Broadcast 7 December 2005, the third instalment examines the spiders and others that produce silk. Attenborough visits New Zealand's Waitomo Caves, which are inhabited by fungus gnats whose illuminated larvae sit atop glistening, beaded filaments to lure their prey. The ability to spin silk developed early in the invertebrates' history, being first used as an adhesive. The female lacewing still applies it in this way, to suspend its eggs from plant stems. Spiders first employed it as a sensitive trip line to detect movement, and Attenborough illustrates this by encouraging a trapdoor spider. The speed with which it appears causes the presenter to jump in surprise. The webs spun by orb-weavers are complex and can comprise up to 60 metres of silk and 3,000 separate attachments. A time-lapse sequence reveals their intricate construction. The largest are made by Nephila and can be several metres across. The venomous redback spins three-dimensionally, and fixes vertical lines that suspend its unlucky meals in mid-air. Meanwhile, the bolas spider swings a length of silk with a sticky blob on the end, with which to snare passing moths. Argiope exemplifies the dangers of mating that are faced by some male spiders: unless they are careful, they can be consumed by the females. The courtship of the wolf spider, though less risky, is one of the more elaborate. Its nesting habits are discussed, along with the eventual birth of its young, which cling to their mother's back.
Production of the series took around two years, during which time filming took place around the world, from the Amazonian rainforest to Costa Rica, Australia, Malaysia, Hungary, Switzerland and many more locations, including the United Kingdom. To follow and understand the various species looked at throughout the series, the production team consulted with some of the foremost experts on invertebrate life. In certain instances, their help proved invaluable, particularly when coming across particularly dangerous species or societies. In other instances, the specialists helped to provide some of their most recent discoveries, enabling the makers to showcase in rich detail the complex processes through which invertebrates may interact with their environment, as well as the regular processes of all animals in the wild, such as their mating rituals and hunt for food. Many of the creatures' interactions were not only filmed for the first time, but were also recorded with such extraordinary magnification that scientists who studied them were able to answer specific questions that observance with the naked eye had hitherto rendered impossible.
Filming of mayflies was made possible by their appearance on the last day.
As always, time and money constraints played a huge part. The filming schedules had to be arranged to fit in with expected dates of major events that were planned to be included, such as the emergence of the North American cicadas or the mass emergence of mayfly in Hungary. As is usual in the preparation of a nature documentary, not everything went to plan, due to the unpredictable nature of the subject matter. Although filming took place over several years, time constraints still meant that some scenes almost weren't filmed, and a few never materialised at all. For instance, the simultaneous mass emergence of the mayfly in Hungary did not occur until the deadline day for its filming, as David Attenborough had to be in Switzerland the very next day to film the mating of wood ants. Using expert advice, the team had come to film at the time of the annual emergence, but the problem of the unusually wet spring had delayed the event. Luckily, on the very last day conditions were perfect, and the mayfly emerged — apparently in one of the more impressive manifestations of recent times. Because of these kinds of occurrences being largely dependent on environmental factors such as temperature or moisture, it was nearly impossible to tell exactly when they would happen. Instead the producers had to rely on expert estimates, but even these could be completely unpredictable. So although the mayfly appearance was captured, others were missed, such as the advent of a type of moth in Arizona (despite the camera crew camping out in the area twice, two weeks at a time). Sometimes subjects were so small that it would have been impossible to film them in the wild. Instead, the construction of a complete habitat in a studio allowed easy pursuit of their actions, allowing the camera to capture them throughout their day. This technique was used on the wolf spider, for example, which provided some 200 hours of film — notably including its courting ritual. Despite the arm span of the grown spider being no more than 1cm, even the newborn arachnids are shown in tight close-up as they climb on to their mother's back.
Filming also involved entering rough environments. To film the giant centipede, a team had to endure a dark cave whose floor was covered with guano, beetles and cockroaches.