The morphology and mechanics of flight in insects in the millimeter range, such as fruit flies (Drosophila melanogaster is 2–3 mm) and mosquitos (about 4 mm), have been well-studied. In contrast, the flight of tinier insects has remained a mystery. This recently changed with an elegant study by an international group lead by Sergey Farisenkov, Dmitry Kolomenskiy, and Alexey Polilov [Reference Farisenkov1]. They studied the flight of the miniature featherwing beetle Paratuposa placentis.
Specimens were located on fungi in the Joint Russian-Vietnamese Tropical Research and Technological Center. Farisenkov et al. constructed a morphological model based on data gained from light, confocal, and scanning electron microscopy. The beetles were less than half a millimeter in length, which is similar to the size of some unicellular organisms such as Amoeba proteus. The length of each of the two wings was about the same as the body length. The wings consist of a stalk (petiole) and a structure resembling a bristle, which has a narrow wing blade with a fringe of several setae covered with secondary outgrowths (Figure 1).
Figure 1: External morphology of Paratuposa placentis. Scanning electron micrographs showing relative size of P. placentis (a) and Amoeba proteus (b), wing of P. placentis (c), and part of a seta (d).
The mass of the bristled wing was about 1% of the body mass of the beetle. Farisenkov et al. calculated that a wing of the same area, if membranous (as found in most flying insects), would have an approximately six-fold greater mass. The bristled wing maintained the needed aerodynamic properties because the physics of creating lift is different at sub-millimeter dimensions than it is on a larger scale. Specifically, the viscosity of air is proportionally greater than inertia at a tiny scale. This was demonstrated by the ratio of inertial forces to viscous forces (Reynolds number).
Having described the microscopic morphology of the beetle's wing, Farisenkov et al. developed a kinematic model using synchronized high-speed (almost 4,000 frames per second) videography taken from two perpendicular positions. After a three-dimensional reconstruction and sophisticated analyses of the beetle and its flight, the group discovered that the wings did not move in a strict up-and-down fashion but rather a figure-of-eight loop. The wings clapped above and below the body, just when the wings reversed direction. The unusually large horizontal and vertical excursion of the wings during flapping poses a peculiar flight dynamics problem that would destabilize the flight pattern. To limit this problematic effect on the insect's body and stabilize the beetle in flight, the modified hardened forewings (elytra) were deployed synchronously with the wing beats to counteract the destabilizing effect of lift. The authors determined that this was a novel flight style.
Farisenkov et al. also performed additional studies that demonstrated the efficiency of the design and motion of the bristled wing of P. placentis. This study helps to explain how extremely small insects have preserved good aerial performance during miniaturization over more than 300 million years. No doubt this is one of the factors responsible for their evolutionary success!
