Webless spiders reverse bungee jump to catch prey

For all locomotory tasks, speed and precision necessarily trade off. This trade-off is particularly crucial for ambush predators, a predation strategy that relies on the element of surprise to capture prey: They must be both fast and precise. In PNAS, Aceves-Aparicio et al. describe the hunting sequence of the webless spider Euryopis umbilicata that hunts a large and dangerous prey, Camponotus ants (1). The E. umbilicata attaches a single silk line to the bark of a eucalyptus tree and waits for a wandering ant to come its way (Fig. 1). When the ant comes close, the spider quickly reorients the silk toward the ant, tumbles over, and tags the ant with the viscid silk before landing on the bark again (Fig. 1). This first phase of the attack, which involves tumbling over and tagging the ant, occurs rapidly, taking less than a second. In contrast, the second phase, where the spider circles the ant, entangling it with silk before biting, detaching, and carrying it away, takes much longer (637 ± 371 s). Surprisingly, these spiders have a remarkable predation success of 85% of all encounters, and if the spiders tagged the ants successfully the capture success rate is 100%, something which is very rare among predatory systems.

Fig. 1.

An image of a webless spider, E. umbilicate, (A) waiting for prey on the bark of a eucalyptus tree and (B) in the middle of an attack on a large and dangerous prey, a Camponotus ant. The black arrow points to the silk thread. Image credit: Alfonso Aceves-Aparicio (1).

How do these spiders achieve this remarkably high success rate? Speed is often a key component of predation strategies (2). Faster attack means that prey has less time to initiate an escape response. During the tumble, the Euryopis spiders achieve rapid speeds of 25 cm/s in less than 100 ms. Although speed is clearly the key for this ambush predator, how do spiders target the ant with such precision at these rapid rates? First, the spider must detect an approaching ant. Early detection would allow the spider to reorient toward the prey and hence increase its precision of tumble direction. Arachnids possess a variety of extremely sensitive visual and mechanosensors that would enable exactly that (3, 4). Spider legs contain multiple thin, mechanosensory hairs called trichobothria (5). Experiments with spiders of the genus Cupiennius show that the trichobothria detect the slightest wind movement and help in reorienting toward prey (6). The energy required to activate these hair-like sensors is less than the energy contained in a single quantum of green light (6). These hair sensors respond to movements between 40 and 600 Hz, with hairs of different lengths responding to different frequencies. Interestingly, variation in the length of hair, as small as 1.5 mm, results in this wide range of frequency tuning. Another mechanosensor that could help detect ants are the substrate-borne, vibration-sensitive slit sensilla on spider legs (7). These occur either individually or in groups. The latter are arranged as parallel lines, called lyriform organs because of their resemblance to the strings of a lyre. The substrate vibrations are transmitted to these organs via the leg exoskeleton, where the slit-like structure concentrates the strains around these organs, causing the underlying neuron to fire. Each leg contains this sensor and, depending on the vibration patterns across the eight legs, spiders reorient their posture (8). In addition to these remarkably sensitive mechanosensors, spiders also possess a highly specialized visual system (4). Spiders have four pairs of eyes, one pair of principal eyes in the anterior medial position and the other three secondary eye pairs located in the anterior lateral, posterior lateral, and posterior medial position. The size and location of the eyes on the cephalothorax are typically tuned to their behavioral ecology. In fact, some species of spiders have the full eight eyes, whereas others have reduced or lost some pairs. However, the two kinds of eyes are implicated in different functions: The principal eyes typically provide high spatial acuity and color vision, whereas the secondary eyes provide peripheral vison with a role in motion vision and shifting gaze to important targets such as potential preys (4, 9). Interestingly, the principal eyes of several spider families have retinal muscles that can move the retinas behind a fixed lens (9). Hence, spiders can reorient their gaze or follow a potential target without having to move their head or reorienting their body.

Having detected a potential prey, the spider must next reorient its silk thread and body to jump and tag the ant. Spider locomotion is powered by eight legs whose extension is controlled by hydraulic pressure, whereas flexion is controlled by direct muscle contraction (10). Several organisms use passive mechanical mechanisms to achieve movements that are ultrafast and hence at the limit of direct neural control. One such example is the latch-mediated spring actuation which helps achieve rapid movements in short timescales (11). This involves storing energy in an anatomical part that acts as a spring, converting the stored elastic energy into kinetic energy when a latch is released. Indeed, one known example is the slingshot spider, which uses its web, an external tool as the spring, and its anterior legs as the latch (12). Release of the stored elastic energy in the small sections of the coiled silk thread allows the slingshot spiders to achieve velocities up to 4.2 m/s within 6 ms. Newer methods in markerless tracking of limb joints have enabled kinematic measurements of body and leg joints in high resolution during natural behaviors (13). Tracking a spider moments before and at the initial launch of an attack would help us understand how the Euryopis spiders achieve these rapid and yet highly precise directional jumps (14). Kinematic measurements will also help us understand the energetic considerations of the attacks, especially as the spiders target and are equally successful at catching the ants in all directions (1).

Finally, to successfully target a moving prey, the spider might also need to predict the prey’s movement and launch an attack in the expected position of the prey. Insects such as dragonflies and killer flies pursue their flying prey (tiny fruit flies) to intercept and capture them (15). An interception mechanism of pursuit requires the predator to maintain a constant angle with respect to the prey’s future location (15–17). Although not as fast as Drosophila, ants are also wandering, and this raises the question whether spiders are predicting the direction of movement of ants. Indeed, one out of the five unsuccessful encounters occurred when the ant changed it heading direction (1). Another curious observation in this paper is that the spider attack was often initiated upon the direct contact between the ants and spider’s legs or its silk thread (1). Like the spider trichobothria, insects also possess hair sensors which are extremely sensitive and are used for escape (5, 18). This raises the question of why the ant does not detect the presence or movement of the spider. The ant behavior suggests that the spiders might be using chemical mimicry or camouflage to hide in plain sight. This brings us back to the idea that these bungee jumpers might be using speed to catch their prey. It is interesting that these spiders exclusively target highly dangerous prey, the Camponotus ants, in the field and brings into focus the several trade-offs that are often at play in these predation strategies such as the sensorimotor systems underlying the behavior and the danger posed by the prey in addition to other factors like hunger, access to prey, etc. (2, 19, 20).