Most animals risk their own lives when hunting for prey. Many spiders reduce this danger by spinning orb webs out of sticky silk, to net unsuspecting insects. However, this doesn't mean that capturing their next meal is problem-free. Spiders face a choice – do they stick to a web design that will capture small insects with little nutritional value and allow bigger, more satisfying prey to escape with minimal damage to the web, or do they try their hardest to capture the more enticing, tastier, larger insects, even though it could mean frequently suffering damage if the gamble doesn't pay off? Andrew Sensenig from Tabor College, USA, decided to investigate (p. 3388).
Sensenig and his colleagues collected spiders from 10 different species of the orb weaver clade, Araneoidea, and allowed them to build webs back in the lab. Under the watchful gaze of a high-speed camera the team then launched a series of tiny balsa wooden blocks at the webs. To mimic different insects found in the wild, they varied the mass (30, 100 and 300 mg) and the speed (1.3–5.5 m s−1) of the projectiles.
For all projectiles, the team found that as speed increased, the probability of catching and retaining the balsa wood blocks decreased. This was not surprising: the bigger and faster the insect or block is, the more energy it has to break through the silky net. But Sensenig explains that even during breakages, the web will absorb some of the projectile's energy. To find out how much, the team measured the speed of the balsa blocks before and after passing through the web. Knowing the mass and speed, the team could then work out the kinetic energy of the block before and after its collision. They found that the faster the projectile had been travelling, the more energy the web absorbed. In fact, the energy-absorbing ability of the web increased sixfold over the range of speeds they investigated. The results suggest that spiders will invest in a web that maximises its chances of catching a big meal.