This 16-millimeter distance seems to be the ideal placement for the honeybee to make contact with the surface with any one of its feet. The foot that first contacts the surface varies by landing orientation. A horizontal platform initiates a hindleg landing, while a vertical platform initiates a foreleg landing.
For a visual of this process, check out this video.
“Although landing is a crucial part of insect flight, it has attracted relatively little study. Here, we investigate, for the first time, the final moments of a honeybee’s (Apis mellifera) landing manoeuvre. Using high-speed video recordings, we analyse the behaviour of bees as they approach and land on surfaces of various orientations. The bees enter a stable hover phase, immediately prior to touchdown. We have quantified behaviour during this hover phase and examined whether it changes as the tilt of the landing surface is varied from horizontal (floor), through sloped (uphill) and vertical (wall), to inverted (ceiling). The bees hover at a remarkably constant distance from the surface, irrespective of its tilt. Body inclination increases progressively as the tilt of the surface is increased, and is accompanied by an elevation of the antennae. The tight correlation between the tilt of the surface, and the orientation of the body and the antennae, indicates that the bee’s visual system is capable of inferring the tilt of the surface, and pointing the antennae toward it. Touchdown is initiated by extending the appendage closest to the surface, namely, the hind legs when landing on horizontal or sloping surfaces, and the front legs or antennae when landing on vertical surfaces. Touchdown on inverted surfaces is most likely triggered by a mechanosensory signal from the antennae. Evidently, bees use a landing strategy that is flexibly tailored to the varying topography of the terrain.” (Evangelista et al. 2010:262)
“The reason for this behaviour becomes apparent in Fig. 8, which shows the variation, with platform tilt, of the perpendicular distance from the base of the antennae to the platform, during hover. This distance is remarkably constant, at 15.7±1.5 mm (mean ± s.e.m., N=266 landings), as the tilt of the landing platform is varied from 0 deg. to 180 deg. It is the only measured variable that remains constant, irrespective of the orientation of the landing platform. This means that landing bees hover at a fixed distance from the platform, irrespective of the tilt of the platform and therefore irrespective of the region of the eye that views it. Thus, hovering bees are capable of evaluating and adjusting the distance to the platform, irrespective of whether the platform is horizontal, vertical or inverted.” (Evangelista et al. 2010:266)“The general observation that the orientation of the flagella varies with the tilt of the platform, reveals that the hovering bee is able to sense the tilt of the platform before any mechanical contact is made with it… At touchdown, the tilt of the platform, and the orientations of the body and the antennae together determine which appendage of the bee’s body makes initial contact with the landing surface…Depending on the tilt of the platform, this can be a single appendage, or a combination of appendages that make contact simultaneously. For surfaces that are close to horizontal (0–30 deg. tilt) the hind legs, often in conjunction with the middle legs, are the first to make contact. As the platform tilt increases towards the vertical plane, the middle legs and front legs are also incorporated into the touchdown. Once the platform tilt exceeds 120 deg., the first contact with the landing surface is made predominantly with the antennae. It should also be noted that in most cases it is the antenna on one side that initially makes contact with the surface, e.g. the right antenna alone, rather than both antennae simultaneously. This is because the approach is not always exactly perpendicular to the surface, although it usually deviates from the normal direction by less than 10 deg. in yaw…” (Evangelista et al. 2010:267)“The most interesting finding of the present study is that, during the final hover phase, bees keep the distance from their head to the landing surface amazingly constant at a value of about 16 mm, irrespective of the tilt of the surface. Presumably, this is the perfect distance from which to initiate the final landing manoeuvre, namely, extending the legs for touchdown. From about 16 mm away a bee can just reach the surface with one or more of its appendages, be it merely extending the hind legs onto a horizontal surface, or grabbing an overhead surface by extending the forelegs over its head.” (Evangelista et al. 2010:268)