Science

The Many Fascinating Secrets of Moths

Modern-day scientists are investigating moths’ behaviours to glean insights about improving some aspects of human life.

Moths on a screen. Credit: Geetha Iyer

Moths on a screen. Credit: Geetha Iyer

…our attitude towards life
is come easy go easy
we are like human beings
used to be before they became
too civilised to enjoy themselves…

The Lesson of the Moth, The Lives and Times of Archy and Mehitabel, Don Marquis p. 96

Since 2012, a citizens’ science initiative called Moth Week has spread around the world. It is observed in the last week of July, when many enthusiastic ‘moth-ers’ gather at the edge of forests and woodlands, within campuses, in and around villages and, often, simply in their backyards, and watch out for moths throughout the night. Moth-watching is simple. Just put up a white screen (like a bedsheet or a dhoti), light a bulb to cast a bright white light on it and kick back with a beverage.

Sufi literature is replete with moth symbolism. The insects’ affinity for the flame fired the imagination of the poets, who used them often in poetry, panegyric and parody – but this isn’t all that makes them beautiful. Modern-day scientists are investigating moths’ behaviours to glean insights about improving some aspects of human life. For example, many moths from the Saturnidae and Erebidae families do not feed as adults. The energy they require – to transform from caterpillar to pupa, then to emerge as a full-fledged moth and finally to find a mate and lay their eggs – comes solely from what they eat at the caterpillar stage.

Tiger moth. Credit: Geetha Iyer

Tiger moth. Credit: Geetha Iyer

While it has continued to remain an enigma, hawk moths are an aerodynamic group of moths that has attracted the attention of researchers.

Eran Levin, at the Tel Aviv University, and a team of other researchers made an interesting observation in February 2017 when studying hawkmoths that were feeding on nectar. These moths are nectar-feeding insects and pollinate night-blooming flowers. When feeding on the nectar using their proboscis, the moths have to hover next to the flower, and such hovering requires a lot of energy. As a result, the moths’ rate of respiration during hovering is high, leading to the production of free radicals. Free radicals are atoms or groups of atoms that are formed when oxygen reacts with certain molecules. They are highly reactive and, within a living body, can damage DNA and destroy cell membranes. To counter this, hawkmoths need antioxidants. Where can they find it? Not in the nectar (it has no antioxidants) or, for that, from any of the food that it consumes. Instead, Levin & co. found that nectar-feeding hawkmoths subverted their metabolic pathway to oxidise sugars from the nectar to their advantage.

An emerald moth of the family Geometridae. Credit: Geetha Iyer

An emerald moth of the family Geometridae. Credit: Geetha Iyer

Living things that respire aerobically – i.e. using oxygen – use the Krebs cycle to oxidise sugar to release energy (the Krebs cycle is a set of chemical reactions that help release energy from glucose during respiration). But the hawkmoths were found using an alternate cycle, called the pentose phosphate pathway, for oxidising the sugar-rich nectar. This pathway came into play as soon as the moths stop flapping their wings. And in addition to energy, this pathway produced the antioxidants necessary to fight off the free radicals. Effectively, the moths had found a way out of their predicament without looking for other sources of food.

A hawkmoth. Credit: Geetha Iyer

A hawkmoth. Credit: Geetha Iyer

Evolutionary biologists like to quote the echolocation example to explain the coevolution of species. Every species has its own way of avoiding predation and increasing self-preservation, and moths are particularly good at it. They can detect the presence of a bat from a distance ten-times that at which the bat can sense the moths. This is because, while the ears of a bat are able to sense only a part of the echo of the sounds they emit (20-60 kHz), the ears of a moth can detect a larger portion. According to a 2014 study, a moth can detect a bat at a distance of 20-100 m whereas a bat can detect a moth at 1-10 m.

The moths’ ears host an organ called the tympanal organ. It consists of membranes that stretch above a volume of air and a clump of sensory cells located below this volume.   The ears of moths have evolved multiple times, and their location and structure on the body of the moth varies widely. In the superfamily of Noctuidae, the ear is generally made of two sensory cells and can be located in the meta thorax (the thorax is the part of the insect below the head and made up of three segments; the third segment is the meta thorax). In geometric, pyralid and drepanid moths, the ears are found at the base of the segments of the abdomen and are made of four sensory cells. In hawkmoths, the ears are placed on the proboscis. But in spite of such differences, all moths can tune in to the 20-60 kHz range. (The larger moths are also more tuned to lower frequencies than are smaller moths.)

Some species of tiger moths possess sound-producing organs called tymbals. They look like tiny blisters and are formed from parts of the cuticle of the meta thorax. Both male and female tiger moths use these tymbals to produce a sharp, clicking sound when they sense bats in the vicinity, to alert others of their kind. According to the 2014 study cited above, these sounds could also be to startle the bats or jam their frequencies. Jamming is a moth’s way of disrupting the cries that bats produce to echolocate the moths. The frequency of a bat’s cry increases as it approaches the moths. In return, the moths produce bursts of clicks to disrupt the bats’ sounds and escape death.

A moth of the Noctuoidae superfamily. Credit: Geetha Iyer

A moth of the Noctuoidae superfamily. Credit: Geetha Iyer

The sound-producing organs in the moths of the Noctuoidae superfamily are also used for communicating amongst themselves. This sound is sometimes strident, sometimes like a percussion and at others like a loud and thick click. In many of the other families of moths, the sound is produced largely by the male of the species. In pyralid moths, the males also ‘sing’ courtship songs as they near potential mates. Unfortunately, the female noctuid moth cannot distinguish between the songs of rivalling males. Overall, their courtship behaviours are not fully understood because moths are nocturnal creatures, and their rituals are harder to observe in the dark. So there is still much work that needs to be done to  understand the significance of moth acoustics.

And in the same vein, there are many secrets the moths are yet to reveal. Apart from scientists, interested citizens could also play an important role in uncovering these secrets from wherever they are – a context in which Moths Week assumes further significance.