This is how the smallest beetles fly (and fly very well)
Beetles of the family Ptiliidae, which includes the smallest non-parasitic insects, fly as fast as their much larger relatives, although forces of viscous friction are of the same order of magnitude for them as inertial forces. We describe their flight style and show how their bristled wings work.
Our team studies microinsects (arbitrarily defined as insects smaller than 2 mm long). The purpose of our work is to shed light on miniaturization, the evolutionary trend towards extreme diminution, which independently took place in a few orders of insects, leading to the emergence — among beetles, wasps and some others — of species in which the adult is less than half a millimetre long, which is even smaller than many unicellular protists, such as Amoeba proteus. Surprisingly, these tiny microinsects are multicellular animals capable of complex movements and advanced behaviour. As we recently discovered, their flight is not only more peculiar but also more efficient than anybody could imagine only several years ago.
The effects of miniaturization on the structure and functions of body systems remains poorly understood. It is known that larger flying animals generally fly faster than smaller ones, but it is not necessarily so. Investigating this issue, we recently found that miniature featherwing beetles (representatives of the family Ptiliidae) fly as fast as much larger representatives of related beetle families. Their maximum accelerations are also surprisingly high, and in terms of the number of body sizes covered in flight per second, they surpass all animals for which this parameter has been measured to date. And yet the mechanism of their flight remained unknown — until now.
Our study combines modern morphological methods, 3D reconstructions of the movement of body parts in flight and new approaches in computational aerodynamics. All these methodologies allowed us to understand the mechanism of flight that helps the smallest beetles to excel at flight. We studied this mechanism using the example of one of the smallest beetles, which is also one of the smallest free-living (non-parasitic) insects, Paratuposa placentis, whose adult body length is less than 0.4 mm. Our study shows that the surprisingly efficient flight of these beetles is facilitated by the following adaptations. First, their wings are extremely light, because, unlike the wings of large beetles, these have a very narrow wing blade with a fan of long peripheral bristles (a condition known as ptiloptery, which means feather-like wing). Moreover, in the smallest beetles these bristles are covered with outgrowths (making the bristles brush-like), which increase the aerodynamic efficiency of the bristles with almost no increase in wing weight. Second, these beetles have a flight style previously unknown and described by us for the first time in this paper. Like other flying beetles, Paratuposa placentis uses for flight the hind wings, which at rest are folded under the rigid elytra (the modified fore wings). The feathery hind wings move along an unusual trajectory, shaped like a broad figure-of-eight, and make rowing movements alternating with claps both above and below the body of the beetle.
The studied beetles were captured in Vietnam, at the Joint Russian-Vietnamese Tropical Research and Technological Center. They were placed in a transparent chamber, and then their flight was filmed on two high-speed video cameras. From the video recordings, 3D reconstructions of the movements of the wings, elytra and body were created, which made it possible to perform accurate aerodynamic calculations using special software. The structure of the wings was studied using scanning electron and confocal laser microscopes. Such a comprehensive investigation allowed us to provide the first detailed description and analysis of the flight mechanism for a microinsect.
For insects as small as these featherwing beetles, the forces of viscous friction are quite high relative to the inertial forces and weight of the body and of particular body parts of these insects. As a result, during flight air sticks to the peripheral bristles of their wings due to high viscous friction and closes the gaps between them, barely passing through between the bristles. Being much lighter (and thus subject to much smaller inertial forces) than a membranous wing of the same size, the bristled wing rows almost as well, without letting much air through, like the feather of a bird. At the same time, under such conditions, the lift generated by the wing is insufficient to support the body weight. This is why a considerable part of the aerodynamic forces created by the tiny beetle in flight is due to the drag of the wings moving at high angles of attack. This flight style is in many ways similar to the swimming of miniature crustaceans, such as fairy shrimps (Anostraca) or water fleas (Cladocera): the wings, much like the branched legs or branched antennae of the crustaceans, make rowing movements, and then collapse and return to their original position for the next stroke. The elytra of ptiliid beetles move much more vigorously, at a greater angular amplitude, than in most larger beetles. We found that moving this way the elytra serve as inertial brakes, preventing the body from too much oscillation under such conditions where the wings, moving along the above-described unusual trajectory, tend to create rotational movements in the body.
Now we plan to study the flight of other miniature insects in as much — or more — detail. We have already started with some such insects, using the same set of methods. The wing apparatus of other tiny insects is arranged somewhat differently, because their miniaturization took place independently. So, we expect our further research to reveal more mysteries of microinsect flight. Studying the aerodynamics of miniature bristled wings is an important objective, because similar air flow conditions are typical for many miniature things, both animate and inanimate. New knowledge about the flight of microinsects helps to better understand their biology, dispersal potential and roles in ecosystems. In addition, the principles of the flapping flight of insects are already being used by engineers in the design of experimental unmanned aircraft. Miniaturization is a widespread trend not only in the evolution of certain groups of animals, but also in the development of technology, and in the distant future, knowledge about the flight of microinsects may help engineers create flying devices as small as the smallest flying insects — or at least as small as some of the larger microinsects.