Biology’s Fascination with Six

Yes “6”, not sex (and sometimes also “5” too)

In terms of their total number of species and number of individuals, the Hexapoda (“hexa” from the Greek “hex” and meaning “six”) are the most common creatures on Earth.  They are animals with 6 legs and because of their abundance they have to be called highly successful. They include, of course, all the insects and that appears to show that having 6 legs must have been an advantage over having fewer or more. And indeed, insects seem to have evolved from ancestors with more than three pairs of legs, but a further reduction seemed unnecessary and even unwanted, for the 6 legs, with always two on one side and one on the other on the ground, provide them with a stable tripod resting stance and a balanced gate (now often copied by robotic vehicles to operate in difficult terrain). The six legs can also buffer an insect’s fall from a height and even with one or two legs lost or injured, an insect can, albeit more slowly, crawl away.

But insects are, of course, not the only organisms in which “six” plays a role. The sea anemone subclass Hexacorallia contains six orders including the stony, reef-forming coral species, all of which possess a six-fold structural symmetry. Six-fold symmetries are also familiar from the honey-combs constructed by honey bees and from the nests of wasps and hornets. The arrangement of the facets in the compound eyes of insects is based on hexagons. The outer cuticular surface of some tiny hexapods, known as springtails, feature beautifully regular hexagons that resemble the lattice structure of graphite and, turning our attention to some internal building blocks, we find more examples of hexagonal organizations, for example in the stacks of retinal microvilli, which when cross sectioned exhibit  a honey comb pattern. The all-important carbohydrates like sugars, starch, cellulose and chitin are all based on the hexagonal shapes of their building blocks, namely a hexagon of 6 carbon atoms.

Although there are plants with flowers (or leaves like clover) with a six-fold symmetry, the number five is also popular in Nature: just look at your hands and feet! Or cut the fruits of an okra (my favourite vegetable) and take a look: you will notice its pentameric  (frem Greek “penta” meaning “five”) shape. A huge number of flowers exist that have five petals; in fact nearly 40% of the flowering plants are pentameric and a little over 10% are hexameric. The most famous examples of five-armed animals are, of course, the starfish. Belonging to the animal phylum known as Echinodermata, all its members in fact, including sea urchins and sea cucumbers are pentameric.

When I was the director of the Electron Microscopy Unit at the University of the West Indies and used our scanning electron microscope to examine various structures, I came across many examples of pentagonal shapes. There were the shapes and surface patterns of some insect eggs (also recently reported for the mosquito Psorophora albipes by Cecilia F. De Mello and co-workers), the compact structures of some pollen grains, and the outlines of some diatoms. Sadly, I never had a chance to examine samples of the unicellular, microscopic deep sea coccoliths, but they, too, are also known to contain many examples of specimens with pentagonally-shaped calcium carbonate plates. In the rare cases where a bregmatic skull bone was present in humans, it was reported to have been of roughly pentagonal shape. In pathological conditions like Alzheimers, amyloid fibrils may be developed and the latter contain pentagonal protein components.

All mammals have five “holes” in their head: two ears, two nostrils and a mouth and generally speaking we distinguish five brain regions, namely the cerebrum, diencephalon (with thalamus and hypothalamus), mesencephalon (= midbrain), metencephalon (= cerebellum) and medulla. We also speak of the “five senses”, i.e.  sight, sound, smell, taste, and touch, but also invoke a “sixth sense” when we describe an ability to perceive something that does not seem to involve the ‘regular’ five senses. Although there are actually more than 6 senses, there you are: we believe that Nature just loves sixes and fives.

© Dr V.B. Meyer-Rochow and, 2021.
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