Xanthopan praedicta, also known as Darwin's hawkmoth, is a favorite among entomologists for its extraordinary proboscis (mouthpart), and among biologists in general for being predicted by Charles Darwin. Now it turns out the hawkmoth is even more remarkable than anyone realized, having the capacity to jam the sonar of the bats that prey on it. Advantageous as this is, it traps the moth in two constant races for survival.
Bats use sonar to find their way at night, and insectivorous bats have refined this capacity to be able to detect prey in flight with superb precision. Some moths have found ways to interfere with the sonar, and in the journal Biotropica, University of Florida PhD student Juliette Rubin has shown this extends to the remarkable X. praedicta when it hears bat sonar or is physically handled.
In 1862, Darwin was sent a package of orchids, including a specimen of Angraecum sesquipedale from Madagascar. A. sesquipedale is notable for what Darwin called its “astonishing length”, with nectar buried almost a foot (0.3 meters) deep. In words he might have written differently today, Darwin added: “Good Heavens what insect can suck it?"
Darwin predicted Madagascar must host a moth with a foot-long (0.3 meter) proboscis. Alfred Wallace supported Darwin's claim, and predicted it would be a relative of Xanthopan morganii, an African moth with the then longest known proboscis.
Wallace lived to see the discovery of a Madagascan moth with the required equipment for A. sesquipedale fertilization. The find was originally considered to be a subspecies of X. morganii, and named Xanthopan morganii praedicta. However, in recent years some entomologists have argued it is a species in its own right, known as Xanthopan praedicta.
Rubin uses that nomenclature, but her focus is on the moth's sonar evasion powers. Last year she was co-author of a preprint reporting that around 20 percent of moth genera have found ways to jam bat sonar using high-density ultrasound, causing bats to misjudge their location.
It takes a large moth to hold such a proboscis, even if it is curled up when not in use, and that makes a tempting meal for bats. Rubin has now shown that male moths deal with this threat by issuing the longest ultrasound burst yet reported from a moth. Rubin thinks this not only jams the bats sonar, but deters other predators, such as lemurs. The females, and two related species, do not produce ultrasound responses.
Naturally, the bats do not take this lying down (nor hanging down) and are constantly evolving sonar less suited to being jammed, creating an evolutionary arms race.
The moths and bats are of course enemies, so it makes sense they would compete like this. However, the orchids need the moths to reproduce, raising the question of why they have made it so difficult.
The answer is thought to be that if any old insect can pollinate a particular flower, they can also pollinate other flowers with equal ease. A generalist is unlikely to go straight from one flower to another member of the same species and will shed the pollen it collects on others with no reproductive value. Some plants deal with this by flowering at unusual times of the year, but there are only so many spots in the calendar.
A. sesquipedale instead has a flower so deep only a specialist can reach it. A moth with a foot-long (0.3 meter) proboscis doesn't want to waste its mighty organ on just any flower, and mostly goes from one A. sesquipedale to another, spreading the pollen where it matters in the process and maintaining the species. However, to ensure X. praedicta's loyalty, A. sesquipedale has had to keep lengthening its flower, forcing the moth to keep up.
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