A balancing act

So, if convergence is going to be a guiding principle in understanding evolution, then of all the areas worth investigating one of the most interesting must surely be to look at what constraints, if any, accompany the development of sensory organs. It is here, if anywhere, that we can approach the wider problem of the evolution of nervous systems, brains, and perhaps ultimately sentience. And this in turn might give some clues as to whether indeed intelligence is some quirky end point of the evolutionary process or whether in reality it is more-or-less inevitable, an emergent property that is wired into the biosphere.

A similar question has been succinctly posed by David Sandeman.8 He writes, 'it may be of interest to ask whether there are sensory organs that are unique and not convergent... One of the most bizarre and ingenious equilibrium systems that have arisen is without doubt the halteres of the dipteran flies' (his emphasis).9 The halteres, it may need to be explained, are the organs for balance in the flies. They are located on the back of the fly and resemble tiny drumsticks. In flight they swing continuously to act as gyroscopes that serve to detect angular accelerations. It has long been realized that the halteres are derived from the pair of hind-wings of the fly (most insects have two pairs of wings), and are indeed, in Sandeman's terminology, 'bizarre and ingenious'. Surely these qualify as a unique biological invention? Not a bit of it. Sandeman's question was rhetorical. He reminds us that very similar structures are found in another group

figure 7.1 Electron micrograph of a strepsipteran insect, in the form of the male emerging from its host. Note the drumstick-like halteres arising from behind the head, as well as the compound eye, the arrangement of which is convergent on that of the trilobites. (Photograph courtesy of Dr J. Kathirithamby, University of Oxford.)

figure 7.1 Electron micrograph of a strepsipteran insect, in the form of the male emerging from its host. Note the drumstick-like halteres arising from behind the head, as well as the compound eye, the arrangement of which is convergent on that of the trilobites. (Photograph courtesy of Dr J. Kathirithamby, University of Oxford.)

of insects known as the strepsipterans (Fig. 7.1), which on molecular evidence do not seem to be closely related to the dipteran flies.10 Strepsipteran halteres are different, however, because they are derived from the pair of fore-wings. They are found only in the males, whose wings are put to use for a short mating flight that they undertake once they emerge from their host, such as a wasp, in search of the females that (with few exceptions) remain permanently within the host.11 The parasitic strepsipterans differ, therefore, quite markedly from the dipterans, yet it is clear that their halteres act in the same gyroscopic fashion.12 However bizarre they may seem, halteres are not a unique invention. Indeed, there is evidence for yet other convergences. One is in an African beetle that has modified its wing-cases (known as the elytra) into haltere-like organs;13 and a fourth example can perhaps be found in a group of hemipterans known as the coccoideans.14 The coccoideans are probably most familiar for the swollen galls the larvae form in plants, but it has been observed that in some of the adult males the hind-wings are 'reduced to haltere-like structures'.15

Swinging tiny drumstick-like structures in the air is one way of keeping a stable course, and man-made gyroscopes adopt a somewhat similar principle. For animals, a much more widespread method of achieving balance is by means of structures known as statocysts, which typically consist of some grains of a mineral attached to fine hairs. This arrangement is sensitive to gravity, the nervous connections to the hairs being stimulated according to how the heavier grains fall. Statocysts have evolved repeatedly, and are found in groups as diverse as the jellyfish, crabs, and cephalopods.16 Indeed, a comparable structure is even known in the group of single-celled organisms, the ciliates.17

In humans and other vertebrates balance is achieved by an extension of the statocyst principle, employing mineral grains known as the otoconia. These are located within sac-like structures housed within the inner ear. Immediately adjacent to these sacs are the so-called semicircular canals, which are also involved in maintaining a sense of balance. Such a sophisticated system might at first be thought to be without a parallel. Primitively, that is in the jawless fish, the lamprey, there are two canals, but in the more advanced vertebrates there are three canals, set at right angles to each other. The canals are filled with fluid and contain sensory hairs that by registering angular accelerations transmit information, albeit in a way not fully understood, to the brain to permit effective and controlled balance. Unique? No, because a convergent arrangement is found in the swimming crabs, although here the arrangement is directly derived from the more primitive arrangement of the statocyst.18 In comparison, however, these statocysts are strongly modified, and within the canals hair-like receptors monitor the fluid flow induced by the crab's movement. Even so, why should a crab in this respect imitate a fish? As so often when discussing convergence, the mode of life gives the necessary clue. These crabs are very agile swimmers, capable of rapid manoeuvre in pursuit of their prey. Balance is clearly at a premium, and it is not so surprising that they have arrived at a system strongly reminiscent of the one seen in the more primitive vertebrates with their two semicircular canals. As might also be expected, these canals are filled with fluid. In contrast to the vertebrates, however, this fluid is composed of sea water and is incorporated each time the crab moults its skeleton. At this juncture it has to secrete a new skeleton, including a pair of the balancing organs which, once filled with sea water, are then sealed to the outside world. In conjunction with a sophisticated visual system these balancers allow these active and fast-swimming crabs to perform their adroit manoeuvres.

The senses, five or otherwise, may be separately identifiable, but the various inputs received by the animal through the nervous system will require appropriate integration. In the case of balance in the swimming crabs an acute visual control is an unsurprising corollary. In reviewing this linked system in the crabs and comparing it with the vertebrates Sandeman remarks that they have both 'achieved the same end using similarly constructed sense organs',19 albeit from very different origins. Sandeman continues, however, by noting that notwithstanding the sophisticated sense of balance the overriding factor in this system is the effective capture of light, and so it is to the topic of the eye that we now turn.

Was this article helpful?

0 0

Post a comment