In his 1879 account of wanderings in the Orient, the travel writer James Hingston describes how, in West Java, he was treated to a bizarre experience:
I am taken by my kind host around his garden, and shown, among other things, a flower, a red orchid, that catches and feeds upon live flies. It seized upon a butterfly while I was present, and enclosed it in its pretty but deadly leaves, as a spider would have enveloped it in network.
What Hingston had seen was not a carnivorous orchid, as he thought. But the reality is no less weird or fascinating. He had seen – and been fooled by – an orchid mantis, Hymenopus coronatus, not a plant but an insect.
We have known about orchid mantises for more than 100 years. Famous naturalists such as Alfred Russell Wallace have speculated about their extraordinary appearance. Eschewing the drab green or brown of most mantises, the orchid mantis is resplendent in white and pink. The upper parts of its legs are greatly flattened and are heart-shaped, looking uncannily like petals. On a leaf it would be highly conspicuous – but when sitting on a flower, it is extremely hard to see. In photos, the mantis appears in or next to a flower, challenging the reader to spot it.
Hiding in plain sight?
On the face of it, this is a classic evolutionary story, and a cut-and-dried case: the mantis has evolved to mimic the flower as a form of crypsis – enabling it to hide among its petals, feeding upon insects that are attracted by the flower. Cryptic mimicry by predators is well known. For example, crab spiders camouflage themselves against a flower, and can change from yellow to white to match their host flower.
The orchid mantis is something of a poster child for such cryptic mimicry. So obviously true is this evolutionary story that it is often discussed today as established fact.
No one seemed to have noticed that there has been no evidence to support this hypothesis. Orchid mantises are actually very rare in the field, so their behaviour is hardly known about, except in captivity. For example, nobody knows exactly which flower the mantis is supposed to mimic.
Now a set of new studies by James O'Hanlon and colleagues shows quite clearly that we’ve been getting it wrong all this time. While it is indeed a flower mimic – the first known animal to do this – the orchid mantis doesn’t hide in an orchid. It doesn’t hide at all. And to an insect, it doesn’t even look particularly like an orchid.
A deadly lure
O'Hanlon and colleagues set about systematically testing the ideas contained within the traditional view of the orchid mantis’ modus operandi. First, they tested whether mantises actually camouflage amongst flowers, or, alternatively, attract insects on their own. For a flower-seeking insect, as predicted, the mantis’ colour pattern is indistinguishable from most common flowers.
However, when paired alongside the most common flower in their habitat, insects approached mantises more often than flowers, showing that mantises are attractive to insects by themselves, rather than simply camouflaging among the flowers.
“We can clearly observe insects, like bees, diverging from their flight paths and flying right towards this deceptive predator,” O’Hanlon told me. “These beasties are marvellous for this kind of question, because we can observe a dynamic interaction between predators and prey.”
This phenomenon, known as aggressive mimicry, occurs in other animals. The Bolas spider releases chemicals that imitate sex pheromones released by female moths seeking a mate. Male moths, with their elaborately plumed antennae, can detect these pheromones from miles away, and are lured in to their death. Carnivorous Photuris firefly females can mimic the flash-responses of a different species of firefly, attracting amorous males who find themselves on the menu.
Next the researchers assessed where mantises chose to sit. Surprisingly mantises did not choose to hide among the flowers. They chose leaves just as often. Sitting near flowers did bring benefits, though, because insects were attracted to the general vicinity – the “magnet effect”.
Any old flower
When they compared the mantis’s shape and colour with flowers from an insect’s perspective, the predator did not resemble an orchid or indeed any particular species of flower, but rather a “generalised” flower. This fits with what we already know: some of the best mimics in nature are imperfect mimics with characteristics of several “model” species.
Placing experimental plastic models out in the field, the researchers found that mantis colour was much more important than shape in attracting insects. They believe that mantises may not actually precisely mimic a particular kind of flower. Instead they may exploit a loophole created by evolutionary efficiency savings within the insect brain.
Jumping to conclusions
As humans with giant, hyper-developed brains capable of abstract thought, we have the luxury of being able to make decisions using all the available information. After a few seconds of scrutiny, what initially looks like a flower because of its colour begins to look suspicious – and once we spy bug eyes and a vaguely insectoid outline, the game is up: it’s a mantis.
But a tiny insect zipping around on the move, with its compact brain, cannot afford such cognitive extravagance. It has a shortcut – a rule of thumb: anything matching colour X is a nectar-containing flower. More colour equals bigger flower, with potentially more nectar. No cross-checks, no two-step authentication. The mantis takes advantage of this shortcut by using “sensory exploitation”. It is a concentrated mass of the right colour - a supernormal stimulus. The insect classifies the mantis as a giant nectar-filled flower and approaches to investigate – to its doom.
“This work is terrific,” Martin Stevens of Exeter University, an expert on animal deception and mimicry, unconnected with the work, told me. “It’s wonderful to see something that Wallace and others discussed so long ago, finally tested experimentally.”
This is not the first species to lure in prey with sensory exploitation. The white crab spider reflects strongly in the UV, making it highly conspicuous to wandering insects. But the spider still “hides” on a flower. Surprisingly a flower with a crab spider is more attractive than one without: the crab spider mimics UV-reflective floral patterns that guide insects to nectar. But the orchid mantis is the first animal ever shown to mimic an entire flower, attracting insects on its own.
There is one remaining issue, though: if the mantis can attract insects by sensory exploitation alone, muses Stevens, then: “Why have body parts that look like petals? My guess is that the pollinators are initially attracted at a distance through sensory exploitation, but then more accurate mimicry kicks in at close range, when the insects can inspect the mantis more closely for what it is.”
Greg Holwell, who coauthored the study, told me: “What this work really emphasises is that working on a completely unstudied species can produce fascinating results. Getting out there and starting with some solid natural history helps to generate hypotheses that you can subsequently test with field experiments, and can lead to the discovery of completely novel phenomena.”
“While important discoveries are made from laboratory research on model species like fruit flies, every species has an exciting story to tell and can help shape our understanding of how the natural world works.”