Mordellidae


The Mordellidae are a family of beetles commonly known as tumbling flower beetles for the typical irregular movements they make when escaping predators, or as pintail beetles due to their abdominal tip which aids them in performing these tumbling movements. Worldwide, there are about 1500 species.

Anatomy

The apparent tumbling movements are composed of a series of very rapid separate jumps. They result from the beetle's efforts to get itself back into take-off position for flight when it has been in either a lateral or dorsal position. Each individual jump should be considered an extended rotation, performed by one leg of the third leg pair. Depending on whether the left or the right metapodium is used as the leg that provides the leverage for take-off, change occurs in the direction of the jump. The energy for propulsion varies with the beetle's immediate muscle work, so that jump lengths and heights vary, with rotation frequencies recorded up to 48 per second around the gravitation centre of the body's longitudinal axis. Additional revolving around the transverse axis effects spiralling somersaults that are perceived as tumbling. While the pintail is of no significance for the jump, the meta-trochanter-femur has a great capacity for free rotation. This capacity is due to a screw joint that connects the base of the metacoxa to the head of trochanter. The nut gradient is 21 degrees. Technically similar jumps, though less powerful, can be observed in the families Melandryidae and Scraptiidae. Their coxa-trochanter-joints are of similar structure. It is inferred that the capacity of a tumbling form of locomotion is rooted in a common phylogeny and can therefore not be ascribed specifically to Mordellidae. Other authors have pointed out the speciality of the screw joint. Weevils of the genus Trigonopterus, in the Asian tropical rainforest, for example, have an even tighter connective construction in the coxa-trochanter joint than is found in Mordellidae. This construction, however, does not facilitate an equally high angular velocity of the torque. The joint here serves to provide a better grip on plants and easier climbing.

Systematics

This family has two living subfamilies - Mordellinae and Ctenidiinae - and a prehistoric one known only from fossils. Another fossil genus, Liaoximordella, was previously treated as distinct monotypic family Liaoximordellidae, but is now regarded as very primitive and probably basal member of the Mordellidae.