biology zoology blog benno meyer rochow wings

Reductions and Concentrations

Advantages of having less

One of the most frequent comments I have to scribble on the margin of my students’ essays, assignments and reports is “condense” or “shorten and compress”. It’s exactly what evolution has done (via the survival of the fittest) with certain organs and structures of the animal body. In a way it’s the opposite of what I had written in a different blog about duplications and repetitive structural elements (the million of identical nephrons in the kidney come to mind, the hundreds of identical legs in some millipedes ring a bell and even the dozens of identical teeth in the mouths of dolphins may be remembered). Therefore, how about the opposite? It’s actually easier to find examples for reductions of structural entities in animals and we can almost use examples from the same animal groups mentioned earlier in connection with duplications.

Take the legs of vertebrates for instance: they are very short in seals and kin and hind legs are completely gone in whales. Some lizards like the blindworm (Anguis fragilis) have lost their appendages and in snakes, the lack of legs is actually the diagnostic feature. Toes in numerous species of birds and mammals have often become reduced in number and in horses, there is just the “middle finger” left to support the animal. The dentition of many mammals has also seen reductions and in some armadillos and the scaly pangolins (and the Australian platypus and echidna) there are no longer any teeth at all. And birds, whose ancestors have been relatives of crocodiles and Tyrannosaurus rex (namely reptiles), they have lost their entire dentition in the course of evolution. The New Zealand kiwi bird, in addition, has no wings at all and just some minuscule bony arm fragments are left in its skeleton to indicate that its ancestors did have wings.

Talking about wings leads us to insects, whose normal number of wings is 4 arranged in two pairs on the meso (middle) and meta (hind) thoracal segments of the three parts of the thorax. But archaic dragonflies of the carboniferous period about 300 million years ago had also a pair of small wings on the first thoracal segment and thus 6 wings altogether. Wings can be useful, but if a small island is one’s home and winds constantly blow, it’s an advantage not to have any wings at all and so it’s not surprising to find that species of wingless insects dominate on small windswept islands. Soil-inhabiting species like termites and ants have also given up their wings (with the exception of the reproductive castes) and for parasitic insects inhabiting the fur of mammals (and sometimes the hair of humans) wings are useless and more an encumbrance than an asset; thus, evolution has favoured wingless fleas and lice. What’s interesting is that very occasionally one does come across specimens of insects with tiny wings that belong to species that are normally wingless like stick insects. Internally, the tendency to compress and reduce, to “streamline” and to concentrate, is particularly obvious with regard to the insects’ nervous system. Being arranged like a ladder with regular rungs in each segment, the insect nervous system resembles that of an earthworm in the more archaic insect groups. In those, however, deemed to be advanced with smart individuals like honey bees, wasps and even flies, neuronal cells are concentrated in closely apposed, often fused ventral ganglia; the ancestral ladderlike organization with repetitive structural elements is then hardly discernable.

There is no animal taxon in which there aren’t species that show at least some kind of regression, compression and concentration of organs or structures. Gills in sharks are the last example I want to mention. The more primitive, i.e. ancient species of sharks have 7-gill slits, but the more advanced forms possess only 5 pairs of gills and thus 5 gill slits. Obviously, greater efficiency needs to compensate for the loss in numbers and that is the driving principle in all of these (and many other examples) of reductions and concentrations. We can see it in other areas too: for example in universities in which a number of smaller departments combine to form a single (and hopefully) more efficient unit. And I see it in my students’ essays, assignments and reports when I go over their revised versions: short and sweet is neat.

© Dr V.B. Meyer-Rochow and, 2019.
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