Spiders, beetles, millipedes, for sure. Moths? Maybe. Butterflies, like the lemon pansy (Junonia lemonias) in the photo above? A stretch. Crabs and lobsters. Certainly not; that’s food! Have we reached the limits of the folk classification of visible bugs? Consider. Spiders are perhaps more closely related to horseshoe crabs than to beetles. And if you think sea lice are bugs, then their close relatives, the shrimps shouldn’t be exempted. So let me go with arthropods instead: those invertebrates with an exoskeleton and jointed legs and segmented bodies. (Ands are powerful things, easily lifting fifty times their weight in sentences. After all we have jointed limbs, and segmented vertebrae. But we are not arthropods, because we are not invertebrates.) I’ll go with this, because it gives me a reason to finally read two papers (this and this) that I’d been meaning to for a while.
When did arthropods come into being? Darwin noted an uncertainty: “For instance, I cannot doubt that all the Silurian trilobites have descended from some one crustacean, which must have lived long before the Silurian age, and which probably differed greatly from any known animal.” Darwin’s intuition has been vindicated by the discovery of new fossils which pushed the origins of arthropods beyond the Silurian period (445-420 million years ago) into the Cambrian (535-490 million years ago). I took this photo of a fossil arthropod, a trilobite, in Shanghai’s Museum of Natural History. That bug was the size of my hand! The species, Sinoptychoparia tuberculata, is known from this single specimen from 515 million years ago, preserved in a sheet of stone from China’s Guizhao province. The oldest fossils of arthropods that we know of are not more than about 550 million years old, embedded in the proliferation of animal forms that is called the Cambrian explosion. This roughly agrees with genetic information.
Of all the forms of living beings known and recorded, arthropods are the most varied. But the living species of arthropods are just a small fraction of all their extinct cousins. All of today’s arthropods are either crustaceans, insects, myriapods (millipedes, centipedes and their relatives), or chelicerates (spiders, hermit crabs, and related species). But there are many groups of animals which seem to be closely related: tardigrades (which recently failed to colonize the moon) and velvet worms certainly, but also roundworms. In Darwin’s time it was expected that arthropods must have evolved from the much older group of roundworms, the annelids. The biggest discovery since Darwin’s days is that genome analysis shows that arthropods do not come from annelids. I think that is my biggest take-away from the first paper. These genomic studies have completely rearranged the branches of the tree of life around arthropods into a form that Darwin would not have suspected.
Insects evolved from cave-dwelling crustaceans about 480 million years ago, late in the Cambrian period (that’s the headline of the second paper). Beetles began to develop about a 130 million years later. They have had time to evolve into a variety of shapes. So many, in fact, that J.B.S. Haldane remarked that “If one could conclude as to the nature of the Creator from a study of creation it would appear that God has an inordinate fondness for stars and beetles.” All insects have a pair of antennae to smell with, but the one I saw on the beetle on the wall outside my flat was really spectacular. This photo was taken in October 2019, and I saw another specimen in November 2022. So, whatever it is (help me if you can, as the Beatles implored), it is not uncommon. There are many ideas, but no certainty yet, about how antennae developed.
If you wander through the fossil section of a museum, you are likely to see insect fossils similar to today’s lacewings and dragonflies. All have two pairs of wings like modern insects. The earliest known fossils of winged insects are a little more than 300 million years old, but genomic studies now show that insect flight arose about 400 million years ago. So, one should expect fossil hunters to discover even older specimens. I’ve written earlier about how a butterfly is grounded by a predator taking a bite from its forewings, but it can continue to fly with reduced manoeuvrability even after losing large parts of its hindwings. Flies seem to have only one pair of wings, because the hindwings are reduced to small appendages called halteres. They lose control over their flight paths if the halteres are lost. Beetles have converted one pair of wings into a hard cover, and still retain an ability to fly. I wonder whether dragonflies and damselflies can also keep aloft without using their hindwings.
Ants are fascinating. With the wonderful cameras that many of us carry in our pockets, I’ve been looking at ants in detail for some years now, without being able to identify them. These have elegant striped bodies which were quite hard to see at first because of the lack of contrast with the flower they are clambering over. When we think of pollinators, ants are not the first to pop into our heads. We think first of bees and butterflies. Interestingly, both these families have their origins before the rise of the flowering plants. Their spectacular diversity, however, comes with the explosion of flowers about a 140 million years ago. Ants also date from that time.
The origins of spiders and related groups of animals still remains to be understood fully. Early ancestors of today’s spiders are visible in the fossil record in the middle Cambrian. Animals that we would perhaps recognize as spiders may have lived about 400 million years ago. They have had time to evolve into the many lifestyles we recognize today: the orb weavers, the jumpers, or the ambush hunting crab spiders, like the one in the photo above. Arthropods are an old order of animals, filling a variety of niches across the world. Even insects are much older than flowering plants. So tales of the insect apocalypse are overblown. If we heat our world beyond our limits, we might carry some arthropods into extinction with us (lobster claws could become rare), but far from all.