Broadcast 14 December 2005, the penultimate episode focuses on the relationships between invertebrates and plants or other animals. It begins with ants and aphids: the former 'herd' the latter and protect them in return for secreted honeydew. The activities of gall-inducing insects are described, using the example of the oak tree. Many plants recruit insects to aid pollination, offering nectar for doing so, and some predators have adopted camouflage to take advantage of this, such as the crab spider. Stick insects rely on ants to hide their eggs underground for them in safety. In the Californian desert, the blister beetle's larvae congregate on a stem and, by releasing a pheromone, attract a male digger bee on the lookout for a female. They climb aboard their visitor and eventually transfer to its mate, which will in turn unwittingly deposit them in its nest — providing sustenance. An orchard spider is shown enduring a parasitic wasp grub, which injects its host with a hormone that deranges it and halts the spinning of webs. The grub then sucks the liquid from the spider's body and uses the remaining silk to form its cocoon. Fairy wasps are so small that they can lay their eggs inside those of water beetles — and can even mate while inside them. The tiger beetle larva ambushes ants by plugging its burrow with its head and pouncing. However, this doesn't work with Methocha, an ant-like wasp, which avoids the jaws of the beetle larva, paralyses it with a sting, and lays its eggs on the host. After dragging the paralysed larva deeper into the burrow, the entrance is carefully plugged and concealed. Ants defend their colonies fiercely; however Alcon blue butterflies manage to get their young inside the ants' nests by giving their young a scent exactly like that of the ant larvae; as a result the caterpillars are treated as if they were in fact ant larvae. However, this strategy is not flawless. Ichneumon wasps break into the ant colonies and release chemicals that make the ant guardians attack each other; the wasp then injects two of her eggs into the butterfly catterpillars. However, the ants seem to save at least one caterpillar as one of the pupae is later shown hatching into an adult Alcon blue butterfly.
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.