The strange origin of the treehopper “helmet”

May 5, 2011 • 5:38 am

Last November I posted pictures of some bizarre treehoppers (membracids, a family in the order of true bugs, Hemiptera) that had strange structures branching from the top of their bodies. Because these insects were so incredibly weird, it proved to be the most linked-to post I’ve ever written.  At any rate, go back for a minute and look at some of those things.   And here are some more, taken from a new paper in Nature by Benjamin Prud’homme et al. (click to enlarge):

We’ve seen the one at the upper left before: it’s Bocydium globulare.  The function of these “helmets” isn’t always clear: some of them may deter predators (e.g. left images, rows 2 and 3), others may serve as camouflage (last row, left and middle), and still others may mimic distasteful or dangerous insects like ants (lower right).  Regardless of their function, the Prud’homme paper has made a significant contribution to understanding where these “helmets” come from.

They are highly modified homologs of the insect’s wings: an ancient winglike structure that was long repressed but appeared again to take on a variety of new functions as a helmet.

The “helmet” appears on the first segment of the insect’s thorax (“T1”), and is the only known dorsal appendage on that segment in insects (wings of modern insects are always on the last two segments, T2 and T3).  Prud’homme et al. suggest, with good evidence, that the helmet is a pair of fused wing primordia: remnants of the early structures that gave rise to modern wings.

How do we know these are wing homologs?  Prud’homme gives several lines of evidence:

  • The helmet is not simply an outgrowth of the thorax, but is connected to it, as are the wings and other appendages, by a “complex articulation”: a flexible joint.   Here’s a cross section of a dissected treehopper: the joint between thorax and helmet (top, red box) is similar to that between the wings and thorax (bottom, blue box):
  • The development of the helmet is similar to that of the wing:  both “unfold” on emergence (many times I’ve watched the wings of newly eclosed flies unfurl from nubbins to full wings, a remarkable process that takes only a minute or two).
  • There are other morphological similarities between wings and helmets:  both consist of two layers of cells connected by columns, and both are suffused with a complex network of veins.
  • The genes that are expressed in the developing helmet are the same as crucial genes expressed in the developing wing, including Nubbin, Distal-less, and homothorax.

If the helmet is a re-expression of a repressed wing structure, does that mean that early insects had more than two wings? Well, even some modern insects have more than two wings: dragonflies and bees, for example.  But these wings are always, as I mentioned, on the second and third thoracic segment.  We know that insect flight evolved about 350 million years ago (insects are the only flying invertebrates), but fossils are scanty. One theory, espoused by Jarmilla Kukalova-Peck and supported by more recent genetic work, suggests that wings evolved from gill plates in early insects (I’ve written before about the likelihood that the ancestors of modern insects were closely associated with water).  In one of her papers, Kukalova-Peck gives a reconstruction of a Paleozoic mayfly nymph (an aquatic life stage), showing winglike structures—gill plates—on every segment of the thorax and abdomen:

From Kukalova-Peck, J., J. Morphology 156:53-125 (1978)

Another theory is that wings arose from branches of the ancestral insect limb.   Both theories, though, posit that there were more than one or two ancestral structures that were winglike, and that the evolution of the two or four wings in modern insects involved genetic repression of these ancestral features.  The gene Scr (sex combs reduced) seems to be involved in this repression: when it’s inactivated in some insects, extra wing primordia form on T1.

The authors thus posited that somehow, in the last 100 million years (the time when membracids arose), the Scr gene lost its ability to repress wing promordia in the membracid lineage, allowing the helmet to evolve.  To test this, they actually inserted the membracid Scr gene into Drosophila (which has a normal, “repressive” Scr), expecting that perhaps wing primordia would then arise on the first segment of the fly thorax. They didn’t.  They thus suggest that other genes—genes normally repressed by Scr in membracids—have lost their ability to be repressed, and these genes are involved in making the helmet.

Here are the authors’ conclusions from the paper. Even if you’re not a biologist you should be able to understand them, for if you haven’t, I have not written clearly enough!

Our results show that treehoppers have evolved a T1 dorsal appendage, thereby departing from the typical winged-insect body plan, by expressing a developmental potential that had beenmaintained under the repression of a Hox gene for 250Myr. This argues that the constraint preventing extra dorsal appendage formation in insects is not developmental but rather selective. We submit that morphological innovations can arise from the deployment of existing but silenced developmental potentials, therefore requiring not so much the evolution of new genetic material but instead the expression of these potentials.

The breadth of morphological diversity in helmets that has evolved in less than 40 Myr (ref. 27 and C. Dietrich, personal communication) is unusual for an appendage. The pace of appendage evolution is generally slow, probably because of the strong selective pressure associated with their role in locomotion. This is particularly true for the wings, and we speculate that, initially alleviated from functional requirements, the recently evolved helmet was free to explore the morphological space through changes in its developmental program.

As your reward for reading this far, here’s another really weird membracid (Heteronotus sp., from Ecuador), photographed by Alex Wild and taken from his wonderful website, Myrmecos.  You can see both the helmet and the wings.


Prud’homme, B. et al. 2011.  Body plan innovations in treehoppers through the evolution of an extra wing-like appendage.  Nature 473: 83-86.

44 thoughts on “The strange origin of the treehopper “helmet”

  1. Fascinating stuff. Even for a non-biologist like me 🙂

    WEIT: Religion discounting, kittehs, interesting evolutionary biology, and now with sunrises.

    It’s why I keep coming back.

    1. Oops! Fixed thanks. The gill plates were on the abdomen, too, but obviously not the ones that became wings.

  2. If the helmet is a re-expression of a repressed wing structure, does that mean that early insects had more than two wings?

    Wait–Two pairs of wings (on T2 and T3) is the ancestral state for winged insects; your flies have a vestigial second pair (halteres) and beetles have converted the front pair to their protective carapace, but many others (lepidopterans, e.g.) retain all 4.
    I don’t think anybody’s suggesting that basal fliers had three pairs of wings. It’s an atavism if you think of the proto-wing ‘gills’ but not as ‘wings’.
    I’d guess it’s less an atavism and more a mutant activation of the wing-building toolkit in the first thoracic segment where such genes are not normally expressed.

    Very cool regardless; thanks for the post.

    1. note, btw, that the reconstructed mayfly nymph bears gills on all but the first thoracic segment (w/ the front pair of walking legs).

      Also, the origin of the ‘gills’ was probably as part of a branched appendage, so there is no real dichotomy there.

  3. What?!? (Spit-take) Biology?!?!

    Don’t worry, we non-biologists are very happy with your clarity. Thanks.

  4. Really interesting. I suppose that because they ‘hop’ with the help of wings rather than fly (do they?), the excrescences do not bother them aerodynamically as they might otherwise so can be selected for or rather not selected against – anyone? Had I but time I would do a literature search!

    1. I mean I ‘suppose’ them to be clumsy fliers because of the shape, unlike something as gorgeously slim as a dragon fly..

      1. Yes–I’d love to see one moving, or flying !(probably would fall off my bike if I actually saw one–although it appears the awesome treehoppers are all in Brazil. *sigh*)

  5. Slightly off-topic, but can someone explain this “true bugs” business to me? Is it actually the official stance of entomologists that speakers of ordinary English have been misusing the word “bug” for centuries before the recognition of Hemiptera as a formal taxonomic order?

    1. From AM Science Dictionary:

      The word bug is often used to refer to tiny creatures that crawl along, such as insects and even small animals that are not insects, such as spiders and millipedes. But for scientists the word has a much narrower meaning. In the strictest terms bugs are those insects that have mouthparts adapted for piercing and sucking. The mouthparts of these bugs are contained in a beak-shaped structure. Thus scientists would classify a louse but not a beetle or a cockroach as a bug. In fact, scientists often call lice and their relatives true bugs to distinguish them better from what everyone else calls “bugs.”

      I think that Heteroptera was the order called true bugs before it was combined with Hemiptera. Don’t know when that happened (anyone know), but it doesn’t look like the use of the word bug, meaning insect, is that old.

      But treehoppers are not in the suborder Heteroptera so are they really considered true bugs? My old entomology book says only Heteroptera are true bugs. Of course, this book says there are only two suborders in Hemiptera and Wikipedia says there are quite a few more. Those crazy lumpers and splitters confuse us all!

      Entomology and etymology in one comment, what fun.

      1. In ordinary usage, “bug” applies not just to insects but to arthropods of all kinds, including lobsters and crawfish.

        If the argument is that the technical meaning came first, and only later did the word come into popular usage, I don’t think that’s the case. According to Wiktionary, “bug” derives from “bugge” meaning “beetle”, at least a century before Linnaeus.

        I suppose on some level scientists are entitled to assign narrow technical meaning to common words. But if such assignments needlessly defy common sense (as in the assertion that cockroaches are not bugs), doesn’t that tend to undermine public trust in science?

        1. Most of what I read said that bugge meant “something frightening” or “ghosts/ghouls” in the 14th century.

          But after more checking on its etymology, bug was used to refer to bedbugs in 1620.

          Your point is interesting, why would entomologists call some bugs “true bugs” and not others. Perhaps the bedbug connection is more important than it seems (they do look like little beetles), they are Heteropterans. I don’t, however, think this undermines science. I don’t see entomogists berating people who use the common term “bug”.

          Now I really want to know why Heteropterans are the only “true bugs”. Can’t find anything specific about why this is the case. On the face of it, it does seem silly to take a commonly used word and make it exclusive. Perhaps it wasn’t quite as commonly used when biologists began classifying “bugs”. And of course, Linneaus wasn’t the first to describe organisms, he just popularized the binomial system.

          There must be someone reading who can enlighten me on this subject. I want a good reason why other insects aren’t considered “bugs”.

          By the way, Jerry, thanks for posting this.

            1. I only learned “boggart” by reading Harry Potter. But that was the first thing I thought of when reading about bugge’s original definition.

      2. “scientists would classify a louse but not a beetle or a cockroach as a bug.”

        Unfortunately, lice are not members of the order Hemiptera and therefore not “True Bugs” or bugs (ss) if such a term had any restricted utility in any case. Clearly the AM Dictionary is “challenged” (retarded).

    2. I have asked around, but aside from the origins of the word “bug” listed below, still haven’t found the answer. I will check with the Entomological Society on Monday–I know they will know who made the official “true bug” designation. (They have a special committee just for naming things.)

      1. The ESA, and a couple of other taxonomists I consulted, remain stumped. I think I may have gotten May Berenbaum interested in tracking the “true” bug designation down, though. If anyone can do it, May can 🙂

  6. I rather liked this post. I can’t help but wonder if the expressive scr failed to work in drosophila because either downstream elements have been lost to mutation or drift, or have been re-purposed as something else, and is no longer available (or able) to be expressed.

  7. I’ll show my total ignorance on this fascinating topic: can someone tell me how closely related lbsters& prawns are to these insects? The drawing of the MayflyNymph looks remarkably like a prawn!!

    Shocking that all this wonderful knowledge is under attack in the USA in the 21st century…

    Wonderful website!!

    1. I’m a bit late to this party, but

      1)on “true bugs” — the insistance on this is by way of “bug” = bedbug,Cimex lectularis, which is a species of Hemiptera-Heteroptera. As is so often the case with official common names, this is a losing battle, so I just say “hemipteran”.

      2) Sean’s comment (9) leads to another very recent achievement of DNA-based research — insects are indeed an offshoot of Crustacea, and among living crustacea. They’re closest to a group callet Remipedia, which live in marine caves.

  8. “Well, even some modern insects have more than two wings: dragonflies and bees, for example. ”

    Not just some, all winged insects have four wings in principle. In flies the second pair has just changed to halters; in beetles they have become the shields.

  9. A new paper disputes this hypothesis:

    Mikó I, Friedrich F, Yoder MJ, Hines HM, Deitz LL, Bertone MA, Seltmann KC, Wallace MS, Deans AR. 2012. On dorsal prothoracic appendages in treehoppers (Hemiptera: Membracidae) and the nature of morphological evidence. PLoS ONE 7(1): e30137.

    DOI: 10.1371/journal.pone.0030137

  10. Amazing creatures. Is it me or do these little fellows resemeble trilobites a lot? Both of them have elobarted and fanciful “horns” and well, weird looking with some variants that really seemed to be there for no apparent reason. perhaps they both evolved such features for the same purpose?

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