Once Proud Wings

The bodies of whales and sirenians abound in historical relics that we notice because they live in a very different environment from their land-dwelling ancestors. A similar principle applies to birds that have lost the habit and equipment of flight. Not all birds fly, but all birds carry at least relics of the apparatus of flight. Ostriches and emus are fast runners that never fly, but they have stubs of wings as a legacy from remote flying ancestors. Ostrich wing stubs, moreover, have not completely lost their usefulness. Although much too small to fly with, they seem to have some sort of balancing and steering role in running, and they enter into social and sexual displays. Kiwi wings are too small to be seen outside the bird's fine coat of feathers, but vestiges of wing bones are there. Moas lost their wings entirely. Their home country of New Zealand, by the way, has more than its fair share of flightless birds, probably because the absence of mammals left wide open niches to be filled by any creature that could get there by flying. But those flying pioneers, having arrived on wings, later lost them as they filled the vacant mammal roles on the ground. This probably doesn't apply to the moas themselves, whose ancestors, as it happened, were already flightless before the great southern continent of Gondwana broke up into fragments, New Zealand among them, each bearing its own cargo of Gondwanan animals. It surely does apply to kakapos, New Zealand's flightless parrots, whose flying ancestors apparently lived so recently that kakapos still try to fly although they lack the equipment to succeed. In the words of the immortal Douglas Adams, in Last Chance to See,

It is an extremely fat bird. A good-sized adult will weigh about six or seven pounds, and its wings are just about good for wiggling about a bit if it thinks it's about to trip over something - but flying is completely out of the question. Sadly, however, it seems that not only has the kakapo forgotten how to fly, but it has also forgotten that it has forgotten how to fly. Apparently a seriously worried kakapo will sometimes run up a tree and jump out of it, whereupon it flies like a brick and lands in a graceless heap on the ground.

While ostriches, emus and rheas are great runners, penguins and Galapagos flightless cormorants are great swimmers. I was privileged to swim with a flightless cormorant in a large rock pool on the island of Isabela, and I was enchanted to witness the speed and agility with which it sought out one undersea crevice after another, staying under for a breathtakingly long time (I had the advantage of a snorkel). Unlike penguins, who use their short wings to 'fly underwater', Galapagos cormorants propel themselves with their powerful legs and huge webbed feet, using their wings only as stabilizers. But all flightless birds, including ostriches and their kind, which lost their wings a very long time ago, are clearly descended from ancestors that used them to fly. No reasonable observer could seriously doubt the truth of that, which means that anybody who thinks about it should find it very hard - why not impossible? -to doubt the fact of evolution.

Numerous different groups of insects, too, have lost their wings, or greatly reduced them. Unlike primitively wingless insects such as silverfish, fleas and lice have lost the wings their ancestors once had. Female gypsy moths have underdeveloped wing muscles and don't fly. They don't need to, for the males fly to them, attracted by a chemical lure which they can detect at astounding dilutions. If the females were to move as well as the males, the system probably wouldn't work, for by the time the male had flown up the slowly drifting chemical gradient, its source would have moved on!

Unlike most insects, which have four wings, the flies, as their Latin name Diptera suggests, have only two. The second pair of wings has become reduced to a pair of 'halteres'. These swing about like very high-speed Indian clubs, which they resemble, functioning as tiny gyroscopes. How do we know that halteres are descended from ancestral wings? Several reasons. They occupy exactly the same place in the third segment of the thorax as the flying wing occupies in the second thoracic segment (and the third too in other insects). They move in the same figure-of-eight pattern as the wings of flies. They have the same embryology as wings and, although they are tiny, if you look at them carefully, especially during development, you can see that they are stunted wings, clearly modified - unless you are an evolutiondenier - from ancestral wings. Testifying to the same story, there are mutant fruit flies, so-called homeotic mutants, whose embryology is abnormal and who grow not halteres but a second pair of wings, like a bee or any other kind of insect.

What would the intermediate stages between wings and halteres have looked like, and why would natural selection have favoured the intermediates? What is the use of half a haltere? J. W. S. Pringle, my old Oxford professor whose forbidding mien and stiff bearing earned him the nickname 'Laughing John', was mainly responsible for working out how halteres work. He pointed out that all insect wings have tiny sense organs in the base, which detect twisting and other forces. The sense organs at the base of halteres are very similar - another piece of evidence that halteres are modified wings. Long before halteres evolved, the information streaming into the nervous system from the sense organs at their base would enable fast buzzing wings, while flying, to act as rudimentary gyroscopes. To the extent that any flying machine is naturally unstable, it needs to compensate with sophisticated instrumentation, for example gyroscopes.

Halteres on a cranefly

The whole question of the evolution of stable and unstable fliers is very interesting. Look at these two pterosaurs, extinct flying reptiles, contemporaries of the dinosaurs. Any aero-engineer could tell you that Rhamphorhynchus, the early pterosaur at the top of the picture, must have been a stable flier, because of its long tail with the ping-pong bat on the end. Rhamphorhynchus would not have needed sophisticated gyro-control, such as flies have with their halteres, because its tail made it inherently stable. On the other hand, as the same engineer could tell you, it would not have been very manœuvrable. In any flying machine, there is a trade-off between stability and manœuvrability. The great John Maynard Smith, who worked as an aircraft designer before returning to university to read zoology (on the grounds that aeroplanes were noisy and old-fashioned), pointed out that flying animals can move in evolutionary time, back and forth along the spectrum of this trade-off, sometimes losing inherent stability in the interests of increased manœuvrability, but paying for it in the form of increased instrumentation and computation capability - brain power. At the bottom of the picture on the previous page is Anhanguera, a late pterodactyl from the Cretaceous era, some 60 million years after the Jurassic Rhamphorhynchus. Anhanguera had almost no tail at all, like a modern bat. Like a bat, it would surely have been an unstable aircraft, reliant on instrumentation and computation to exercise subtle, moment-to-moment control over its flight surfaces.

Anhanguera didn't have halteres, of course. It would have used other sense organs to provide the equivalent information, probably the semicircular canals of the inner ear. These were indeed very large in those pterosaurs that have been looked at - although, a touch disappointingly for the Maynard Smith hypothesis, they were large in Rhamphorhynchus as well as Anhanguera. But, to return to the flies, Pringle suggests that the four-winged ancestors of flies probably had long abdomens, which would have made them stable. All four wings would have acted as rudimentary gyroscopes. Then, he suggests, the ancestors of flies started to move along the stability continuum, becoming more manœuvrable and less stable as the abdomen got shorter. The hind wings started to shift more towards the gyroscopic function (which they had always performed, in a small way, as wings), becoming smaller, and heavier for their size, while the forewings enlarged to take over more of the flying. There would have been a gradual continuum of change, as the forewings assumed ever more of the burden of aviation, while the hind wings shrank to take over the avionics.

Worker ants have lost their wings, but not the capacity to grow wings. Their winged history still lurks within them. We know this because queen ants (and males) have wings, and workers are females who could have been queens but who, for environmental, not genetic, reasons failed to become queens.* Presumably worker ants lost their wings in evolution because they are a nuisance and get in the way underground. Poignant testimony to this is provided by queen ants, who use their wings once only, to fly out of the natal nest, find a mate, and then settle down to dig a hole for a new nest. As they begin their new life underground the first thing they do is lose their wings, in some cases by literally biting them off: painful (perhaps; who knows?) evidence that wings are a nuisance underground. No wonder worker ants never grow wings in the first place.

Rhamphorhynchus (top) and Anhanguera (bottom)
0 0

Post a comment