Eucraniini (Ocampo, 2005)

Palestrini, Claudia, Barbero, Enrico & Roggero, Angela, 2020, The evolution of the mouthpart structures in the Eucraniini (Coleoptera, Scarabaeidae), Organisms Diversity & Evolution (New York, N. Y.) 20 (3), pp. 451-465 : 461

Eucraniini and allied tribes

Since the Eucraniini are endemic to a circumscribed, extremely arid area in Argentina, the adaptation to those harsh conditions led to profound rearrangements of mouthparts features linked to the food provisioning ( Ocampo 2007, 2010a, b). The Eucraniini MOS ( Figs. 3 and 4 ) seems to be particularly adapted to the use of small—and dried— dung pellets of small mammals, which are the most common source of food in that region for coprophagans. The fact to have a dried diet does not require filtering the dung from impurities before ingestion, thence the total lacking, or at least massive reduction of pubescence on the epipharynx in Eucraniini . In the here examined Dichotomiini and Phanaeini , the epipharynx is quite more hairy, since they eat fresh dung.

The MOS evolution related to the food processing constitutes an exaptation present in all the Eucraniini ( Figs. 3 and 4 ). A clypeal tooth also was detected—although differently shaped—in the phylogenetically close Dichotomiini and Phanaeini tribes, in which both epipharyngeal tooth and EP (i.e., the modified pariae regions of the epipharynx) are absent ( Figs. 5 and 6 ). Thus, these two tunneler tribes feeding on wetter dung than Eucraniini carry a moderately large clypeal tooth on the ventral margin of the clypeus, which ostensibly is the only common MOS trait displayed by all of them ( Figs. 4 , 5 , and 6). In the three tribes, the clypeal tooth is so differently shaped and located, which may be regarded as a structure independently evolved. In Eucraniini , the epipharyngeal and clypeal teeth can be acting in synergy to hash food before ingestion, as seen in Eucranium , which fragments large pieces of dung before carrying them to the nest ( Ocampo and Philips 2005).

The epipharyngeal pariae are greatly modified, elongated, and tapered anteriorly to surround both epipharyngeal and clypeal teeth and concurs to collect and move food particles. The two, symmetrical anterior flaps in the modified epipharyngeal pariae could be involved in the feeding mechanism. The thick and long setae of the chaetopariae which can likely canalize small fragments of dried dung facilitate their passage to the digestive tube.

The differences among the clypeal tooth models could indicate the different diet choices which were vouched for those tribes ( Ocampo and Philips 2005; Ocampo and Hawks 2006). Likely, the shape of the clypeal tooth in Eucraniini (adapted to a dry diet) is apt to smash, while in Dichotomiini and Phanaeini (feeding on wetter dung) is fit to slash. Furthermore, the clypeal tooth can be involved in maintaining the correct position of the epipharynx.

The movements of the epipharynx in Eucraniini are likely to be constrained by the clypeal tooth, which fits into the anterior margin of the epipharynx, at the same time reinforcing its action for a better hashing of the dried dung. It can be hypothesized that all the mouthparts evolved in synergy in response to feeding needs, which is supported by the variations of mandibles, maxillae, and labium cooperating in the feeding behavior of Eucraniini . Further information on the shape and position of the mouthparts within the head can be found in the Online Resources (Appendix 6, the movie of the head tomography of Anomiopsoides heteroclyta ). Here, the exact position of the epipharyngeal and clypeal teeth can be easily evaluated, as well as the relationships among these structures and the labium and maxillae complex. In Phaneini and Dichotomini, a wider variation pattern allowed for a clypeal tooth, while in Eucraniini , the MOS vary only slightly (see above for more details).

Figures

Fig. 3 Eucraniini, ventral side of clypeus and epipharynx. ET epipharyngeal tooth, CT clypeal tooth; EP pariae regions of epipharynx. a–c Glyphoderus sterquilinus Westwood. b–d Anomiopsoides heteroclyta (Blanchard)

Fig. 4 Eucraniini, ventral side of clypeus and epipharynx. ET epipharyngeal tooth, CT clypeal tooth; EP pariae regions of epipharynx. a–c Eucranium simplicifrons Fairmaire. b–d Ennearabdus lobocephalus (Harold)

Fig. 5 Phanaeini, ventral side of clypeus and epipharynx. Oxysternon conspicillatum Weber

Fig. 6 Dichotomini, ventral side of clypeus and epipharynx. Dichotomius diabolicus (Harold)