‘Under cover of the night’ nocturnal Lepidoptera

What moths (and some butterflies) get up to under cover of the dark

Most of us know that moths and butterflies are from the same insect order (Lepidoptera). The differences between the two have been discussed many times… every rule has many exceptions, making it hard to pin down exactly what those differences are. A commonly used distinction is that butterflies are day-flying (diurnal) and moths are nocturnal. This is a fairly easy one to shoot down because there are many, many diurnal moths. Less well known are the nocturnal butterflies.

Whether they are moths or butterflies, the behaviour of nocturnal species is not easy to observe. That’s a consequence of their antics happening under cover of darkness. Our senses are tuned to daylight; perceiving (and hence observing) nocturnal behaviour does not come easily to us. There is a lot going on, and some of it is only recently discovered, via the use of newly developed technology.

Night butterflies

To begin with, let’s look at some nocturnal, or to be more accurate, crepuscular (dusk- and dawn-active) butterflies. Kloof has several of these; they are among our most striking insects.

The Bush Night-fighter, Artitropa erinnys erinnys, is the most brightly coloured of our ‘night butterflies’.

Bush Night-fighter larvae feed on Dracaena aletriformis, a common plant in our area.

‘Skipper’ butterflies in the family Hesperiidae are often seen flying on dull, cloudy days. A few of them take this to extremes and are full-on nocturnal insects. They are sometimes seen at dusk, but they fly at night and even come to lights, just like moths do. Like most Skippers, the Bush Night-fighter larva creates a shelter from a leaf of its host plant, and lives inside it, coming out to feed.

Female Strelitzia Night-fighter, Moltena fiara, resting on a leaf of her host plant, Strelitzia nicolae.

Strelitzia Night-fighter larva inside its shelter made of a folded-over leaf.

The Strelitzia Night-fighter is much more ‘moth-like’ than the Bush Night-fighter and is even harder to spot as it flits around the host plant in the darkness. Fortunately for the photographer, the larvae of both species are easy to find and rear in captivity. I have tried many times to shoot them in flight around my Strelitzia plants. The only image I ever managed to capture was on a high-end smartphone’s ‘ultra slo-mo’ video app. This was in such dim light that the visual noise rendered the video unusable.

Common Evening Brown, Melanitis leda, female. The inset arrow shows the ear (the tiny opening at the base of the forewing costal vein).

This butterfly really IS common; it’s found all over the Indian Ocean rim from Cape Town to Australia. If you go into the thick forest anywhere on the East Coast (including here in Kloof), you are likely to flush one of these sleeping beauties. They are mostly active at night but are very wary and wake up at the slightest sound. Why? Because they can hear!

Many butterflies in the Nymphalidae (brush-foots) have tiny but effective ears situated in their ‘armpit’ where the forewing meets the body on the underside. Charaxes and Browns have them. It’s the reason why they can detect movement so quickly, even in dim light or darkness. The sound frequency they can detect is in the same range as ours.

Black-haired Bush Brown, Bicyclus safitza safitza, male.

These little butterflies can often be seen skulking around in the shade of trees; they prefer to fly in dull weather, or even dusk. They also have ears at the base of the forewing underside. The swollen veins at the leading edge are connected to these, and probably function as sound amplifying devices – rather like bats’ ears.

And talking of bats…

The ‘arms race’ between bats and moths

Talking back

Chiroptera, the insect-eating bats, are well-adapted to hunting nocturnal insects – their main prey. They are equipped with sonar; their voices generate chirps that ‘paint’ an insect in flight in the same way that a radar system ‘paints’ an incoming missile or fighter jet with radio waves. The bat uses the sound reflection from the prey by using its finely tuned ears to interpret the echoes via neural pathways in the brain. By emitting chirps at an increasing rate and sensing how the time lags between chirp and echo are changing with time, the bat knows how big the prey is, how close it is, how fast it’s moving, and in what direction. How, I hear you ask, can a moth escape that?

As far back as 1877 it was suspected that moths can detect bat sonar, and take evasive action as a result, from observing behaviour. If you watch moths around a tall light pole where bats can be seen hunting them, you can see them taking evasive action – but is it by sight or by hearing? Hearing was covered in this article.

In it, I touched briefly upon the two-way aspect of communication channels between moths and bats. Scientists’ ability to accurately measure ultrasonic pulses has improved dramatically in recent years. We’ve known for a long time that insects can produce sound at frequencies audible to our ears. Crickets, cicadas, and grasshoppers come to mind, and these can be as deafening as the frog choruses we hear in local wetlands. As mentioned in my previous article, the Death’s-head Hawkmoth, Acherontia atropos, can produce audible sound through its proboscis. As recently as 1992 it was reported that other hawkmoth species can do this, and that males of several others could produce a sound by rubbing (stridulating rather like a cricket) parts of their genitals against the underside of the abdomen (I am not making this up)! These discoveries were made by the quite crude technique of catching the moth and holding it to the researcher’s ear, whilst holding its wings over its back and (gently!) squeezing its body to irritate it.

Oleander Hawkmoth Daphnis nerii is one of the species that produces an audible sound when handled roughly. It’s not a squeak like the sound made by a Death’s-head.

That early study found that many hawkmoths had genitalia structures that appeared to be stridulating even if they produced no audible sound. It was speculated that they might be emitting ultrasound. Now we know that they were – and the ultrasound is effective in jamming the bats’ sonar, preventing them from completing their ‘attack runs’ on the moths. To work, the moth needs to detect the bats’ pulses before the reflection has found its way back to the bat, so it has time to activate the jamming signal.

This behaviour has evolved independently at least seven, and possibly ten, times in the 65 million years that bats and moths have been in this ‘arms race’. Ears (tympanal organs) are found in hawkmoths (mouthparts), inchworms (Geometroidea) in the abdomen, tiger moths and their relatives (thorax), butterflies (wings) and others. Many, if not most, of these have evolved to have stridulating organs as well. Tiger moths can use two separate strategies to evade bats. They can jam the bats’ sonar like the hawkmoths do or emit clicks that advertise the fact that they taste nasty and might make the bat ill. It works rather like the bright colouring on diurnal tiger moths: ‘aposematic’ patterning that tells a predator this is something to avoid. This is usually learned behaviour – the bird or lizard must experience the bad taste first and learn to associate it with a nasty experience.

Apart from the hawkmoths’ noisy genitals, many moths have ‘tymbals’ like cicadas have – in the thorax in tiger moths, for example. These are pleated membranes that emit a sound when buckled by muscular action. There is evidence that as many as 50% of all nocturnal lepidoptera have evolved some kind of anti-bat defence.

Most of these work in the 20-50 kHz ultrasound band, and the moths emit sounds in response to bat sonar in the same range. Other insects, like beetles, mantids, crickets, katydids, lacewings, and locusts, do the same thing. Interestingly, some moths that are deaf, like the ermines, have evolved wing tymbals, which produce sound bats can hear, situated on their hindwings, near the base. They appear as scale-free patches at the base of the wing.

Dotted Lesser-Ermine Ethmia coscineutis

Photo: Hermann Staude

A tiny moth that can emit sound in the same range as bat hearing is the Dotted Lesser-Ermine.

Renowned lepidopterist David Agassiz recording call of Dotted Lesser-Ermine

Photo: Hermann Staude

The Dotted Lesser-Ermine is one moth whose ultrasound call has been recorded, as shown here. The sonogram is visible on the laptop screen – very similar to what has been used to record bats’ sonar clicks. It has yet another mode of sound generation, this time from its forewings.

Small Ermine, Yponomeuta africanus

Photo: Hermann Staude

Mounted specimen of Small Ermine, Yponomeuta africanus

Photo: David Agassiz

Small Ermine has also been recorded using ultrasound, which is produced by the hindwing buckling under action of the flight muscles. When the hyaline (clear, scale-free) patch is twisted and buckled, tiny pleats in its surface produce an effect rather like when rigid plastic film is crumpled by hand – but much more accurately, and with directional effect!

Research has shown that these tiny moths are as bad tasting as a tiger moth; their larvae feed on plants that contain similar toxins, which are metabolised, and persist into adulthood. Their sounds closely mimic tiger moth clicks, even though they are created using different mechanisms.

Baur’s Frother, a distasteful tiger moth

Baur’s Frother showing its aposematic colouring and acrid defensive secretion

Baur’s Frother Is clearly an unpleasant mouthful. I can attest to this, since the specimen in the photo was found resting on a wall in the Kloof Village Mall when it was acting as a huge moth trap a few years ago. I got some of that froth on my finger and I smelled it. It was bitter and nasty! These moths are as likely to be found in the day as at night, so although they haven’t yet been tested for ultrasound emissions, it’s likely that they emit aposematic clicks like the many tiger moths that are known to do so.

The calls of Small Ermines have been compared with those of tiger moths, and the sound signatures are identical. In ‘Müllerian mimicry’, distasteful creatures amplify their deterrent effects on predators by resembling one another. Indeed, some smaller tiger moths (albeit larger than the Small Ermine) have the same black-spots-on-a-white-ground pattern and are also called Ermines (showing how confusing ‘common names’ are!). However, at night the visual deterrence falls away, and sound does the job that bright colours do.

African Plain Tiger Danaus chrysippus orientis

White-barred Telchinia Telchinia encedon

Many butterflies exhibit visual Müllerian mimicry, like the African Plain Tiger Danaus chrysippus orientis and the White-barred Telchinia Telchinia encedon. Many diurnal moths have similar patterns as well. Some moths fly both at night and by day and have both defence mechanisms.

‘Stealth clothing’

There’s another aspect to moths, that is a different kind of superpower. Have you ever wondered why so many moths are very furry?

If you have deep carpets and thick curtains in your house, have you noticed that sounds are muffled compared to a room with bare floors and windows? Soft furnishings absorb sounds. Owls’ feathers have soft edges so their prey can’t hear their wingbeats.

The latest warplanes have coatings that absorb radar pulses and are so shaped that they don’t reflect radio waves, preventing an enemy radar from ‘painting’ them. That way, they don’t show up on screens or trigger the planes’ automatic attack manoeuvring and weapons targeting.

Sonar works like radar; submarines and fishing vessels use under water it to detect reflected images of enemy vessels, or schools of fish. Bats use it to detect aerial prey. They have fine-tuned sonar pulse generators on their faces, and ears that can pick up surface detail we can only guess at. Remember this has had 65 million years to evolve into near perfection. But bats don’t have it all their own way. Recent research is indicating that the scales on moths’ bodies form a kind of stealth shield that absorbs the sonar pulses from bats or reflects the image of something that isn’t a moth – like a bit of leaf or bark. Many moth body scales look like hairs, but they have a similar attachment mechanism to wing scales.

We know that butterflies have multiple layers of scales on their wings. Often the top layer is brightly coloured, with duller ones underneath. The top picture here is a freshly emerged Natal Russet, Aloeides penningtoni, a butterfly often seen in local grasslands. The bottom one has been around a few days, and in flight, its rapid wingbeats have knocked off the top layer of scales. It’s not the colour that’s faded, it’s an underlayer with duller red colouring and less of the silvery markings. We’re not sure if this is because the fresh ones must attract the opposite sex, so they need to be conspicuous, but as they age, the colour might attract predators…

This Venus Turntail moth Caligatus angasii has wonderful daytime camouflage. To us, the camouflage patterning is beautiful, but to a bird it looks like a… bird dropping. It isn’t hard to imagine that those elaborate gauntlet shapes on the legs, and the soft, multi-layered scales around the head, are there to confuse a bat’s sonar. The wing scales have a ‘ripple’ pattern at a micro scale, that is just about visible on this photo. Those patterns will break up and absorb a sonar pulse in the same way that a fighter plane’s radar absorbing paint will prevent detectable reflections.

This Cabbage-tree Emperor moth, Bunaea alcinoe, has an effective deterrent to diurnal predators that get past its cryptic wing markings. Those lurid orange eyes are so like the real eyes of an Eagle Owl that any mouse would run a mile when the moth lifts its forewings to reveal them.

If you look closely at the wing scales of a Cabbage-tree Emperor, you can see multiple layers of hair-like scales overlying corrugated ones. These will absorb bat sonar chirps, and any reflection will be scattered in many directions by the corrugations and will appear blurry and indistinct to a bat’s sonar detectors (ears).

Night flower pollination

White flowers with heady scents often open only at night, and these are usually moth pollinated.

The long spurs on these Mystacidium orchid flowers contain the nectar hawkmoths need to fuel their high energy flight. The moths have very long tongues (proboscis) that allow them to get all the way to the bottom of the spurs to suck up the nectar. This forces the moth’s head to contact the anthers and pick up pollinaria, which they carry to the next flower in search of food, which allows the plant to reproduce. The whiteness of the flowers helps the moths to locate them in the dark of the night. Their scent is a ‘broad’ attractant that draws the moths in, but they need to see them to get their tongues into the nectaries.

This Natal Sphinx moth, Macropoliana natalensis, shows two adaptations to night life. It has dense hair-like scales over the body, wings, and legs, to absorb bat sonar and not provide reflections the bats could use to ‘see’ them. The huge compound eyes of hawkmoths can see in very dimly lit conditions – well enough to see a white orchid flower. The moth can see well enough to aim its proboscis down the spur (whose entrance is small). It’s been shown that these moths can follow the movements of these flowers in quite strong air currents well enough to aim the tongue accurately.

Oleander Hawkmoth and Accented Hawk, Nephele accentifera

Both hawkmoths show the large, coiled proboscis between the ‘labial palpi’. These are sensory organs on the head, shielding the proboscis. They cover typanal organs that assist the moth in detecting bat calls, as well as carrying scent detectors.

Convolvulus Hawkmoth, Agrius convolvuli, nectaring on a Baobab flower in Botswana

Photo: Leigh Kemp

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.