If you’re running at 120 body lengths per second, as some species of diurnal tiger beetles (Carabidae: Cicindelinae) can, it would be rather difficult to see everything around you (Figure 1). Motion blur becomes a problem, as it substantially degrades visual contrast and compromises a beetle’s ability to detect obstacles in its path. The fast-running diurnal tiger beetles compensate for this by periodically stopping to reorient itself, especially when hunting down prey. However, entomologists Zurek and Gilbert showed that these insects also perceive the world via antennal touch mechanosensation, which is not affected by motion blur. Moreover, this mechanosensory perception method is both necessary and sufficient for navigation while running.
Figure 1. Hairy-necked tiger beetle, the fast-running diurnal tiger beetle used in Zurek and Gilbert’s study.
Adaptive use of mechanosensory perception in animals with poor vision is common, especially among nocturnal species. Insects living in dark environments use vibrissae and antennae to actively perceive environmental stimuli, including dimensions of a nest site and/or surrounding conspecifics. Diurnal tiger beetles, however, have great daytime vision, but are functionally blind while moving at top speed. Zurek and Gilbert were interested in whether mechanosensory perception can compensate for poor vision due to motion blur, as it does for low light levels, and allow for effective spatial navigation. To answer this question, the researchers first recorded the runs of 20 beetles, each for 10 trials in four different obstacle conditions with varying obstacle heights from 2mm to 4mm; a total of 800 runs. Then, the beetles were separated into four groups, with various physiological impairments (blindness or no antenna) and no impairment. These beetles were subjected to an additional 40 runs in with either obstacles of high contrast or low contrast. Success in obstacle conditions was determined by avoidance of an abrupt collision with the obstacle. Successes with and without antennal contact and failures were averaged in each group (Figure 2).
Figure 2. Run outcomes, where the control group is the left-most group that is unimpaired and exposed to low-contrast obstacles. High contrast obstacles are marked by a black rectangular box above the beetle. The blinded group is shown with red eyes and the right-most two groups represent the antennectomized groups exposed to different contrast obstacles. Bars represent success/failure rate or runs with and without antenna contact with the obstacle.
Analysis of these runs revealed that presence of antennas and lower obstacle height led to more successful runs, whereas beetle vision or visual contrast of the obstacle had practically no effect on successful runs. Specifically, antennectomized beetles succeeded in as low as 40% of obstacles, while blindness did not affect obstacle success rate. Antennectomized beetles also decreased their ground clearance and pitch angle, suggesting that they may alter their posture to engage antennal contact with the ground in instances when the antenna is short. Such behavior has also been found in cockroaches, which use antennal-ground contact during wall-following. Altogether, the data suggested that the tiger beetles may rely almost exclusively on antennal mechanosensation to avoid collisions while running.
Source:
Zurek, D. B., & Gilbert, C. (2014). Static antennae act as locomotory guides that compensate for visual motion blur in a diurnal, keen-eyed predator. Proceedings of the Royal Society B: Biological Sciences, 281(1779), doi: 20133072.
