04/19/2020: Six-spotted tiger beetles (Cicindela sexguttata) are some of the earliest insects whizzing around. Desperate to heat up, these metallic blue-green micropredators are easy to spot glinting on the tops of concrete surfaces and rocks. In high temperature conditions, flight and leg muscle performance greatly increases, putting them on par with the fastest-running insects proportional to their body size. But fortunately for me, the cool climate makes these runners more feasible to catch and observe.
Early anecdotes of tiger beetle behavior were atypical for insect predators. As opposed to a dragonfly that intercepts a fly in one swoop, tiger beetles will stop in their tracks numerous times before attempting subjugation. It’s easy to think that beetles may be optimizing their pouncing position through successive stops, similar what a jumping spider might do. However, the stop-and-go motion is incredibly regular, indicating that tiger beetles may be compensating for some limitation of their sensory system. The prevailing explanation for these strange locomotor patterns stems back to a paper in 1997.
Tiger beetles face major challenges in visual discrimination when overtaking prey in high speed pursuits. During a chase, apparent motion across the beetle’s compound eye results from movement by the prey item as well as self-generated image motion. At greater velocities, it is increasingly difficult to disentangle these two sources of motion and their interaction with the background environment. You can imagine that for a 2cm long insect moving at 0.5–2.5 meters per second, the whirlwind produced by both parties makes localizing a tiny prey item seem unlikely… even more so, when thinking of variable wind and light conditions and the remarkable neural processing speed required. For aerial predators, the mid-air hunting arena provides them with a more uniform background and better image contrast, reducing many of these constraints for their visual systems. For terrestrial hunters, on the other hand, leaf litter environments are more heterogeneous with many physical obstructions that limit visibility.
High velocity running greatly decreases the reliability of tracking moving prey. Instead of modulating their speed, tiger beetles employ intermittent stop-and-go motion. By stopping the body entirely, image movement induced by the beetle itself is eliminated, and the beetle only needs to calculate the prey’s position and relative angular velocity. Presumably, this allows for decision-making to occur during pauses before making fairly unidirectional sprints. This ‘stop-and-go’ locomotor pattern is the most commonly observed hunting behavior in tiger beetles. However, if the speed of prey is below a certain threshold, beetles can continuously track them and the jagged movements disappear entirely. If tiger beetles are truly unable to localize fast-moving prey during sprinting bouts, once stopped they must rely on their prey to make a move— which is exactly the case. When prey movement ceases during the short time interval that the tiger beetle is moving, both parties remain at a standstill. This situation is a nightmare for all the small arthropods out there: for any evasive maneuver you do, the beetle achieves a faster one in your direction.
All tiger beetles in this post photographed under controlled conditions after capture [6]
07/30/2020: The largest individual of around two dozen Carolina tiger beetles (Tetracha carolina) I found scurrying around two nights ago. In addition to being quick maneuverable runners, some tiger beetles in the genus Tetracha are known to secrete cyanide when subdued. Their rainbow iridescence might be a conspicuous warning coloration to advertise their distastefulness to predators. Another cool feature is that the two spots on the elytra resemble the color pattern of velvet ants, which are armed with a powerful sting. It could be that tiger beetles are a Müllerian mimic; both velvet ants and tiger beetles benefit from predators’ increased exposure to and association of the twin spots with an unpalatable prey item.
