Ento-musings from the University of Kentucky Department of Entomology

Monday, October 7, 2013

Look, Kids! The Rhipicerids Are Back!

Well, it's that time of year againlate September/early October—and the beetle Sandalus niger makes its only annual appearance anywhere near where we humans are likely to observe it. The genus itself is the only North American member of the family Rhipiceridae (Dascilloidea), with 5 species known from across the continent. Their adult life is ephemeral (3 days at most), and consequently they are seldom seen.

Female S. niger are largish, clumsy insects, about an inch in length, with a pleasing dark mahogany color. The well-flying males are half the females' size and more oblong in outline, possessing beautifully pectinate antennae. Both genders are only active on sunny autumnal afternoons when the temperature exceeds 15°C—gravid females sit on tree boles (most frequently those of elms) without movement, awaiting buzzing clusters of paramours attracted by pheromonal broadcasts. 

Once a female is inseminated (often more than once), she slowly ascends the trunk, probing the substrate with her ovipositor, sometimes climbing to a height of 10 meters or more before laying her eggs in gaps or holes in the elm-bark. Why elm? It is a popular food tree of the dog-day cicada Tibicen pruinosa (Cicadidae): and it so happens that S. niger (and by extension all rhipicerids) are parasitoids of juvenile cicadas. Many of the eggs (which occur in cumuli) are washed away by rain; this is compensated for by the quantities in which they are laid: one brood numbered 16,864.

Only the first instar larvae of S. niger have been studied by science: they are miniscule, swift triungulins, apparently adapted for promptly seeking out their nymphal hosts. But aside from an empty pupa that was found within a cicada's exuvium, the ontogeny of this distinctive species remains a mystery.


Elzinga, R. J. (1977). Observations on Sandalus niger Knoch (Coleoptera: Sandalidae) With a Description of the Triungulin Larva. Journal of the Kansas Entomological Society, 50(3), 324-328. Retrieved 8/7/13 from http://www.jstor.org/stable/25082945?seq=4             

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