Lough Muree, County Clare, Ireland.
Many such debatable questions raised by the anatomy of these creatures still wait universally agreed answers. However, they all have six legs and tripartite bodies and these characteristics clearly link them to that great and varied group of land invertebrates, the insects. They appeared many millions of years after the earlier groups were well established. Geneticists have now shown that collembolla, as well as the insects, including the silverfish, are all closely related to one particular group of water-living crustaceans, the remipedia (the name means ‘oar-foot’), which today are found only in the pools and streams of caves.
The primitive insects must have found some of their food by climbing the trunks of the early tree ferns and horsetails. The ascent was doubtless relatively easy. The climb down, involving long detours over the upward-pointing leaf-bases, may have been very much more laborious and time-consuming. Whether or not the prevalence of such obstacles had anything to do with the next developments, we cannot be sure. It is certain, however, that some of these primitive insects did develop a much swifter and less laborious method of getting down. They flew.
We have no direct evidence of how they achieved flight, but the living silverfish provides a clue. On the back of its thorax it has two flap-like sideways extensions of the chitinous shell that look as though they might be the rudiments of wings. The early wings may not have served initially for flight. Insects, like all animals, are greatly affected by body temperature. The warmer they are, the quicker the energy-producing chemical reactions of their body can proceed and the more active they can be. If their blood were to be circulated through thin flaps extending laterally from the back, they could certainly warm themselves very effectively and quickly in the sunshine. If, furthermore, these flaps had muscles at their base, they could be tilted to face squarely to the sun’s rays. Insect wings do originate as flaps on the back and they do, initially, have blood flowing in their veins, so such a theory seems very plausible.
However this may be, insects with wings appeared some 350 million years ago. The earliest so far discovered are dragonflies. There were several species, most about the size of those living today. But for the dragonflies as for millipedes and other groups that have pioneered a new environment, the absence of competition allowed some early forms to develop to an enormous size, and dragonflies eventually appeared with a wingspan of 70 centimetres, the largest insects ever to exist. When the air became more thickly populated, such extravagant forms disappeared.
Living dragonflies have two pairs of wings which have simple joints to them: they can only move up and down and cannot be folded back. Even so, they are highly accomplished flyers, shooting over the surface of a pond in a blur of gauzy wings at up to 30 kph. At such speeds, they need accurate sense organs if they are to avoid damaging collisions. A tuft of hair on the front of the body helps them to check that their motion through the air is straight, but their primary navigational guidance comes from huge mosaic eyes on either side of the head, which provide superbly accurate and detailed vision.
Because of this dependence on sight, most dragonflies do not fly at night, although there are some that migrate vast distances over the oceans, flying from India to Africa and stopping off at the islands of the Maldives along the way. All are daytime hunters, flying with their six legs crooked in front of them to form a tiny basket in which they catch smaller insects. That fact alone makes it clear that they must have been preceded into the air by other herbivorous forms which, judging from the primitive nature of their anatomy, were probably related to cockroaches, grasshoppers, locusts and crickets.
The presence of these large populations of insects, whirring and buzzing through the air of the ancient forests, was eventually to play an extremely important part in a revolution that was taking place among the plants.
The early trees, like their predecessors, the mosses and liverworts, existed in two alternating forms, a sexual generation and an asexual one. Their greater height posed no problem for spore dispersal: if anything it was a help, since up in the treetops spores were more easily caught by the wind and carried away. The distribution of sex cells, however, was a different matter. Hitherto, it had been achieved by the male cells swimming through water, a process which demanded that the sexual generation be small and live close to the ground. That of ferns, club mosses and horsetails still is. The spores of these plants develop into a thin filmy plant called the thallus which looks not unlike a liverwort and releases its sex cells from its underside where there is permanent moisture. After its eggs have been fertilised, they grow into tall plants like the previous spore-producing generation.
On the ground, the thallus is clearly vulnerable. It is easily grazed by animals; if it dries out it dies; and the very success of the asexual generation with their arching fronds cuts it off from life-giving light. Many advantages would follow if it too could grow tall, but this would require a new technique for getting the male cell to the female.
There were two mechanisms available – the ancient, rather hazardous and capricious method that distributed spores, the wind; and the newly arrived messenger service, the flying insects, which were now regularly moving from tree to tree, feeding on the leaves and the spores. Plants took advantage of both mechanisms. About 350 million years ago, some appeared in which the sexual generation no longer grew flat on the ground, but up in the crowns of the trees. One group among these plants, the cycads, survives today and shows the development at a particularly dramatic stage.
Cycads look superficially like ferns, with long coarse feathery fronds. Some individuals produce tiny spores of the ancient type that can be distributed by the wind. Others develop much larger ones. These are not blown away but remain attached to the parent. There they develop the equivalent of the thallus, a special kind of conical structure within which eggs eventually appear. When a wind-blown spore – which now can be called pollen – lands on an egg-bearing cone, it germinates, not into a filmy thallus for which there is now no need, but into a long tube which burrows its way down into the female cone. The process takes several months. Eventually, when the tube is complete, a sperm cell is produced from the end of the tube. It is a majestic ciliated sphere, the largest known sperm of any organism, plant or animal, so big that a single one is visible to the naked eye. Slowly it makes its way down the tube. When it reaches the bottom, it enters a small drop of water that has been secreted by the surrounding tissues of the cone. There it swims, slowly spinning, driven by its cilia, as it re-enacts in miniature the journeys made through the primordial seas by the sperm cells of its algal ancestors. Only after several days does it fuse with the egg and so complete the long process of fertilisation.
Another group of plants adopting a similar strategy to the cycads arose at about the same time. These were the conifers – pines, larches, cedars, firs and their relations. They too rely on the wind to distribute their pollen. Unlike the cycads, they produce both pollen and egg-bearing cones on the same tree. The process of fertilisation in a pine takes even longer. The pollen tube requires a whole year to grow down and reach the egg, but once there, it contacts the egg cell directly, and the male cell, after descending the tube, does not tarry in a drop of water but fuses directly with the egg. The conifers have at last eliminated water as a transport medium for their sexual processes.
Common hawker dragonfly (Aeshna juncea) on Brackish Moss National Nature Reserve, County Armagh, Northern Ireland.
Cones of the Eastern Cape giant cycad, or bread tree (Encephalartos altensteinii). Present day cycads are survivors of a group dating back 300 million years. Most families died out during the Cretaceous period, 80 million years ago. Cycads are of great evolutionary interest due to their reproductive system, considered to be the forerunner of flowering plants. The cones are the reproductive structures and can be male or female, producing seeds to form new plants.
They have also developed one
