In his 1879 account of wanderings in the East, the travel writer James Hingston describes how he was treated to a bizarre experience in West Java:
I am guided through his garden by my kind host and shown, among other things, a flower, a red orchid, which catches and feeds live flies. While I was present, he grabbed a butterfly and closed it in its beautiful but deadly leaves, as a spider would have wrapped it in a 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 an orchid mantis and was deceived by it, Hymenopus coronatuswhich is not a plant but an insect.
We have known orchid mantises for over 100 years. Famous naturalists such as Alfred Russell Wallace have speculated about their extraordinary appearance. The orchid mantis eschews the dull green or brown of most mantises and is resplendent in white and pink. The upper parts of its legs are very flattened and heart-shaped, and look eerie like flower petals. On a leaf it would be very noticeable, but when you’re on a flower it’s extremely hard to see. In photographs, the praying mantis is often shown in or next to a flower, challenging the reader to recognize it.
Hide in plain sight?
At first glance, this is a classic evolution story and a ready case: the praying mantis evolved to mimic the flower as a form of cryptis, allowing it to hide among its petals and feed on insects it attracts . through the flower. Cryptic mimicry by predators is well known. For example, crab spiders camouflage against a flower and can change from yellow to white to match their host flower. The orchid mantis is sort of a poster child for such cryptic mimicry. This evolutionary story is so blatantly true that it is often discussed as an established fact today.
No one seemed to have noticed that there was no evidence to support this hypothesis. Orchid mantises are actually very rare in the field, so their behavior is only known from captivity. For example, no one knows exactly which flower to mimic the praying mantis.
Now a series of new studies by James O’Hanlon and colleagues make it very clear that we have been wrong all along. While it is indeed a flower mimic – the first known animal to do so – the orchid mantis does not hide inside an orchid. It doesn’t hide at all. And to an insect, it doesn’t even look like an orchid.
A deadly attraction
O’Hanlon and colleagues began systematically testing the ideas behind the traditional view of the orchid mantis’ method. First, they tested whether praying mantises actually camouflage themselves between flowers, or whether they attract insects themselves. For a flower-seeking insect, as predicted, the color pattern of the praying mantis is indistinguishable from most common flowers. However, when combined with the most common flower in their habitat, insects approached mantises more often than flowers, showing that praying mantises are attractive to insects in their own right. The camouflage does not rely on them hiding near flowers.
“We can clearly see insects, such as bees, deviating from their flight paths and flying straight at this rogue predator,” O’Hanlon told me. “These beasts are great for questions like this because we can observe a dynamic interaction between predators and prey.”
This phenomenon, known as aggressive mimicry, also occurs in other animals. The Bolas spider releases chemicals that imitate sex pheromones released by female moths looking for a mate. Male moths, with their elaborately feathered antennae, can detect these pheromones from miles away and are lured to their deaths. Carnivorous phototuris firefly females can mimic the flash responses of another firefly species, attracting amorous males to the menu.
The researchers also assessed where praying mantises chose to perch and found that they chose not to hide among the flowers. They chose leaves just as often. Sitting near however, flowers brought benefits, as insects were attracted to the general environment – a ‘magnet effect’.
Any old flower
When they compared the shape and color of the praying mantis to flowers as they would be viewed from an insect’s perspective, the predator did not resemble an orchid or any particular type of flower, but rather a “common” flower. This is consistent with what we already know: some of nature’s best mimics are imperfect mimics with features of several “model” species. By placing experimental plastic models in the field, the researchers discovered that the color of the praying mantis was much more important than its shape in attracting insects. They conclude that praying mantises may not exactly mimic a particular type of flower. Instead, they can exploit a loophole created by a cognitive efficiency in the insect brain.
As humans with giant, hyperdeveloped brains capable of abstract thinking, we have the luxury of being able to make decisions based on all available information. After a few seconds of close examination, what initially looks like a flower due to its color starts to look suspicious – and once we see bug eyes and a vague bug-like outline, it’s game over: it’s a praying mantis.
But a small insect flying around on the go, with its compact brain, cannot afford such cognitive extravagance. It has a shortcut – a rule of thumb: anything matching color X is probably a flower that contains nectar. More color equals bigger flower, with potentially more nectar. No cross-checks, no 2-Step Verification. The praying mantis takes advantage of this shortcut by using “sensory exploitation”. It is a concentrated mass of the correct color – a supernormal stimulus. The insect classifies the mantis as a giant nectar-filled flower and approaches to examine it.
“This work is amazing,” said Martin Stevens of Exeter University, an animal deception and mimicry expert not connected to the work. “It’s great to see something that Wallace and others discussed so long ago has finally been experimentally tested.”
This isn’t the first species to lure prey with sensory exploitation. The white crab spider is highly reflective in the UV, making it very noticeable to wandering insects. But the spider still “hides” on a flower, relying on its attraction. The UV reflection makes a flower with a crab spider look more beautiful than one without, but the flower is still needed. The orchid mantis is the first animal ever shown to mimic an entire flower and attract insects itself.
There’s one more problem, though: If the praying mantis can only attract insects through sensory exploitation, Stevens muses, “Why have body parts that look like petals? My guess is that the pollinators are initially attracted from a distance by sensory exploitation, but then more accurate mimicry occurs up close, when the insects can more closely inspect the mantis for what it is.”
Greg Holwell, who co-authored the study, told me, “What this work really highlights is that working on a completely unstudied species can produce fascinating results. Going out and starting with a solid natural history helps to generate hypotheses that you can then test with field experiments, and can lead to the discovery of completely new phenomena.”
“While important discoveries are made through laboratory research on model species such as fruit flies, each species has an exciting story to tell and can help shape our understanding of how the natural world works.”
Behavioral Ecology2015. DOI: 10.1093/beheco/aru179 (About DOIs).
This article was originally published on The Conversation.