Long live Lepidoptera!

How long does a butterfly live for?

This is something people often ask me when I give talks or lead a walk. There are several ways to answer it. The simplest way is to point out that few adult butterflies, or moths, or any insect for that matter, will exist in that state for more than a year. And the adult stage is merely the final stage in a sequence of stages. Moreover, the adult cannot moult or regrow broken wings or legs; once it is broken, it usually falls to a predator, or falls to the ground to die of starvation, and rot down. So, the familiar concept of a lifespan measured in years, as we are used to, does not hold for most insects. It is more a question of, how many lifetimes fit into a year?

Vertebrates such as birds, reptiles, and mammals have two stages – the egg, and the finished article. It can be argued that there are juvenile stages, but once a bird or a reptile has hatched from the egg, it retains its form until fully grown, and till it dies. Reptiles shed their (inflexible) skin periodically, allowing room for expansion. Birds moult, and mammals do too (as all dog owners know!) but the skin is not shed in stages – it grows with the animal and its cells are replaced gradually. The skeleton grows at a rate that matches that of the body, and the size increase is gradual, taking time to reach full scale. Mammals also have an egg stage, but that is (with some exceptions) hidden inside the mother. Like birds and reptiles, a new-born mammal or marsupial has all the features of the adult, on a smaller scale, and it also retains its basic form until fully grown. Fish are slightly different in that newly hatched individuals are referred to as ‘larvae’ – they carry the yolk sac from the egg with them during the first stage of life, but they are still recognisable as fish.

Amphibians are perhaps the exception because they have a free-swimming ‘larval’ stage – tadpoles. Like reptiles they moult, and as they grow, legs develop and they change shape. But they remain vertebrates – the skeleton is retained, not shed.

Vertebrates’ support structure is a skeleton made of mineralised bones, hidden inside the body and formed inside the embryo as it develops inside the egg. It is known as an ‘endoskeleton’ (‘endo’ = ‘inside’).

Butterflies and moths are invertebrates and as arthropods they have a rigid ‘exoskeleton’ (‘exo’ = ‘outside’) holding the liquid contents. This is not elastic, as is the skin of birds and mammals, but has a limit beyond which it cannot stretch. So rather like reptiles, they must moult to allow growth, but there is no gradually expanding endoskeleton. Some arthropods, such as spiders and scorpions, maintain their body shape and appearance at each moult, only growing larger as the internal organs expand to stretch the exoskeleton. Others have so-called ‘larval’ or ‘nymphal’ stages, whose physical shape is quite different. In some, such as crustaceans, the larva may be a microscopic free-swimming organism, and the progression to full adulthood may take several months or years. Some insects, such as wood-boring beetles or cicadas, have larval or nymphal stages that can last years, but the adults are relatively short-lived.

However, it’s butterflies and moths we are discussing here.

Life expectancy of a ‘typical’ butterfly

Butterflies and moths – Lepidoptera – are a remarkably diverse group but all have one thing in common – metamorphosis (change in shape). Their adults mate, the females lay eggs, which hatch into larvae (or caterpillars). These then feed and put on weight as they do so. Every so often, when the non-stretch skin has become too tight to allow further growth, the larva moults and a new one, now with a loose skin, crawls out of the old one. Depending on the species this happens three to four times, sometimes six or more, and the caterpillar grows in a series of stages, known as ‘instars’. I recently raised a local butterfly from the egg to the adult, and the whole process took 7 weeks from egg laying to release of the adults.

Female Pirate, Catacroptera cloanthe ovipositing on a Blue Bushviolet (Barleria obtusa) at Crestholme Nature Reserve.

I watched her lay two eggs and took them home with some of the host plant to rear.

Eggs of the family Nymphalidae (‘Brush-footed Butterflies) are varied in form. This Pirate egg is a typical ribbed egg of the subfamily Nymphalinae.

First instar Pirate caterpillar, about one day old; egg hatched after 5 days. Length – about 3mm

Second instar Pirate caterpillar now 11 days old. Length – about 8mm

Third instar Pirate caterpillar now 14 days old. Length – about 12mm

Fourth instar Pirate caterpillar now 19 days old. Length – about 20mm

Final (Fifth) instar Pirate caterpillar now 28 days old. Length – about 50mm

Pirate pupa 33 days after being laid as an egg. Length – about 22mm

Male Pirate emerged 46 days after being laid as an egg

The female Pirate that hatched from one of the eggs collected on 20 Feb 2021, released exactly 7 weeks after being laid as an egg in the same spot.

This male Pirate was photographed on the 24th of September in 2016 – a really old soldier who has been around a bit. He probably hatched from a pupa the previous April and had been sleeping rough, fighting battles in the air and drinking whatever he could find in the dry season. Even then, he’s still a handsome little beastie with his fugitive purple sheen still visible.

What does this tell us about the life expectancy of a butterfly? Well, for the individuals I reared, not a lot. I won’t know how long the adults I released survived for, but it was the 10th of April when I released them – and as you can see, scruffy males are still around in September. So, add on 4-6 months, and you have a total lifespan of about 6-8 months for that generation.

Instead of lifespan, for butterflies and moths (and other insects), we talk about generational length. That is, the time lapse between an individual being laid as an egg, and its own mating and oviposition of the next generation. It follows that for this generation of Pirates, the generational length would be somewhere between 2 and 8 months depending on when the offspring laid eggs. As we know that there is usually a peak of Pirate activity in spring, another in late summer, and another in autumn, and they overwinter as adults, that makes sense. The same would hold for other Nymphalids. As adults they tend to be long-lived and robust, and they use high energy sugar-rich foods such as nectar and fermenting fruit. The tatty Pirates (and Gaudy Commodores) we usually see in September and October most likely emerged as adults during the previous April.

The spectrum of ‘generational length’

We all know that many butterflies only appear once a year, and not in two or three seasonal peaks. Others may be common for a while during rainy conditions, then vanish during a drought, only to be seen the next time it rains – which can be more than a year later. And some are seen nearly all year round but have peaks in the numbers seen. What is their generational length?

One of the best-known local butterflies that has a truly short adult life is the Yellowish Amakosa Rocksitter, Durbania amakosa flavida. Colonies exist in the hills around Kloof, and its life history was covered in an earlier edition of Leopard’s Echo.

This rare butterfly appears annually around November, and individuals can be seen as late as early January. We usually see the males (like this one) first, then females later. To begin with they are strongly marked and deeply coloured, and as time goes on, they start to look, well… tatty. The adults cannot feed; they lack functional mouthparts. They exist to find a mate, copulate and then the female lays her eggs. Once this is done their task is over. They probably only live as adults for two to three weeks, and as time goes on, new adults emerge and the population peaks in December. The eggs are laid on sandstone rocks on which grows the species of lichen on which the larva feeds. As could be expected, this is low in nutrients; the larva will grow very slowly through the instars, taking 11 months to grow to full size and pupate. This species therefore has a generational length of 12 months.

Another butterfly that only appears once a year is the Bush Beauty, Paralethe dendrophilus indosa. It’s confined to the eastern cool forests of South Africa and occurs at a higher altitude as one moves north. Adults begin to appear in late February, it is most often seen in late March to April, and they cease to fly in May.

This is a skulking butterfly that lurks in the shady parts of forests and comes out occasionally to bask in the sun. Those of us fortunate enough to live next to one of the wooded kloofs in our area may see them in our gardens. The female lays her eggs among forest grasses, and they hatch into caterpillars that grow slowly as they feed on the nutrient-poor grass leaves. The adults normally live for two to three weeks, getting more ragged as time goes on. Unlike Rocksitters, they feed readily – most often on fallen fruits. They are particularly fond of banana flowers that are ‘going over’.

This emergence of adults within a tight time frame is known as a ‘brood’. Such species, which have a 12 month generation length, are also known as ‘univoltine’. Many butterflies have more than one brood a year. Those with two are known as ‘bivoltine’. More than two, ‘multivoltine’.

Heliconiinae, the long-wings, are numerous all over the tropics and we have plenty in Kloof. These eggs of Dusky Telchinia, Telchinia esebria, have been laid en masse on a leaf of the host plant, Climbing Nettle, Urera trinervis.

Telchinia larvae, like their relatives the Acraeas, are gregarious. Here you can see that two batches of larvae have joined together; the small ones are second instar, the larger ones, fourth.

 In the cool months of June and July Telchinia larvae can be found sheltering in silken tents, like this.

Telchinias often pupate far away from the host plant; almost as if they are aware of their distasteful nature and deliberately do it out in the open to advertise it. This one pupated on a friend’s luridly painted garden wall!

Dusky Telchinias have several colour forms. The orange and black form esebria is often seen in Durban.

The commonest local form of Dusky Telchinia is the cream-banded form protea.

Dusky Telchinia is one of our commonest and most familiar Heliconiines, and adults may be seen almost year-round in the warmer areas. Numbers drop in the cooler months, but they increase as summer comes around, and autumn population explosions are common. So, one asks, what is the generational length here? Well as you can probably imagine, it’s short – but variable.

The eggs can take less than a week to hatch. Faster in hot weather; slower in cool. Heliconiine larvae can grow extremely quickly when the conditions favour them. I once collected some fresh eggs of Blood-red Acraea, Acraea petraea, and three days later they had darkened, and I could see the larvae inside them. Eight days later they were in third instar; two weeks later they had pupated, and the first adults emerged a week later:

From this

To this inside four weeks.

Of course, there were multiple batches of eggs laid by several females at the same time, and larvae of all stages crawling all over the place. A generational length of 4-6 weeks, most likely. So why aren’t we inundated with countless millions of Acraeas and Telchinias all the time? Why is it that most of the year, we only see the occasional adult, and their condition varies from dewy fresh to old and tatty? And then, usually in late summer or autumn, a massive burst of thousands of adults sometimes appears everywhere?

The answer: predator-prey interactions

I covered this in an earlier issue of Leopard’s Echo here and here.

The last year’s enforced ‘house arrest’ in my Gillitts forest garden has led me to think that here is a cycle. Prey numbers increase, which leads to predator numbers increasing. But eventually the prey wins the numbers battle. And then it starts all over again.

August to October: adults that have overwintered from the previous late summer surge become active as the days get longer, they mate and lay eggs on the abundant plant growth that follows the early rains. Butterfly and moth numbers increase, and some univoltine species have their only emergence. Bivoltines have their first brood. But at the same time, predators are waking up. Birds arrive from migrations, and nesting begins. Nestlings require insect food. Wasps and flies hatch from pupae formed last summer, mate, and start looking for prey. Spiders hatch from eggs and start to grow.

Result – lots of eggs and caterpillars, but most get eaten quickly. Wasp and flies increase, begin to peak, and win the numbers game against the herbivores. Birds raise one or two broods of young.

November to January: peak plant growth and what look like optimum conditions for butterflies and moths. But the predators keep the upper hand, and most lepidoptera are in their larval stages, feeding voraciously. Most bivoltines are in the larval stage. But they keep getting eaten!

February to May: slowly but surely the herbivores gain the upper hand. Birds and mammals complete their rearing activities and may migrate, taking themselves out of the race. Wasps and flies pupate; spiders become prey themselves, and bivoltine lepidoptera have their second (and larger) brood. Butterfly and moth numbers increase; suddenly there are Acraeas, Charaxes and other species everywhere.

June and July: as the weather cools and cold-blooded insects become less active, adult butterflies find places to hide. They come out on warm days and feed on any fruit or nectar available, but they are waiting for the first rains and plants to start sprouting…

This of course only applies to the coastal littoral area where we live. In the mountains and cool grasslands, and tropical rainforests, the cycle would be different.

But wherever you are, it’s not how long a butterfly lives for that counts – it’s how many generations it can fit into a year.

Steve Woodhall is a butterfly enthusiast and photographer who began watching and collecting butterflies at an early age. He was President of the Lepidopterists’ Society of Africa for eight years, and has contributed to and authored several books, including Field Guide to Butterflies of South Africa and Gardening for Butterflies. His app, Woodhall’s Butterflies of South Africa, is described as the definitive butterfly ID guide for South Africa.