Thoughts on Arthropod Classification
And why there are no such things as Shrimp... probably
One of my favorite topics and deepest academic interests is the sometimes convoluted world of biological classification, a rather esoteric branch of natural history which remains a comforting window into how much we still don’t understand about the world around us. Classifications, which are ideally meant to represent evolutionary history, can change in the blink of an eye with new evidence, or simply from observing from a different perspective.
I have included by way of illustration for this essay a few of Ernst Haeckel’s spectacular plates from Kunstformen der Natur (1899-1904) and one of Maria Sybilla Merian’s marvelous paintings from the 17th century, which remain among the most beautiful pieces of scientific art ever created. Every species illustrated is perfectly accurate (so far as they were understood at the time), and yet there is also…something else, a hard to define aesthetic quality which the majority of natural history illustrations lack.
When Carl Linnaeus began the practice of formal taxonomy more than 300 years ago, he was faced with a mind-boggling diversity of animals and plants that most scholars before him had not given much thought to. His solution was logical, and more or less the same as what good taxonomists do today: he found traits or ‘characters’ that united organisms into groups, and placed these groups into bigger groups, and so on. All mammals have hair, all land vertebrates have amniotic eggs, all vertebrates have a backbone protecting a dorsal nerve chord.
But relationships are not always so obvious. The largest group of animals- by far- is the arthropods: insects, spiders, crabs, shrimp, millipedes etc. All these groups share so much in common that their relatedness has rarely been seriously questioned (to Linnaeus they were all ‘insects’). But within that gigantic assemblage, how do they all fit together?
Today, most arthropods belong to three groups: crustaceans, insects, and arachnids. The latter two are, for the most part, very well defined, in the sense that the majority of insects are unequivocally insects and not something else. Crustaceans, on the other hand, are more difficult. They are the oldest of the modern arthropods, with a fossil record stretching back as far as any animal group, and also the most morphologically diverse. While there are more species of insects than crustaceans, the difference between different subgroups of ‘shrimps’ is often far greater than between different insects. Barnacles, prawns, brine shrimp and mantis shrimp are good examples of the extreme variation found in this group.
For much of the 20th century, zoology textbooks showed ‘family tree’ diagrams indicating a close relationship of insects to millipedes, centipedes, and their relatives, the ‘Myriopoda’: ancient, fairly primitive arthropods with long multi-segmented bodies. And indeed, millipedes do share a number of traits in common with insects. They have only one pair of antennae, their legs lack the upper branch of trilobites and crustaceans, and they have various adaptations to terrestrial life, including a waxy cuticle and so-called tracheae or breathing tubes. The problem is that these are exactly the sort of traits any air-breathing arthropod would need, and so they may tell us less about the historic relationships of these groups than they do about the difficulties of adjusting to life on land. Similarities between insects and crustaceans were also noted at an early date, but remained more or less ignored for most of the 20th century- presumably because they complicated the simplified view of ‘evolution as progress’ which unfortunately dominated biological science during that era.
However, over the last 20 years, the old idea that insects are close relatives of lobsters and shrimp has come back. Details of anatomy, including the complicated compound eyes and the structure of the nervous system, point towards this relationship, which is also backed up by genetic studies. In fact, it may well be that some ‘crustaceans’ are closer to insects than they are to other crustaceans. This would, in a sense, make insects a sort of specialized terrestrial branch of the class- not highly advanced millipedes, but more like flying shrimp! We don’t actually know with complete surety which theory is correct, because we can’t look back millions of years into the past and see exactly what was going on. Perhaps new fossils or a careful look at some obscure anatomical feature will shift the perspective back to myriopods. I myself like the notion that insects derive from crustaceans, perhaps because it pleases my rather contrarian personality for the less obvious answer to be true. Nature isn’t obliged to be easily understood.
Put plainly, evidence currently points towards there being no natural category of ‘shrimp’. The prawns we eat are closer taxonomically to insects than they are to the brine shrimp floating about in desert pools. And yet most people today shrink at the thought of ‘eating bugs’. Apparently ‘sea bugs’ are fair game!
But whatever the relations within the arthropod lineage, it is in any case one of such disparate adaptations as to defy belief, a truly baroque flowering of life into its most fantastic forms. A beautiful metallic blue butterfly floating through the Amazon rainforest shares much in its genetics, its archaic history and even features of its anatomical structure with a barnacle clinging to the side of a blue whale gliding through the depths of the ocean. A tiny featherwinged beetle (Ptiliidae), half a millimeter long, living cozily within the spore tube of a forest fungus, is kin to a giant spider crab slowly ambulating across the cold, dark seafloor of the north Pacific.
All life is related. We are all made of the same stuff, breath the same air, drink the same water, and share many of the same genes. For me personally, the study of nature is a meditative, inspiring activity, through which one can glimpse our connection with all life. This is why taxonomy has always fascinated me. Rather than a dry, library-style ‘catalogue of life’, I see it as the charting of a ‘map’ of life’s long and tangled journey. It is in this way a humbling act, gazing backward into the ‘abysm of time’ as Shakespeare said, and trying to piece together glimpses of a great pattern that connects us with all life on earth.
The best science and scholarship must be done for its own sake, out of the love of the subject in question, out of the deep curiosity which is such an important facet of our humanity. The study of nature- true science- is more like writing a symphony than designing a computer, and when science is hijacked by the technologist mindset it loses sight of its true purpose. There is one reason to study beetles, or crustaceans, or flowers: because they are intrinsically beautiful and fascinating, and because knowledge of that beauty can, like great works of art, connect us to something profound and sublime beyond ourselves.
A sincere thank you to all my readers for your continued interest. Much more to come!
D.K. DeYoung
How wonderful. As a student of biology, this is amazing and inspiring to read. Going to sit down and review my paleontology notes now, lol.
What a wonderful essay! I love my copy of Haeckel's book, but from the perspective of an artist rather than a scientist. It's all one!