotum, elyt. ap. umb—apical elytral hump, istr. st—sutural interstria, labr—labrum, lm. prn—lateral margin of pronotum, mand—mandible, ocu—eye, pa. prn—posterior angle of pronotum, pl. prn. proc—posterolateral pronotal process, proc. ceph—frontoclypeal horn or tubercle, scut— scutellum, st. elyt—sutural margin of elytron, str. elyt I—elytral stria 1, tub. ocu—eye tubercle, umb. cpr—central pronotal bulge, umb. hum—humeral elytral hump.

The anterior margin of the frontoclypeus is more or less semicircular in most taxa. The frontoclypeus has a marked continuous border; the border is narrow in most taxa but can be wide and upturned medially in Renorphnus Frolov & Montreuil, 2009. The frontoclypeus is symmetrical in most taxa except for some males of Madecorphnus. In the latter case, the clypeal asymmetry correlates with the mandible asymmetry. The frontoclypeal suture is indistinct. The border between the clypeus and frons may be marked with a horn-shaped process (males of Triodontus and Pseudorphnus) or a low transverse ridge (Madagascan Pseudorphnus carinatus and some African Orphnus) but the comparative morphological data on this account are not available and in this study we do not separate frons and clypeus. The frontoclypeal process of the males is subject to the reasonable allometric variability and is almost absent in some specimens. The material available is not sufficient for a morphometric analysis, but we think that the variability is continuous rather than the males being dimorphic, i.e., the so-called “minor” and “major” males cannot be rigorously distinguished. The frontoclypeal process is always absent in females, although the females of some species of Triodontus may have a minute tubercle in the center of the frontoclypeus. The canthus is small, incompletely divides the eye into smaller dorsal and larger ventral parts. The males of Triodontus and Pseudorphnus have a small tubercle mediad of each eye; development of these ocular tubercles seems to correlate with the development of the frontoclypeal process.

The antennae with 10 antennomeres, externally similar in all taxa. No cases of symphysocery, known in some African Orphnus (Frolov & Akhmetova 2016), have been encountered in Madagascan taxa. The antennal club has three compact antennomeres. The labrum is about two times wider than long, rounded laterally, sinuate anteriorly, densely setose. The apical part of the labrum protrudes past frontoclypeus and is visible in dorsal view. The mandibles are mostly symmetrical, about the same length, normally with 2–4 well-developed scissorial teeth. Exception to this are the males of Madecorphnus, which may have highly asymmetrical mandibles with the right mandible being up to two times or more longer than the left. The maxillae have separate lacinia and galea, which normally bear thick spinules along with the thin setae. The prementum is trapezoidal, densely setose, and somewhat rugose to almost smooth; a few species of Madecorphnus have two conical tubercles in the middle of the prementum. The mentum is about two times shorter than the prementum, concave. The gula is longer than wide, convex, having a shallow longitudinal groove in basal 3/4, and somewhat concave and setose apically. The glossae are feebly sclerotized and finely setose. The basal labial palpomere is about two times shorter than second and third palpomeres.

Triodontus itremoi male in ventral view
Figure 2. Triodontus itremoi, male in ventral view (left legs and abdomen removed): ant—antenna, cav. mscox—mesocoxal cavity, cav. prcox—procoxal cavity, epipl—epipleuron, gul—gula, mand—mandible, max—maxilla, ment—mentum, mscox—mesocoxa, msepim—mesepimeron, msfem—mesofemur, msst—mesosternum, mstar—mesotarsus, mstib—mesotibia, mstr—mesotrochanter, mtcox—metacoxa, mtepis—metepisternum, mtfem—metafemur, mtst—metasternum, mttar—metatarsus, mttib—metatibia, mttr—metatorchanter, ocu—eye, prfem—profemur, propl—propleurae, protib—protibia, prst—prosternum, prstnl—prosternellum, prtar—protarsus, prtr—protrochanter, str. propl—propleural stria.


The pronotum is trapezoidal, more or less rounded laterally. Shape and sculpture of pronotum are subject to sexual dimorphism and allometric variability in males of most taxa. We distinguish the following parts of the pronotum (Fig. 1): anterior margin, usually with a wide, flat or slightly convex border and membranous very anterior part adjacent to the head; base, mostly bordered and with a row of punctures; lateral margins, coarsely punctate and with long dense setae, anterior and posterior angles, disc of pronotum. The pronotal armature consists of anterolateral and posterolateral pronotal processes, sometimes forming lateral pronotal ridges, and a central pronotal bulge, characteristic of the Triodontus males. Pronotal armature, when well developed (in so-called “major males”) is species specific, however such forms are rather rare in the collections and majority of specimens belong to the forms intermediate between “major” and “minor” males. Shape of the pronotal armature is also genus specific. The most complex pronotal armature is found in the males of the genus Triodontus.

The prothoracic legs of the majority of Madagascan Orphninae are of typical scarabaeoid shape, with three rather large outer teeth to facilitate digging in soil. Protibiae of the males lack apical spurs but have a few apical setae on protibia (in the place of the absent spur) that are thicker than the others (Fig. 3A). Females have well-developed, rather long apical spurs. The procoxae of both sexes have a longitudinal hollow on anterior surface of procoxa (Fig. 3E); the hollow is concealed in the coxal cavities while a beetle is walking; it opens only when the prothoracic legs are appressed to the pronotum.


Theelytra are convex, elongate, with marked humeral and apical humps. The first (sutural) elytral stria is well marked in most taxa except for Pseudorphnus; basally it is joined with basal elytral border which is similar to the stria in some species or appears as a row of contiguous punctures in others. The first elytral interval is more or less convex in most taxa. Other striae are distinct in some taxa (either as impressed lines or rows of punctures) or unclear; elytral intervals, except for sutural interval, are flat in most taxa. Punctation of elytra varies.

Triodontus itremoi details 1
Figure 3. Triodontus itremoi. Protibia in ventral view (A), metacoxa in dorsal view (B), metatibia in apical view (C), mesotibia in apical view (D), procoxa in ventral view (E), wing (F); pars. str—stridulatory field, procx. fv—hollow of procoxa; 1a and 2a—anal cells; AA1+2, AAP, AA3b+(AA4+AP1+2), AP4—anal veins; aas—antero-apical sclerotization; cas—costo-apical sclerotization; CuA—anterior cubital vein; js—jugal sclerotization; MP3+4—posterior medial vein; rc—radial cell; RMP—fused posterior radial (RP3+4), anterior medial and posterior medial (MP1+2) veins; SV2—secondary “vein”. Wing venation terminology follows Fedorenko (2009). Not to scale.

The wings are fully developed in all Madagascan species. The wing venation can be described as of advanced Scarabaeidae type (Fedorenko 2009) with reduced costal window, anal vein AA1+2 not reaching the wing margin, open cell 1a with strongly angulate posterior border, and well developed jugal sclerotization almost reaching the wing margin (Fig. 3F).

The scutellum is exposed, semicircular to triangular, with rounded apex.

The metepisternum has a slightly widened anterodorsal angle slightly overlapping the epipleuron, which is somewhat concave in this place. The orifice between the mesocoxal cavities, found in the Neotropical orphnines, is absent.

The mesothoracic and metathoracic legs are similar in shape; the metafemora and metatibiae are usually about 1/8 longer than the mesofemora and mesotibiae. Tibiae somewhat triangular with two apical spurs; the inner margin only slightly concave and with one transverse keel. Metatibial spurs are separated by basal tarsomeres (Fig. 3C); metatibial spurs are adjoining (Fig. 3D).

Longer tibial spur as long as the two basal tarsomeres. Claws 1/3 length of last tarsomere. The stridulatory apparatus is of typical orphnine shape. Stridulatory field is situated basally on the dorsal surface of metacoxa and consists of straight fine keels (Fig. 3B).

Abdomen and external genitalia

The abdomen of the Orphninae has eight tergites and seven or eight sternites (Figs. 4A, B). Tergites 1–6 are weakly sclerotized, flexible, hidden under the wings and elytra. Tergites 7–8 (propygidium and pygidium) are strongly sclerotized, more or less exposed (although not visible in dorsal view). Sternites 1–3 are modified, partly hidden under metacoxa. Sternite 1 is strongly reduced and its delimitation is difficult. Sternite 2 bears plectra and irregular, mostly longitudinal ridges. The plectra are triangular to somewhat rounded, with the apex being highly sclerotized and somewhat upturned. Sternite 3 is the widest, features a few secondary transversal ridges forming metacoxal cavities. Sternites 4–7 are mostly of the same width, relatively narrow medially. Sternite 8 is wider than sternites 4–7 and different in males and females.

Triodontus itremoi details 2
Figure 4. Triodontus itremoi, abdomen (A—dorsal, B—ventral view): cav. mtcox—metacoxal cavity, plec—plectrum, propyg—propygidium, pyg—pygidium, stern2–stern8—abdominal sternites 2–8, terg1–terg6—abdominal tergites 1–6; Triodontus lemoulti, spiculum gastrale (C); Triodontus itremoi,female genitalia (D): burs. cop—bursa copulatrix, coxt—coxite, gl. acc—vaginal gland, gl. rec—spermatheca gland, ovd—oviduct, parapr—paraproct, rec. sem—spermathecal, scoxt—subcoxite, styl—stylus. Not to scale.

The male external genitalia of Madagascan Orphninae have subsymmetrical phallobase, symmetrical parameres, and variably armed internal sac of the aedeagus (endophallus). The phallobase in the Madagascan taxa is of the same type (Frolov 2013b) as in the Afrotropical taxa: it is strongly sclerotized dorsally and with a thin membrane ventrally. The shape of the parameres varies reasonably and in the most cases is species specific. The parameres of Triodontus are of complex shape: they are divided into inner and outer lobes, which may have processes and excavations. The internal sac consists of sclerites of different shape and number. The combination of the characters of the internal sac armature and the shape of the parameres reliably diagnose all species and in many cases, especially in Triodontus and Madecorphnus, are the only characters to separate the otherwise similar species. However, in Triodontus, the shape of the parameres is highly specific while the shape of the large symmetrical sclerite of the internal sac is similar in some species. In Madecorphnus, as opposed to Triodontus, the shape of the parameres is similar in some species while the shape of the internal sac armature is highly specific. Spiculum gastrale is triangular (Fig. 4C).

Female external genitalia consist of a few sclerites connected by membranes. More defined and sclerotized are the subcoxites, covering large vaginal accessory glands, and coxites, bearing styli. All three pairs of sclerites bear long setae apically and have a sensitive function. Other sclerites, proctiger (hemitergites IX), and paraprocts (epipleurites IX), are less distinct.

Sexual dimorphism

The character immediately separating males from females in the orphnines is the absence of the protibial spur in the former. This character is always distinct except, sometimes, for the males of Madecorphnus (see diagnosis of this genus). The two other characters are more general and valid for the majority of scarab beetles: 1) females usually have relatively smaller pronotum, distinctly narrower than elytra, and, 2) the head and pronotal armature is developed only in the males (but is subject to allometric variability: see diagnoses of the genera). Abdominal sternites differ in length. Sternite 8 in males is shorter and sometimes clearly sinuate medially; in some Triodontus species, it has a tubercle or concavity medially. The sexes do not differ in body coloration or pubescence. Also, all the specimens examined, both males and females, had fully developed wings.