A joint multidisciplinary study conducted by researchers from the University of Haifa (Dr. Moshe Gish and Professor Moshe Inbar from the Department of Evolutionary & Environmental Biology) and the Technion  (Dr. Gal Ribak and Research Professor Daniel Weihs from the Faculty of Aerospace, and the Technion Autonomous Systems Program (TASP)), found that  aphids (known as plant lice), which drop from the host plant as a defensive response to danger, are capable of turning themselves over in mid air to almost always land on their feet. The study, which describes the aerodynamic mechanism and the ecological significance of this phenomenon, was recently published in the Current Biology Journal.

Aphids are tiny sap-sucking insects that dwell on plants. The aphids are nourished by plant sap, feeding on several species of legumes, and many species are known to be among the most destructive insect pests on cultivated plants. When aphids sense danger, most often they choose to escape from the host plant by dropping to the ground. Researchers found that immediately after an aphid abandons its host plant, it carries-out a rotational maneuver in mid-air (similar to that done by cats), so that it lands almost always on the ventral side (i.e. right side up, as oppose to landing on its back), not dependant on the starting orientation from which it fell.  The study showed that Aphids dropping from their host plant in an attempt to escape one of its most deadly predators, the seven-spotted ladybug (Coccinella septempunctata), landed on their ventral side in 95% of cases when the drop was made from the height of 20 centimeters, whereas in cases where the drop was made from heights lower than this, its ability to rotate in mid-air was only slightly affected. A fraction of aphids were capable of pulling off the rotational maneuver in mid-air from heights of only a few centimeters. The researchers used special high-speed digital cameras to document the falling process on speeds above 1,000 frames per second, in order to identify the mechanisms used by the aphids in carrying-out the mid-air rotation. It has become clear that already from the beginning of the drop, aphids raise their back legs (in relation to their bodies) and tilt their long antennas forward, to get into a distinct and stable falling position. The researchers built a mathematical model based on air resistance, which demonstrated that during the fall, when the aphid is in its distinct falling position, aerodynamic forces on its body parts (caused by air resistance) cause the aphid to rotate to the point at which an aerodynamic stability is reached that locks the duration of the fall at a fixed angle, whereby its ventral side and legs are pointed in the direction of the ground. In essence, when the aphid’s body is in its distinct falling position, it forms a type of aerodynamic “roly-poly” stance, whereby every deviation from its being stable (during the fall) is automatically and immediately corrected by air resistance, devoid of any action required by the aphid.

The scientists made additional experiments in order to clarify the benefits of an aphid’s ability to land on its ventral side.  Their finding showed that by landing on the ventral side, aphids will have a better chance of being able to cling onto the lower leaves of its host that they collide with on their fall to the ground. For the aphid, the ground is an extremely dangerous place, as it makes them susceptible to surface predators (such as ants), or death of starvation and dehydration. Although the aphid drops from its host plant to escape from dangers, it deliberately tries to do everything in its power to cling to the lower leaves of its host in order to avoid reaching the ground. A theory proposed by researchers, based on the video coverage, and from conclusions drawn by other experiments, suggests that when an aphid lands on its ventral side, sticky pads on the ends of their feet come in contact with the surface of leaves and consequently allows it to cling tightly onto the leaf and stop it from falling. Nonetheless, if an aphid lands on its side or backside, the sticky pads at the ends of their feet do not come in contact with the surface leaves and consequently, the aphid is spewed from the leaf and continues to fall towards the ground.

The mid-air rotational mechanism is very impressive in its simplicity and efficiency, because it doesn’t require from the aphid to act in any way, apart from moving its legs and antennas to the distinctive falling position. The aphid completes its rotation in a very short period of time – in less than two tenths of a second, a phenomenon that is made possible due to its small size (only a few millimeters). In such small masses, the falling speed of an aphid is relatively very low, while the viscosity of the air highly influences the aerodynamic forces on the body. In such instances, quick body rotation ensues, already at the early stages of falling. In contrast to cats, who, owing to their size are forced into making complex maneuvers to ensure they turn over in time before they land, aphids let air resistance and gravity  do the work for them.

The study highlights the significance of the host plant for the aphids that live off them: even upon being forced to escape for fear of becoming instant prey, its adapted mechanism enables the aphid to cling onto the lower leaves of its host and hold onto them for dear life.

Above: The free falling position of aphids. Aphids raise their back legs up and their antennas forward and up to form a stable aerodynamic falling position ensuring a good landing (on their feet). Illustration by Nick W. Sloff