The Moth That Flies Like a Hummingbird

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Hummingbird hawk-moth (Macroglossum stellatarum) visiting a thistle. Video: gifer.com

One of the more curious insects you’re likely to run into as a gardener is the hawk moth, also called sphinx moth, a member of the Sphingidae family. There are over 1400 species found all over the world—big ones, medium ones, large ones—and what impresses most about them is their unique way of flying. Unlike the lazy movement of so many moths, they beat their narrow wings so rapidly, some up to 85 beats per second, that all you can see is a whir, often accompanied by a humming sound. This allows them to hover in the air, move forwards and backwards and travel at great speeds. Some are mistaken for hummingbirds! 

Hawk moth slowed down so you can see its movement. Photo: giphy.com

This hovering capability is only known to have evolved only four times in the animal world, always in nectar feeders: in hummingbirds, certain bats, hoverflies and hawk moths. It’s an example of convergent evolution. 

Hovering requires a huge amount of energy and indeed, hawk moths are heavy feeders, attracted to a specific sugary, energy-rich food: flower nectar. By hovering, they can gain access to flowers not available to other insects, often using a proboscis as long as or longer than their body to suck up the sweet liquid. 

Flowering tobacco (Nicotiana sylvestris) flowers droop scentlessly during the day, but rise up and give off a powerful perfume at night to attract their pollinator, the hawk moth. Photo: longwoodgardens.org

Some hawk moths are nocturnal or crepuscular, but others fly during the day, wowing gardeners with their acrobatics. For many plants, they’re important pollinators: many night-blooming plants give off heady aromas just to attract the nocturnal and crepuscular species. Think of flowering tobacco (Nicotiana sylvestris), a fairly popular annual, with its white, tubular blooms, wonderfully scented at night, but floppy and scentless during the day. 

The extraordinarily long spur of Darwin’s orchid means it can only be pollinated by a specific hawk moth. Photo: http://www.petrensorchidshop.eu

Indeed, for some plants, hawk moths are the only pollinator. The best-known case is that of Darwin’s orchid (Angraecum sesquipedale), whose white flower has a spur nearly 18 inches (45 cm) long that can only be pollinated by a specific hawk moth (Xanthopan morganii) with an equally long proboscis. The flower was discovered first, but Darwin accurately predicted that a corresponding moth would be discovered. You can read more about Darwin’s orchid and its pollinator here.

Behind Every Moth…

Yes, while the hawk moth may fascinate you and you certainly have nothing against its pollinating activities, you may have more trouble putting up with the caterpillar it arises from. Different species have different host plants (most are quite specific to certain plant species or families), but they all eat plants of some sort. 

Tomato hornworm (Manduca quinquemaculata): note the spike at the tip. Amanda Hill, Wikimedia Commons

The five-spotted hawk moth (Manduca quinquemaculata), for example, prefers tomatoes. Its larva is called the tomato hornworm. (Many hawk moth caterpillars have a spike at their tip and are thus called hornworms.) 

Eating people’s plants is not considered genteel behavior from a gardener’s point of view, so any hawk moth caterpillars spotted on a favorite plant (there is rarely more than one per plant) can be handpicked and disposed of or moved to a related plant of lesser value. 

Hawk moth caterpillars tend to be smooth and hairless and come in camouflage shades of green or brown. Some larvae are said to mimic snakes as a protective strategy. When resting, the caterpillar often bends its head down into a praying position, like the Great Sphinx of Giza, as in the photo above, whence the name “sphinx moth.” 

The caterpillars have a propensity for plants that are poisonous to other caterpillars, chewing on the soft new growth. Most then excrete the toxin, although some species retain it and become poisonous to their predators.


Hawk moths: fascinating creatures, aren’t they?

Darwin’s Orchid

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Darwin’s orchid (Angraecum sesquipedale) with its long spur.

I just saw, for the first time in my life, Darwin’s orchid (Angraecum sesquipedale). That was at the annual show of my local orchid society (Les Orchidophiles de Québec), held on April 8 and 9, 2017. It was labelled, but it didn’t need to be. I’ve known about this orchid since I was a teen and avidly read everything I could find about evolution. I recognized it immediately. And how could you fail to notice it? It’s has large white flowers… but more importantly, it’s unique in that it has a long downward-hanging green spur that projects behind the flower.

It’s certainly attractive enough, but its history is even more fascinating.

A Surprising Discovery

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Louis-Marie Aubert of Petit-Thouars. Illustration: Wikimedia Commons

In 1798, the French botanist Louis-Marie Aubert of Petit-Thouars discovered in Madagascar an orchid with large white flowers bearing a very long spur. He only named the plant 25 years later, in 1822, calling it Aeranthes sesquipedalis, later changed to Angraecum sesquipedale. Angraecum comes from the Malay word for orchid, “angrek”, while sesquipedale refers to the very long spur and means “a foot and a half” (45 cm). Indeed, the spur ranges from 10.6–16.9 in (27 to 43 cm), just short of 1 ½ feet (45 cm). If you include the distance from the tip of the spur to the tip of the flower’s lip from which it extends, that would indeed make “a foot and a half”. It it by far the longest spur found on any flower.

Darwin Makes A Prediction

Charles Darwin in 1881

Charles Darwin studied and wrote about

In 1862, a British orchid enthusiast James Bateman sent Charles Darwin, the famous scientist and father of modern biology, blooming specimens of this orchid to Charles Darwin… and he was immediately fascinated.

You see, in flowers, such a spur, also called a nectary or a nectar spur, is a projection of the flower that contains a sugary liquid called nectar… and a flower only produces nectar to attract a pollinator. Thus an insect or other pollinator needs to have some means of reaching into the spur to suck up the nectar and, in so doing, picks up pollen which, one hopes, it will then deposit when it visits another flower of the same species.

But only insects with a long proboscis, a bird with a long beak or an animal with a long tongue have access to flower spurs. That way the plant ensures a certain fidelity in its pollination: it pre-selects pollinators, eliminating those less likely to ensure its fecundation.

But the extremely long spur of Angraecum sesquipedale only contains nectar in the last few centimeters of its extremity. Why was nectar placed so far from the flower’s anthers and stigma, where pollination takes place?

Darwin guessed almost immediately. He quickly began dissecting flowers and testing the mechanism necessary to reach the nectar and release the pollen. He soon announced that it was now certain that in Madagascar there had to be a moth with a proboscis ten to eleven inches (25 to 28 cm) in length and which would then be able to reach the nectar at the base of the spur. If he predicted it would be a moth, that’s because the flower was white and gives off an intense scent only at night, a combination of traits only moths find appealing.

The prediction is greeted with much skepticism by the scientific community of the time. But it turns out Darwin was correct.

The Prediction Comes True

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Darwin’s sphinx moth (Xanthopan morgani praedicta). Photo: Esculapio, Wikimedia Commons

In 1903, 40 years later, and well after the death of Darwin in 1882, such a moth was indeed identified. Darwin’s sphinx moth (Xanthopan morgani praedicta, now Xanthopan morganii) has a long proboscis that it carries coiled. The sphinx first approaches the flower, attracted by its odor, then once it has inspected it, backs up about a foot (30 cm), extends its proboscis, then inserts it into the spur as far as it will go. It’s a bit of a struggle, so it pushes hard against the flower, thus inadvertently picks up pollinia (masses of pollen) that it will later carry to another Darwin orchid, ensuring its fecundation.

Interestingly, even in 1903, the relationship could only be postulated: no one had yet actually seen a Darwin’s sphinx moth pollinate an Angraecum sesquipedale flower. That didn’t happen until… 1992, nearly a century later!

Darwin’s sphinx moth is the only pollinator of what is now called Darwin’s orchid. The two are believed to have co-evolved, each adapting to the shape of the other to ensure a perfect symbiosis.20170412A

Isn’t nature marvelous?20170412A