Nature 2020 December

Waiter, there’s a dipteran in my soup!

I was reading a brand new book all about insects. It will never be a best-seller, nor will Netflix be buying the film rights soon! An opening remark that caught my eye was the question: ‘What is the oldest profession practised by people with their clothes on?’ According to Genesis 2: 19-20 it is a taxonomist! – Adam was given the responsibility to give a name to all the creatures (and presumably plants).

Adam would have had a big challenge when it comes to naming all the insects. There are over a million known species of insects in the world and many more unnamed and thought to yet be discovered. In Britain there are mere 24,000 or so species, but this is ten times more than plants, and eight times the number of other invertebrates, which includes everything from worms and snails to starfish and lobsters.

I think it’s worth starting by defining what is, and what is not, an insect. It used to be possible to say all ‘bugs’ are insects. Some bugs are insects but only certain ones are called ‘true bugs’, like bed bugs, to distinguish them from those other insects loosely called a ‘bug’. But bug has a much wider meaning. Catch a cold and it is said you’ve caught a ‘bug’. In this case a microorganism, usually a bacterium, but not a virus. When a computer program does not run it is said to have a ‘bug’. I refuse to use the term ‘mini-beast’ to distinguish any insect with attitude from those that some television presenters would consider boring.

The ‘rule of six’ was a rubric used to distinguish insects which had six legs from other near relatives which had eight or ten and up to 354 for centipedes or 750 for millipedes! The bodies of adult insects are segmented into three parts, head, thorax and abdomen. In comparison, spiders only have two body parts whereas millipedes may have hundreds. Most, but not all, insects have wings, either one or two pairs. Another characteristic feature of insects, though not unique to them, is metamorphosis. Insects have been around for 400 million years and have evolved considerably to cope with the tumultuous changes in the earth’s climate and geography. Their success is down to their ability to adapt to these changes and metamorphosis is both the means and the evidence for their survival and success whilst other groups of animals have long since disappeared. Many representatives of the primitive forms of insects thrive too. Some inhabit our soil (springtails), others our rivers (mayflies). Uniquely some even float around our skies, at the whim of the breeze, like thrips, or even scurry around our kitchens in the dark, eg silverfish.

It would be reasonable to conclude that our British insect collection comprises a representative cross-section. Annoyingly, we have far more than our share of those species we consider annoying like flies, wasps and fleas. In the UK, flies seem to love living with us humans. They represent 30% of species compared to just 15% worldwide. However, when it comes to the more exotic groups there are some interesting contrasts.

Take, for example, beetles, known collectively as coleoptera. They are the largest group of all insect species in the world, with 350,000, which is about 35%. Beetles only rank third in the UK league table of insect groups, with 4,000 (16.6% of all species). Beetles often live a clandestine life and finding and collecting them became a craze for eccentric Victorian gentlemen who might employ a servant to collect beetles for them. Rare, mutated variations and duplicates were traded for ridiculous prices in the ultimate hobby in an era before postage stamps. Fifty years later when variations and rarity in postage stamps provided a similar fascination for the Edwardians, this was eminently a more civilised hobby than scrambling through the shrubbery for beetles!

 

The insect group comprising ants, bees and wasps (hymenoptera) takes first place in the UK with 7,000 species, (17% of all UK insects). Across the whole world they rank only third. Why is this? It’s all down to climate and geography. Social insects, like bees, wasps and ants are evolutionarily closely related and highly specialised to enable them to successfully compete in all kinds of habitats. Their use of elaborate constructions; hives, hills and inter-connected tunnels and caverns in which they can control the climate is an insurance policy against dramatic changes in environmental conditions. The architecture of these homes is welladapted to cope with extremes of heat, cold and high humidity. In tropical rainforests and arid deserts there are many species of these colonial-living social insects. Colony size rather than species diversification, is more important to the survival of the species in the tropics. In the UK, ants, bees and wasps can be found in every niche and due to their social organisation they are well-placed to cope with our unpredictable climate.

My final thoughts turn to my favourite insects: the butterflies and moths. The lepidoptera are by far the most conspicuous of all insects. The configuration, striation, pigmentation and refractive qualities of butterfly scales provide the colouration. Unique photonic crystals refract the light producing the iridescent blues and greens. Pheromones, which females use to attract males over great distances, are a second feature of their highly evolved status. Only 10% of UK species are butterflies and moths compared to nearer 20% worldwide. This is a consequence of the more diverse, bountiful and nutritious foodplants in tropical regions permitting the caterpillars to invest so much more energy than their UK cousins. In the UK there are only 58 regularly breeding species of Butterfly plus another 12 or so visitors. Moths on the other hand are represented by 2,400 species. The contrast between caterpillars and the adult butterfly or moth is one of the great mysteries, only recently discovered through meticulous observation. It is still not fully understood how the former can transmogrify into the latter in a matter of weeks.

Waiter, there’s a dipteran in my soup!
Sir, according to my taxonomic handbook for waiters it’s a wine fly.
I think he must have taken the pledge!

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