Serendipity plays a pretty important role in scientific advances. It was involved in the discovery of penicillin, microwaves and x-rays. And now, it seems a bunch of old moth scales can be added to that list.
Scientists drilling cores from lake sediments in Germany – hoping to learn about past ecosystems from ancient pollen grains – recently stumbled across a profusion of tiny scales from moth wings. This is significant as the sediments are a whopping 200 million years old, making it the earliest appearance of moths and butterflies in the fossil record.
The very first moths
Moths first evolved over 250 million years ago. At least, this is the best estimate (made using a suite of techniques including genomic sequencing, constructing evolutionary trees and radiocarbon-dating fossils).
Picture the history of the planet as a 24-hour clock. Modern humans evolve only a few seconds before midnight. But the first moths appear before 11pm.
So where did moths come from? Well, the ancestral group is long extinct but are thought to have lived in wet habitats. They gave rise to caddisflies, as well as moths.
The first moths were about the size of a grain of rice. They were probably found in ancient humid forests, where the caterpillars lived in damp soil or moss. The primitive adults did not sip nectar like their contemporary cousins but instead had mandibles (jaws) and fed on pollen grains or fern spores.
Scaling the heights
Due to their delicate wings and buoyant scales, butterflies and moths typically fossilise very poorly. In the German lake cores, Timo van Eldijk’s team found about 70 individuals scales. This raises the question of whether there was anything special about this particular area, or have the tiny scales simply been overlooked in the past? I suspect the latter.
Butterflies and moths are characterised by the scales that cover their wings. The name of the taxonomic order they belong to – Lepidoptera – derives from the Greek word for scale (‘lepido’) and wing (‘ptera’). These scales, which are essentially flattened hairs, distinguish moths and butterflies from their closest living relatives: caddisflies (Trichoptera = ‘hairy-winged’).
Scales are central to the success of Lepidoptera; the infinite medley of intricate patterns and colour combinations they can generate is likely to have facilitated the divergence of the c.200,000 described species of butterflies and moths alive today.
Scales provide the canvas that is used to develop elaborate camouflage tactics or, conversely, enable gaudy colouration to advertise their unpleasant taste to potential predators. Scales may also afford stealth to moths by absorbing sound thus rendering bat echolocation less effective. They have also been shown to improve aerodynamics in migratory butterflies. The scales that cover their bodies are also advantageous, providing a thick fur coat to keep flight muscles above the critical temperature required for flight (a rather toasty 30°c).
Lepidopterous scales often contain pigments that confer colour. But, many brighter shades (especially iridescent blues and greens) are purely the result of structure; the micro-architecture of the scales scatters and reflects light to produce colour.
Broader variation in scale shape and structure is often distinctive, allowing individual scales to be assigned to particular families of butterflies and moths. A varied selection of scale types was noted in the ancient lake sediments, including ones similar to the primitive pollen-munching Microropterix still alive today. Significantly, hollow scales were also uncovered. These are exclusively found in Glossata, a group of Lepidoptera equipped with a tongue-like structure called a proboscis.
Prior to the discovery, this particular group was thought to have evolved over 70 million years later in the Cretaceous period, capitalising on the explosion of flowering plant diversity that occurred at that time. This presents something of a conundrum. What were these lepidopterans using their proboscis for, if nectar-bearing flowers were yet to arise? The authors suggest it was an adaptation to the parched conditions in the Triassic, allowing thirsty individuals to suck up moisture.
The tight interactions formed between flowers and their insect pollinators often drive coevolution, so the finding that proboscis-possessing Lepidoptera were around before flowering plants also raises an interesting prospect. Could moths and butterflies have been a driving force behind the spectacular radiation of flowering plants?
Fluttering through the eras
250 million years of evolution have carefully fashioned different ecological niches for the 125 families of moths and butterflies alive today. While the number of families present in the fossil record remains low until the last 50 million years, the low fossilisation rate means it’s difficult to know for sure if the group diversified as late as this (Lepidoptera are usually thought to have radiated in the Cretaceous period).
Whenever it occurred, the diversification of Lepidoptera was accompanied by pronounced shifts in feeding habits. The early caterpillars fed internally, within soils, stems or woody tissues. It wasn’t until later that some groups ventured above ground to feed on foliage. At first, only staying within the narrow tunnels they mined through leaves. Fossilised patterns produced by these leaf-mining moths date back 100 million years. Such conspicuous clues left by internal feeders could have easily attract predators and parasitoids. The prospect of being a ‘sitting duck’ may be why some groups have sacrificed shelter and embraced the mobility afforded by external feeding.
From the leaves to the roots, different groups of caterpillars have evolved the ability to exploit almost every part of a plant. This vast array of feeding strategies has enabled the divergence of many different families.
But the spectacular diversity within families is largely due to specialisation in eating certain species of plant (many moths and butterflies only feed on a single type of plant). A handful of groups have shunned plants altogether, with some moth larvae preferring to gorge on fungi, other caterpillars, dead animals, or even sloth poo!
In the adults, there have been multiple switches between favouring daytime activity and flying at night. The earliest evidence of a butterfly is dated from the mid-Paleogene, though they probably evolved tens of millions of years earlier.
Intriguingly, many adult moths have lost the ability to feed, relying on food stores accumulated as larvae. Body size has gradually increased and many of the more spectacular groups (including hawk moths and silk moths) only came into existence relatively recently.
In their 250 million years on this planet, lepidopterans have witnessed the emergence and diversification of dinosaurs, mammals and flowering plants; seen the breaking up of the Pangaea supercontinent; and survived two mass extinction events.
The chances are they’ll still be around long after humans have gone extinct. Pretty good going for a fragile group of insects I’d say.