Most animals have a need for speed so that they can stay safe and nourished, defend their homes or find a mate. But what determines how fast an animal can run? Previous research has revealed that body mass is key; bigger animals tend to ou'trun smaller ones. This relationship between speed and mass is well studied in animals with a backbone – vertebrates – whereas it is only recently that similar studies have been conducted in invertebrates such as insects, centipedes or spiders. Spiders are particularly interesting animals to study as their movement mechanism is unique; they lack extensor muscles in two of their three main leg joints. Instead, they use internal pressure to push a blood-like liquid into their legs, which in turn expands the soft tissue within these joints to extend the limbs. Curious to see whether this unique limb extension mechanism limits the speed of spiders as they get larger, Charlotte Boehm and her colleagues from the University of the Sunshine Coast, Australia, headed out into the field to collect some athlete spiders.

Bedecked with head torches, the researchers hand-collected 71 wolf spiders (Tasmanicosa godeffroyi) and 57 huntsman spiders (Heteropoda jugulans) ranging from as small as 5 mg up to 3 g. After returning to the lab, they filmed the spiders running on a racetrack and analysed their footwork. The researchers found that the larger spiders have a significantly faster maximum speed, while their average speed also increases with size but at a lower rate. For example, a spider with a mass of 1 g can run at the same speed as an average human walking (∼1.4 m s−1). What's more, the fact that some spiders were missing legs didn't seem to matter, as long as they had at least five. Looking at the spiders’ footwork to understand whether they also adjust their movement characteristics revealed that as their mass increases, spiders take larger strides that last longer, similar to mammals.

Next, the researchers broke down the spiders’ movements into limb bending – achieved by muscles – and limb extension – achieved by the pressure-driven mechanism. Boehm and her colleagues suspected that the pressure-driven mechanism might affect the spiders’ speed by limiting how fast two of the three main leg joints can extend in bigger spiders. Using software that allowed them to automatically track each animal's posture – the position of the joints on all four pairs of legs – the researchers observed the opposite; larger spiders were just as able to quickly stretch their legs as smaller ones. However, they did note a decrease in how fast spiders bent the joint closest to their claws, potentially because the muscles in that portion of the leg were competing directly against the internal hydraulics.

But do all spider legs play the same role as they scamper? In a final experiment, the authors tracked how quickly the spiders manoeuvred each limb joint, suspecting that their joint use might vary. They spotted that the largest movements occurred in the muscle-mobilised joints for limbs 1 and 3, counting from the head towards the belly, but in the joints with the combined muscle–pressure mechanism in limbs 2 and 4. This alternating pattern of joint use could be a strategy that spiders have developed to avoid collisions between neighbouring legs that would slow them down. However, the authors also discovered another potentially speed-limiting factor; larger spiders have larger bellies that slow down their back legs. While the question of how spiders from even larger-bodied families solve this problem remains unanswered, one thing is apparent: the impact of increasing size on an invertebrate's speed is far more complex than we ever imagined.

Understanding the limits to the hydraulic leg mechanism: the effects of speed and size on limb kinematics in vagrant arachnids
J. Comp. Physiol. A
. doi:10.1007/s00359-021-01468-4