TERMINAL SEGMENT How Many Parts? JOHN ACORN Downloaded from https://academic.oup.com/ae/article/59/4/256/6854 by guest on 28 September 2021 riting this column in Austin, at so straightforward in the textbook (or chil- the annual meeting of the ESA, dren’s book) diagram of the grasshopper: WI’ve been thinking a lot about head, thorax, and abdomen. Butterflies how we teach entomology to kids, and to and moths are easy to interpret as well. “the general public.” The highlight of the So are flies. On every field trip, however, meeting, for me at least, was receiving an you quickly encounter beetles and ants, award (a medal, no less) from the Ento- and again, you have a problem. mological Foundation, recognizing my I stopped talking in terms of main body contributions to inspiring young entomol- parts after the umpteenth enthusiastic neo- ogists, which made me wonder how much phyte proudly showed me the three main of an expert I really am when it comes to body parts of a beetle. From above, almost spreading the word about the fascinating every beetle looks like a textbook three- world of insects. part insect. That’s why I find it so terribly At the very least, I know that I general- disheartening to have to explain that this ly approach an audience of children and is, in fact, a mistake. You have to flip the beginners in a somewhat different way than beetle over, and place great importance on many of my colleagues. For example, I am a dividing line that even the undergrads not as quick to define insects for a general have difficulty tracing correctly, in order to audience—as creatures with “six legs and recognize the “real” divisions. Ants provide Counting main body parts—for example on three main body parts.” Almost any kid can this earwig—is not as self evident as we often a similar problem, even though they prob- recite this proto-scientific meme, along suppose. ably are the most visually tripartite insects with the corresponding fact that spiders you could ask for. The problem, of course, have eight legs and two main body parts. all of the members of the most diverse is that the first segment of the ant abdo- All of this is well and good, until you head family of butterflies, the “brush-foots” or men (the propodeum) is fused with the out into the field and actually look at some Nymphalidae. Sure, they “have” six legs, thorax, as part of the so-called mesosoma, arthropods. That’s when leg and body part but you can’t see all six, since the front and separated from the gaster (the part of counting becomes quite a bit less straight- pair is thin and hairy, and hidden up in the the abdomen that looks like a separate forward. We have recently been reminded setae of the thorax. How about mantids? body part) by the petiole (the thin part of that this is a difficult task even with respect To many kids, they too have four legs, as the abdomen forming the “waist”). Ants, to our own bodies, as the announcement well as two arms—after all, humans have along with their closest wasp relatives, all of a new human body part (a ligament in only two legs, right? Even the eight-legged share this structure. Some ant-mimicking the knee) demonstrates (Claes et al. 2013). spider rule seems to fail for many children, spiders confuse matters even further by Mind you, if you Google “new human body since the pedipalps of a good many spiders possessing an apparently three-part body part,” you quickly discover that “parts” are look a lot like a fifth pair of legs (as they made up of only two “main body parts.” being discovered constantly. should, being serially homologous). So It is a lot of fun to look at insects with Let’s begin, however, with arthropod you wind up, if you are lucky enough to be this type of forced naïveté, don’t you legs. Sure, most adult insects have six legs, dealing with kids who notice such things, think? It makes you wonder just what a but entomologists are acutely aware of spending quite a bit of time explaining the main body part is supposed to be. Does the fact that mite larvae, springtails, and exceptions to the “rules,” and making the Torre-Bueno define it in the Glossary of other non-insect hexapods also bear six. whole subject seem a lot less accessible Entomology? No, he doesn’t. “Main body Sometimes, we add this qualification, but than you promised it would be. part” is more of a folk concept, isn’t it? It with kids, we usually leave it out. Then Legs, however, are nowhere near as con- there are the insect exceptions, such as fusing as “main body parts.” It all seems (continued on page 255) 256 American Entomologist • Winter 2013 Terminal Segment, from page 256 a caterpillar, and explain the concept of References prolegs, as opposed to jointed legs, and Claes, S., E. Vereecke, M. Maes, J. Victor, P. is, on the one hand, a subset of the body how they mysteriously don’t count as legs, Verdonk, and J. Bellemnas. 2013. Anatomy that has a particular function. The head even though we call them “legs.” Legless of the anterolateral ligament of the knee. is analogous to our own, with the prima- larvae round out the picture (especially Journal of Anatomy 223: 321-328. Torre-Bueno, J. R. de la, S. W. Nichols, G. ry sensory organs, a brain, and a mouth. the nearly spherical examples), and the S. Tulloch, and R. T. Schuh. 1989. The The thorax bears the legs and the wings. maggots of cyclorrhaphan flies cap it off. Torre-Bueno glossary of entomology. New The abdomen is full of guts, gonads, and Almost all signs of a maggot’s head have York Entomological Society in cooperation suchlike. On the other hand, there are, disappeared through the course of evolu- with the American Museum of Natural His- in many instances, obvious constrictions tion, except for a pair of feeding hooks that tory, New York. between the “parts,” making them appear project through the mouth at the front of as semi-separate “objects” as well. Phyloge- the thorax (which is itself indistinguish- John Acorn lectures netically, they represent the fusion of many able from the abdomen). None of this is at the University of segments (of the leggy-worm ancestor of scientifically or entomologically mistaken, Alberta. He is an ento- the Hexapoda) to form the head; the spe- or beyond explanation (please don’t think mologist, broadcaster, Downloaded from https://academic.oup.com/ae/article/59/4/256/6854 by guest on 28 September 2021 and writer, and is the cialization of the next three segments for I am making such a grand claim), but it author of fifteen locomotion, forming the thorax; and the does make it difficult to define insects for books, as well as the quasi-distinctness of whatever is left over to a general audience without in turn setting host of two television form the abdomen. By these criteria, mind them up for no end of confusion. series. you, there are still plenty of exceptions. So what is the solution? Perhaps it is Getting back to beetles (and after all, to admit that there are so many insects, there are a lot of beetles), one has to won- of so many different sorts, that it is real- Letter to Editor, from page 195 der why the prothorax doesn’t count as a ly difficult to come up with a simple rule main body part. After all, it is clearly dis- for recognizing them. They are, in psy- enabling Christians to defend their faith tinct from the head, and from the winged chological terms, not a “basic category” and to proclaim the gospel of Jesus Christ pterothorax, and it serves a unique func- because it is impossible to form a com- effectively,” http://www.answersingenesis. tion in that it is the only segment that bears posite mental image that subsumes all org/about). AIG is slated to manage “Ark legs, and only legs. One could easily argue insects and excludes all other organisms. Encounter” once it opens. that both the prothorax and pterothorax If you try to provide a simple definition, Providing lavish state tax-funded sub- are main body parts. Think of beetles, but it becomes impossibly confusing. For sidies for the construction of a self-de- also of dragonflies and damselflies, dipter- example, you might say that an insect is scribed religious theme park is as egregious an flies, heteropterans, and so on. Why “an organism that possesses six jointed a violation of our nation’s founding core not admit to the kids that these insects legs and a distinct head, jointed legs and principle of separation of church and state seem to have four main body parts, with wings, or one that came from something as is teaching biblically inspired “intelli- one often hidden under the wing covers? that once fit this definition, or one that gent design” in public schools, a practice Do any other possibilities suggest them- will soon become something that fits this already resoundingly confirmed as a vio- selves? Well, how about the terminal seg- definition.” How confusing! lation of constitutional law. Were some ments? It’s about time I mentioned that The whole thing, to be honest with you, massive demographic revolution ever to subject here. On some insects—in fact, on makes me wonder why the concept of shift the majority in the Kentucky legisla- many insects—there is an abrupt structur- “insect” is so terribly important to us in ture over to Judaism, I would protest just al transition between the basal abdomen the first place, and why arthropods in gen- as loudly were legislators to propose a and either the genitalia or the segment eral, or the various insect orders, never ban on goetta, the popular Northern Ken- bearing the pincers or other appendag- gained equivalent status, with societies tucky-area breakfast sausage, because its es.