Insects: masters of multiplication

Insects are good at multiplication. They dominate life on earth – in diversity, numbers and volume. It may be hard to believe, but termites and ants alone could account for a quarter of all animal biomass on land.

If you consider that 94% of vertebrates, about 60% of invertebrates, plus about 25% of insects themselves, eat insects, it is perhaps not so very hard to believe after all. They are food for the majority of land and freshwater animals and ensure the pollination of 80% of land plants.

Gecko with a cricket in its mouth. Insects are food for the majority of land and freshwater animals.

Insects are so ubiquitous, that their role in nature has been strangely overlooked and undervalued. But not anymore. Terrifying reports of precipitous plunges in insect populations have made headlines in recent years.

In an article in the Guardian, Environment Editor, Damian Carrington wrote: “regulators around the world have falsely assumed that it is safe to use pesticides at industrial scales across landscapes” and “the effects of dosing whole landscapes with chemicals have been largely ignored.” I thought the world ‘got it’ back in the 1960s and 70s, when DDT was banned. Yet here we are again, surprised that insects are dying when we spray insecticides. Pardon?

People are starting to worry (again), and rightly so. Our own survival is at stake. These creatures, that we took for granted, and whose existence even irk certain people, are suddenly on the long (and growing) list of things we need protect, not destroy.

Luckily, insects can bounce back quickly in numbers, as soon as their natural habitat is restored, and the poisoning ceases – thanks to their ability to multiply. This issue contains stories related to this multiplication process.

We all know about courtship displays in birds. We may have seen male impalas battling it out to win the favour of their dams, and we know that mammals feed their young on milk. But courtship, territorial battles and suckling of young – in insects?

Courtship

Finding and choosing a mate is a vital business, especially if you are one of several closely related species, and if your mate has a good chance of looking like someone else (mimicry), or like a leaf (camouflage). Best to have a clear code.

Darkling beetles (toktokkies) drum their abdomen on the ground. Males start the tapping and then wait for the female to respond. A pair exchange signals until, eventually, they locate each other. Then it’s run and jump, hold on tight.

Mating toktokkie beetles; the courtship was handled remotely via virtual meetings.

Hangingflies are real gentlemen: they wine and dine their ladies with a nuptial gift (ok, it’s a dead insect but to her that is like gourmet steak).

Hangingfly: males offer their females a nuptial food parcel.

Stalk-eyed flies (their eyes really are at the end of those stalks!), are quite territorial. Their territory may only be the size of a big leaf in a sunny clearing, but they defend it jealously against rivals. Two males size each other up. The guy with the longest stalks wins, simple. The loser walks away dejected.

Stalk-eyed flies compete over territory by meeting face-to-face to see who has the longest eye stalks.

Male butterflies often perch obviously in an area, displaying their boldly marked wings, advertising their presence, and chasing off rivals. When a predator comes near, they may snap their wings shut, and ‘disappear’ from view, because often the underside of the wings is coloured in utterly different camouflaging colours.

Butterflies are also rather famous for their courtship dances. Pairs flutter around each other in beautiful ecstasy, before they get down to business.

Mating monarch butterflies.

But would you believe this pair of micropezid flies? The male, standing firm, dressed formally in black, wearing white gloves, seems to choreograph a dance, with clear, stiff, authoritative arm movements, like a little conductor. The female bobs and prances around him, in excited circles, fluttering her wings, an awkward ballerina.

Micropezid flies (my nickname for them is conductor flies), perform nuptial dances.

To mate or not to mate

Males and females often differ in size or colour (this is called sexual dimorphism). There are many examples. In this pair of grasshoppers, the male is much smaller than the female. She is the one who has to produce the eggs after all, he only supplies the sperm. They recognise each other with the help of pheromones – insect perfume.

Sexual dimorphism: male and female are different colours and different sizes.

Glow-worms (a family of beetles) have extreme sexual dimorphism. Females dispense with wings altogether; they do not need them. The male however needs his wings to fly around in search of a mate. The pair signal to each other using their amazing light organs. Each species has their own special code.

Extreme sexual dimorphism. Glowworms are wingless females. Fireflies are the winged males.

Some insects do not need to mate. This Seychelles scale insect fertilizes her own eggs. Mating only occurs every second generation. A mated female keeps some of the male’s sperm and passes it along to her female offspring. Her daughters then use their fathers’ leftover sperm to fertilize their own eggs. This female carries her brood of babies inside a fluted, white wax pouch. Peel the wax back and thousands of young scale insects come to light.

A Seychelles scale insect with her brood of offspring, which she carries around in a waxy pouch.

Aphids go a step further and actually produce clones. They also skip the egg-laying and instead give birth to live young. By the time a female clone is born, it already has the next generation of clones growing inside it. Cloning saves a lot of time. No wonder aphids win the speed-breeding award. Not the egg-to-egg cycle – that award goes to someone else – but through cloning they can multiply faster in numbers than any other insect.

Aphids win the speed-breeding award by giving birth to clones that already have the next generation growing inside them.

The transfer of sperm from male to female, when it does happen, seems pretty straight forward, doesn’t it? But some insects have managed to complicate even this basic event.

Before a male damselfly goes courting, he transfers some sperm into a special pouch on his belly. Then he goes and finds a mate. He clamps firmly onto her neck with special pincers on the tip of his abdomen. Pairs often fly together in tandem like this. Eventually they settle down to business. The female bends her abdomen forward, collecting the sperm from his pouch. Job accomplished!

The complex mating mechanics of damselflies…

Laying eggs

Insects are not stupid: a mother lays her eggs exactly where the hatchlings need to be when they emerge (which is the same place where the mother hatched): the hoverfly lays her eggs among aphids (which her babies will devour), the hippo fly in the reeds around a hippo pond (the maggots will drop down into the mud), the cycad looper lays her eggs on a fresh new cycad leaf, the grasshopper in the ground among bushels of grass.

The dragonfly lays her eggs in the water, her babies are aquatic.

A heavily pregnant mother mantis is having a last big meal before she lays her batch of eggs.

Growth and development

And here come the babies!

In a few minutes these hatching mantids unfold into gangly killers on stilts.

Caterpillars grow from tiny commas to the size of a finger in a few weeks. That is like a mouse growing to the size of a killer whale.

Some insect babies (called nymphs) look not too different from their elders. In this group (the exopterygota) the wing buds show up early and grow slightly bigger with every stage. Adults and nymphs tend to live the same lifestyles and eat the same food.

Like father like son: a baby stick insect sitting on daddy’s shoulders.

Another group (the exopterygota) go through complete metamorphosis. They pupate as one kind of creature and emerge as another. Larvae and adults live utterly different lifestyles, in different habitats, and usually eat completely different food. They are so unlike each other that early scientists did not realise they were the same species. Nor can you blame them!

Mother, and…

…child! See the resemblance? No? Well who would have thought that a scary antlion grows into something as delicately and lovely as a lacewing?

Growing up is a bit problematic for insects. They are arthropods and this means they have hard exoskeletons – sturdy, articulated boxes and pipes that do not grow with them. As an insect gets bigger, it has to shed its skin, and grow a new one, repeatedly.

A grasshopper adult emerges from its last nymphal skin.

Once the adults emerge, their job is to make more. They may have very little time to do that.

Some mayflies have no more than five minutes to court, mate, lay eggs, and die.

Multiplication – or not

The faster this egg-to-egg cycle can be completed, the more generations can fit into a year. Or a season. Or a month. The speed-breeding award for insects goes to mosquitoes. When temperatures are right, they can complete the egg-to-egg cycle in less than a week. Each female lays hundreds of eggs so local populations can grow to millions, seemingly overnight.

Mosquitoes can breed in the smallest patches of standing water. When you find you have a mosquito infestation, don’t assume they come from a natural stream or pond. It is more likely they bred in discarded containers with standing rainwater near your house.

Mosquitoes breeding in a bromeliad.

But not all insects multiply like this. Tsetse flies and louse flies for instance, produce only one egg at a time. The egg hatches inside the uterus, and remains there, fed from ‘milk glands’. The maggot grows bigger and bigger, still inside its mother, until it is larger than the mother herself! When the larva is fully grown, the mother gives birth to her single enormous baby. The maggot pupates, and soon emerges as an adult.

Louse flies are parasites on birds and large mammals. A mother feeds a single growing maggot inside her body from a ‘milk gland’.

Not all insects reproduce quickly either. The slowest-maturing insects are jewel beetles. They are wood borers. Their larvae can take 30 to 40 years to reach adulthood! I don’t know if there is another animal on earth that takes this long to grow up.

Jewel beetle larvae can take between 30 and 40 years to grow to adulthood.

I feel sorry for them. Imagine a beetle mother laying her egg in a lovely little patch of forest back in the 1980s, and her poor child, if it was lucky enough to survive, emerges, looks around, but the trees are gone!

Since my childhood, natural environments have been destroyed at a rate never seen before. The human population has doubled, animal populations have halved.

Yes, in theory insects can bounce back quickly in numbers, given the right conditions. But sometimes they need our help. Say No to insecticides in the garden. Say No to exotic plants. Say Yes to butterflies (and thus to caterpillars). Say Yes to birds (and therefore their prey). 

But now, let’s enjoy summer, and keep our eyes peeled for all the little creatures out there going about their mathematical business.

About the author

Marlies Craig is an epidemiologist who used to research malaria, but now works for the Intergovernmental Panel on Climate Change. Though she did originally study Biology and Entomology, her love affair with insects is very personal. In her book What Insect Are You? – Entomology for Everyone, she shares that passion with young and old, see What Insect Are You?. She hopes to kindle in people of all ages enthusiasm and a deeper appreciation of nature and show them why and how they can make a difference. She recently started a non-profit organisation called EASTER Action focussing on education and action in biodiversity, climate change, and sustainable living, see EASTERaction.org. At this time, EASTER Action is distributing a Covid-19 information booklet to help spread useful information and urge people to do what they can to overcome this crisis.