and can fend for themselves.
Mediterranean/European scorpion (Buthus occitanus) stinging a spider (Amourobius sp.).
Spiders, too, must be extremely cautious in their courtship. Matters are made even more hazardous for the male because he is nearly always smaller than the female. And he prepares for his encounter with his mate long before he meets her. He spins a tiny triangle of silk a few millimetres in length and deposits a drop of sperm on to it from the gland that lies underneath his body. He then sucks it into the hollow first joint of his pedipalp, a special limb at the front of his body. Now he is ready.
The courtships of spiders are beguilingly various and ingenious. Jumping spiders and wolf spiders hunt primarily by sight and have excellent eyes. The courting male, consequently, relies on visual signals to make the female aware of his presence and his purpose. His pedipalps are brightly coloured and patterned, and as soon as he sights a female, he begins to signal with them in a kind of manic semaphore. Nocturnal spiders, on the other hand, depend largely on an extremely delicate sense of touch to find their prey. When they meet one another, they gingerly caress each other’s long legs, and only after a great deal of hesitation do they come to closer quarters. Web-making spiders are sensitive to the vibrations on their silken threads that tell them when a victim has blundered into the web. So when the male of such a species approaches a female hanging, large and menacing, on her web, or lurking hidden beside it, he signals to her by twanging the threads at one side in a special and meaningful way which he trusts the female will recognise. Other species put their faith in bribery. The male catches an insect and carefully parcels it up in silk. Holding this in front of him, he cautiously approaches the female and presents it to her. While she is occupied in examining the gift, he quickly scuttles over her and ties her to the ground with bonds of silk. Only then does he risk an embrace.
All these techniques lead to the same conclusion. The male, having survived every danger, places his pedipalp close to the female’s genital opening, squirts out the sperm and then hastily retreats. It has to be recorded that in spite of all his precautions he sometimes fails to make his getaway in time and the female eats him after all. But in terms of the transmission of his genes, the male’s disaster is of limited consequence: he lost his life after, not before, he had completed his purpose.
While the early segmented animals were perfecting their adaptations for living on land and away from moisture, the plants were also changing. Neither the mosses nor the other early forms had true roots. Their short upright stems sprang from a horizontal one of a similar character lying along the ground or just below it. This construction served well enough in moist surroundings, but in many parts of the world the only permanent water supply lies below the ground. To tap that requires roots that probe deep between the particles of the soil and can absorb the film of water that clings to them in all except the most arid environments. Three groups of plants appeared that possessed such structures, and all three have descendants that have survived without much change: club mosses, which resemble mosses but have stiffer stems; horsetails, which grow in waste patches and ditches and have stems encircled at intervals with rings of needle-like leaves; and ferns.
Wolf spiders (Pardosa sp.), male (right) waving palps in courtship display, Derbyshire, UK.
The ferns, early in their history, had developed a special protein to protect themselves from damage by ultraviolet light, something that had not been a problem for their ancestors since they lived in water where such wavelengths could not reach them. This substance now slowly changed into a material called lignin. This is the basis of wood, and it gave them the rigidity needed to grow tall. So a new kind of competition developed between plants.
All green plants depend on light to power the chemical processes by which they use simple elements to synthesise their body substances. So if a plant does not grow tall, it risks being overshadowed by its neighbours and condemned to shade where, starved of light, it might die. So these early groups used the newly acquired strength of their stems to grow very tall indeed. They became trees. The club mosses and horsetails were still, for the most part, swamp-dwellers, and there they now stood in dense ranks, thirty metres tall, some with woody trunks two metres in diameter. The compacted remains of their stems and leaves today form coal. The great thicknesses of the seams are impressive evidence of the abundance and persistence of the early forests. Other species of both these groups also spread farther inland and there mingled with ferns. These had developed true leaves, large spreading structures with which to collect as much light as possible. They grew tall with curving trunks, like the tree ferns that still thrive in tropical rainforests.
Wood horsetails (Equistetum sylvaticum) Columbia River, Gorge National Scenic Area, Oregon, USA.
The height of these first forests must have caused considerable problems for their animal inhabitants. Once, there had been a superabundance of leaves and spores close to the ground. Now the soaring trunks had raised this source of food high in the sky, creating a dense canopy that cut out much of the light. The floor of these forests was, at best, only sparsely vegetated and great areas may have been entirely without any living leaves. Some of the multi-legged vegetarians found their fodder by clambering up the trunks.
There may have been another factor that induced these creatures to leave the ground. About this time, animals of a completely new kind joined the invertebrates on the land. They had backbones and four legs and wet skins. They were the first amphibians and they too were carnivorous. A description of their origins and fate will have to wait until we have followed the development of the invertebrates to its climax, but their presence at this stage must be mentioned if the scene in these first jungles is not to be misrepresented.
Virtually all of the new-style invertebrate families still survive. Among the most numerous are the bristletails and springtails. Although they are little known and infrequently seen, they are enormously abundant. There is hardly a spadeful of soil or leaf litter anywhere in the world that does not contain some of them. Indeed, the springtails, or collembola, are probably the most abundant arthropods on the planet. Most are only a few millimetres long. Of those new families, only one is commonly noticed – the silverfish that glides smoothly across cellar floors or is occasionally discovered making a meal of the dried glue in the bindings of books. Its body is clearly segmented but it has very many fewer divisions than the millipede. It has a well-defined head with compound eyes and antennae; a thorax bearing three pairs of legs, the result of fusing together three segments; and a segmented abdomen which, while it no longer carries limbs on each segment, retains little stumps as signs that it once possessed them. Three thin filaments trail from its rear end. It breathes like the millipedes by means of tracheae, and it reproduces in a manner reminiscent of those early land invertebrates, the scorpions. The male silverfish deposits a bundle of sperm on the ground and then, one way or another, he entices the female to walk over it. When that happens, she is stimulated to take it up into her own sexual pouch.
There are several thousand different species of bristletails and collembola. They vary considerably in their anatomy and, as is often the case when considering the simpler members of a big group, it is sometimes difficult to decide whether a particular characteristic represents a truly primitive survival or one that has become secondarily reduced to suit a particular way of life. The silverfish, for example, has compound eyes but other members of the group are blind. All lack wings. Some even lack tracheae and breathe through their chitinous skeleton which is particularly thin and permeable. Is this because they never had them or because they have lost them?
Marine springtail/bristletail (Petrobius maritimus) adult
