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Why do flies like eating dog turds? To me, this seems horrible. Why don’t they get sick like a human would?

• This is a larger issue than dog turds. Parents worldwide teach children faeces are dirty, messy, bad and yucky. Thus the stuff itself and all the synonyms for it that we learn as children become lodged in the brain as a bad thing. We do not play with excrement. This extends to all excrement from any source.

Through this process we learn from a young age that excrement may harbour “germs”, which is sometimes true, although not all microbes are germs and therefore bad for you. Nonetheless, the lesson we learn is a good one because much gastrointestinal disease is transmitted by the so-called “faecal-oral” route, making hand-washing after visiting the toilet imperative.

Because I was a paediatric gastroenterologist for almost 40 years, I have looked through the microscope at literally thousands of specimens of human faeces, searching for evidence of maldigestion (undigested fat or muscle), or more commonly inflammation (white and red blood cells). At the microscopic level, excrement is a seething mass of microbes – bacteria, archaea, yeast, and occasionally visible protozoa or fungi all scooting around either actively or by among the debris of digested food particles and cellular cast-offs from the bowel. The microscopic view is almost a work of art as the energetic micro-world recycles debris into life forms that will in turn become food for something further up the food chain.

So to answer the question: flies were never instructed by their parents to avoid faeces, dog or human, and neither were African dung beetles. So they do not object to the odours, hydrogen sulphide, dimethyl sulphide and similar compounds that have been formed by sulphur-metabolising microbes in our colon, or to the appearance of the turd itself. They just see a buffet where much of the digestive work has already been done and they can feed, lay their eggs and relax.

Maybe the Lord of the Flies gets to enjoy the biggest dump.

Adrian Jones, Professor Emeritus, pediatric gastroenterology and nutrition, University of Alberta, Edmonton, Canada

• Sadly, humans in our mindless quest for economic growth rarely appreciate the services the natural world provides free of charge and instead we often seek to trash it.

Luckily for us – or perhaps unluckily when it’s our favourite item of clothing – insects are less discerning and will eat any kind of organic matter, whether it be fur, dead vegetation or excrement. For their part insects profit from a ready supply of food and energy, while we profit because such waste products, which otherwise would build up and bury us, are recycled relatively rapidly back into the food chain.

As far as faeces are concerned, despite having passed through a digestive tract, they still contain enough organic material to feed the larvae of flies, and often the adult flies as well. Effectively dog faeces are composed of material which the dog has not digested, just as there is organic matter in our faeces which humans can’t digest – think sweetcorn. Flies are able to digest the remainder.

“Despite having passed through the digestive tract, faeces have enough organic material to feed fly larvae”

Insects which live in environments with a high microbial load have evolved a resistance to potentially harmful microbes. For instance, they have genes that govern the synthesis of antimicrobial peptides, which, to them, are effectively broad-spectrum antibiotics.

Dogs and humans play host to a great variety of bacteria in the gut which are actually beneficial. The superbug bacterium Clostridium difficile may be present in a healthy gut without causing harm but it can cause problems for hospital patients whose immune systems are weak. A controversial way of restoring gut health after an episode of diarrhoea, say, is to inject a sample of faeces from a close relative into the colon.

So it is not necessarily true that people would fall ill eating dog faeces. I would not recommend the practice, however, because one major danger is that they may contain eggs of flat or roundworms of the genus Toxocara, which present no threat to insects but which can establish themselves in humans.

“Excrement is composed mainly of microbes, but they are not necessarily harmful”

Nonetheless, in cities with many dogs, and particularly in winter when insects feed on faeces less, we can unwittingly ingest a significant amount of airborne bacteria from that source.

Terence Hollingworth, Blagnac, France