The beetle featured in today’s photo is the blister beetle (family Meloidae), Nemognatha cribraria cribraria. The genus Nemognatha and its relatives in the subfamily Nemognathinae are distinctive due to the greatly elongated adult mouthparts that are modified for feeding on flowers. Specifically, parts of the maxillae, or second pair of mouthparts (behind the mandibles) are elongated to allow access to nectar in flowers with deep corollas, while the fairly standard-issue chewing mandibles are used for feeding on pollen. As pointed out by Enns (1956) in his revision of the North American members of the genus, the length of the maxillae seems to be related to the particular kind of flower preferred for feeding by the various nemognathine species, with species exhibiting longer maxillae adapted to feeding on flowers with deeper corollas. In the photo above, the elongated maxillae can be seen tucked underneath the adult and appear to be nearly half the length of the body—other species in the genus have the maxillae as long as the body, or in the case of a Mexican species (N. chrysomeloides) even longer than the body (Enns 1956).
The proboscis-like mouthparts of nemognathine blister beetles are often depicted in entomological texts as an amazing example of sucking mouthparts in Coleoptera, the vast majority of which possess strictly chewing mouthparts. Borrer et al. 1976, White 1983, Downie & Arnett 1996, and Pinto & Bologna 2002 all mention that the mouthparts are modified into an elongated proboscis for “sucking” nectar, and it has been suggested that nectar uptake occurs through a median food canal, formed by concavities on the inner surfaces when the two structures are locked together into a functional unit. However, Wilhemi & Krenn (2012) used scanning electron microscopy and micro computerized tomography to study the elongated mouthparts of three meloid genera: Nemognatha and Gnathium and Leptopalpus. They demonstrated that neither the elongated galeae of Nemognatha and Gnathium nor the elongated maxillary palpi of Leptopalpus formed a median food canal through which nectar is sucked. Furthermore, the filiform galeae of Nemognatha and Gnathium are densely covered with long bristles, suggesting that nectar uptake in these two genera is accomplished by capillary action along the bristles of the proboscis. In all three genera nectar transport is likely aided by musculature around the mouth.
Borrer, D. J., D. M. DeLong & C. A. Triplehorn. 1976. An Introduction to the Study of Insects, Fourth Edition. Holt, Rinehart and Winston, xii + 852 pp.
Downie, N. M. & R. H. Arnett, Jr. (Eds.). 1996. The Beetles of Northeastern North America. Volume II: Polyphaga: Series Bostrichiformia through Curculionoidea. The Sandhill Crane Press, Gainesville, Florida, x + 891–1721.
Enns, W. R. 1956. A revision of the genera Nemognatha, Zonitis, and Pseudozonitis (Coleoptera, Meloidae) in America north of Mexico, with a proposed new genus. The University of Kansas Science Bulletin 37, part 2(17):685–909 [Biodiversity Heritage Library].
Pinto, J. D. & M. A. Bologna. 2002. Chapter 111. Meloidae Gyllenhal 1810, pp. 522–529. In: R. H. Arnett, Jr., et al. (Eds.). American Beetles, Volume 2. CRC Press, Gainesville, xiv + 861 pp.
White, R. E. 1983. A Field Guide to the Beetles of North America. The Peterson Field Guide Series, Houghton Mifflin Co., Boston, xii + 368 pp.
Wilhelmi, A. P. & H. W. Krenn. 2012. Elongated mouthparts of nectar-feeding Meloidae (Coleoptera). Zoomorphology [abstract].
Copyright © Ted C. MacRae 2013