Written by Nancy Miorelli

Well, there’s quite a lot to cover here! So lets get to it!
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This is a great series of questions because I was going to tackle a butterfly coloration question this week, but would have had to talk about development. Now I can talk about development and we can learn all about pretty shiny butterflies next week.

Sooo, how do we make pretty, shiny butterflies?

Do Caterpillars Know They’ll Be Butterflies?

No, caterpillars don’t really know what they’re doing in the same way that we can’t tell our bodies when to start puberty or command our menstrual cycles to happen with our brain power (if that sort of thing happens to you).

Insect development happens through a cascade of hormones that first trigger the caterpillar to get bigger through a series of molts, or instars. Then, with the absence of something called juvenile hormone, the caterpillar turns into a cocoon, and then into an adult. The caterpillar has no more control over this process than you growing hair on your body in your teens.

In the video, I explain the hormonal process of insect development if you’re interested, including some pretty crazy experiments from the 50’s and 60’s straight out of a B horror movie. No worries, you don’t need a biology degree to get through it.

Now onto the rebuttal of the deluge of gibberish that was posted in reply to the initial question.

1) They Don’t Turn Into Goo. Well, Not Entirely

What’s going on in here?

It’s true that if you were to look into almost any insect textbook they would tell you that the larva completely disintegrates into goo, but they’re generally outdated.

Lowe et al. 2013, using CT imaging,  found that not everything disintegrates in the chrysalis when observing a Painted Lady completing development through the pupal stage. In fact, many of the important bits didn’t!

The tracheal tubes, the insect’s main way of breathing are one of these. They stay in the same spot over time and get bigger. This makes sense, because the butterfly is a charismatic creature that flaps about and needs a lot of oxygen to show itself off to potential mates and find food. The gut is also present during the whole cocoon stage but shrinks and becomes more convoluted during development. The authors don’t note much about why the gut shrinks or changes shape, but I’m assuming it’s because adults don’t need to binge eat as much as their former gluttonous caterpillar selves.

In blue, the tracheal tubes get bigger.
In red, the gut shrinks.
But both are still very much there.
Lowe et al. 2013

But what about those pale green structures?
Ah, we’re getting there =)

2) Fated to Fly

This is a diagram of a fruit fly. Fruit flies don’t have a hind pair of wings, and instead have a little knob called a “haltere”. Here you can see the imaginal discs that make up different adult structures.
Diagram by Morata G, 2001

You can really see the wing pads and how the wings develop in this Dark Caerulean Butterfly pupa.
Photos by Horace Tan

There are  structures are called imaginal discs that are present from the time the butterfly is a caterpillar. Imaginal discs don’t break down in the pupa; instead they undergo rapid development and turn into all of the structures the adult butterfly has, but the caterpillar doesn’t.

Some of these structures include the eyes, the antennae, wings, legs, mouthparts, and genitalia.

The wings aren’t so much “ready” in the caterpillar, so much as they’re blobs of tissue that will later become wings during the pupal development. And they’re not so much “floating around” as they have specific locations within the insect.

Imaginal discs are special, because while they are located in certain places, they are predestined to become a specified structure. Imaginal discs are so set in their ways that they’re not even affected by hormones. They can even be transplanted cross species, but not without some problems for the hosts. In early studies, imaginal discs were transplanted and resulted in mutants with legs and antennae coming out of the wrong places.

You can also get body parts in the wrong places if the imaginal disc is coded incorrectly in the first place.  In some cases, the imaginal discs can change jobs which is what happens in the case of Antennapedia. This is a fairly rare occurrence, and for the most part, once the job description is set to a particular imaginal disc, it’s going to do that job forever.

Some coding went really, really, wrong.
F. R. Turner at Indiana University.

3) Are the Brains All There?

It is true that we have shocked caterpillars and the adults remembered the associated smell. Learning isn’t really anything new in insects as we’ve trained wasps and bees to be bomb sniffers and mantises know to sit on hummingbird feeders for free food. However, the fact that the butterflies could retain information through their pupal stage was relatively new to science. Obviously, some sort of neural tissue has to stay intact for this to happen but the central nervous system (CNS) is definitely not unaltered.

Yesss, notice my lack of eyes and wings, but I do have a tiny derpy pair of antennae.
photochem_PA – CC BY 2.0

The important thing to note is that the insect brain is comprised of many parts and while we have some pretty good guesses about what all the insect brain parts do, there is a lot we still don’t understand.

However, the mushroom bodies in particular are responsible for “tasting”, “smelling” with the antennae, learning, and memory. The caterpillars have simple antennae located near the mouth, and form into complex structures in the adult with imaginal discs.** However, the mushroom bodies are present in the larvae, making the association between, presumably the “taste” of the chemical  and the shocking.  Since the mushroom bodies are so integral to the insect, it is unlikely that they go though a lot of change in the pupal stage. Because the mushroom bodies detect  “taste” in the immature, and “smell” and “taste” in the adult,  and  is responsible for learning in all stages, it’s unsurprising that the adult retained this knowledge.

The mushroom bodies are highlighted in this Drosophila head.
Schindelin and Heisenberg, 2006

That being said, a lot of the brain has to be reconnected and revamped during the pupal stage. The optical lobe  connects to a big shiny pair of eyes, the subesophegal ganglion now has a bunch of mouthparts to control,  the mushroom bodies have a fancy new pair of antennae to read, and the nerve clusters along the body  have six functional jointed legs, no squishy back legs, and usually some wings to move about and coordinate. But you can learn about all the other parts of the brain in my video below.

TL;DR?

By ThePoppyTree.

It’s complicated.
No the insect doesn’t turn into goo.  Many important structures stay in tact and just get moved around a bit, like the gut, tracheal system, and part of the central nervous system. Wings aren’t fully formed, floating around in insect goo, but come from specialized tissue within the caterpillars. The caterpillars aren’t sentient enough to know they’re about to transform, they’re just subjected to a cascade of hormones.

Maybe you can train some wasps to find your phone for you.
And yes, insects are very majestic, but I wouldn’t say stupid.

Majestic Indeed

**An earlier version of this article incidcated that caterpillars don’t have antennae at all. Most of this work is done in fruit flies, which don’t have antennae as larvae. Caterpillars, have little short stubby antennae located near the mouth but undergo the same deconstruction and reconstruction from imgainal discs as the legs do.

 References

1. Blackiston DJ, Casey ES, and Weiss MR. 2008. Retention of memory through metamorphosis: Can a moth remember what it learned as a caterpillar? PLoS ONE 3(3): e1736.

2. Halder G, Callaerts P, Gehring WJ. 1995.  Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila. Science 267: 1788-1792.

3. Haynie JL and Bryant PJ. 2005. Development of the eye-antenna imaginal disc and morphogenesis of the adult head in Drosophila melanogaster. Developmental and Cellular Biology 237(3): 293-308.

4. Klowden MJ. 2007. Physiological Systems in Insects. ISBN: 978-0-12-415819-1

5. Lowe T, Garwood RJ, Simonsen TJ, Bradley RS, Withers PJ. 2013. Metamorphosis revealed: time-lapse three-dimensional imaging inside a living chrysalis. Journal of the Royal Society Interface 12(104): DOI: 10.1098/rsif.2013.0304.

6. Mindek G. 1972. Metamorphosis of imaginal discs of Drosophila melanogaster. Wilhelm Roux’ Archiv für Entwicklungsmechanik der Organismen 169(4): 353-356.

7. Ochieng SA, Poole K, Linn CE Jr, Vickers NJ, Roelofs WL, Baker TC. 2003. Unusual pheromone receptor neuron responses in heliothine moth antennae derived from inter-species imaginal disc transplantation. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 189(1): 19-28.

8. Pauls D, Selcho M, Gendre N, Stocker RF, and Thum AS. 2010. Drosophila larvae establish appetitive olfactory memories via mushroom body neurons of embryonic origin. Journal of Neuroscience 30(32): 10665 – 10666.

9. Singh K and Singh NR.  1999. Metamorphosis of the central nervous system of Drosophila melanogaster Meigen (Diptera: Drosophilidae) during pupation. Journal of Biosciences 24(3): 345-360.