Broadcasted on 23 November 2005, the first episode tells how invertebrates became the first creatures of any kind to colonise dry land. Their forerunners were shelled and segmented sea creatures that existed 400 million years ago. Some of them ventured out of the water to lay their eggs in safety, and Attenborough compares those first steps with today's mass spawning of horseshoe crabs off the Atlantic coast of North America. Some animals abandoned the oceans altogether when the land became green with algae, mosses and liverworts. The earliest ground-dwellers were millipedes, which were quickly followed by other species. Springtails are shown to be smaller than the head of a pin and, for their size, can jump immense heights. The velvet worm hunts nocturnally and has scarcely changed over millennia, while the giant centipede can kill instantly and is shown hunting bats in Venezuela. Mating habits are explored, including the unusual ritual of leopard slugs and the meticulous nest maintenance of the harvestman. The arrival of earthworms was of great importance since they changed the nature of the soil, leading to a proliferation of plant life. Despite their aquatic ancestry, many invertebrates, particularly those with no exoskeleton, need a moist environment to keep themselves from drying out. Finally, a creature that has adapted to a desert habitat, the scorpion, is shown as it pursues its dangerous courting dance, followed by the birth of up to fifty individuals.
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.