Anyway, again, sitting out in the field,
we've watched 61 individuals in nature
continuously for six hours each recording on computer
all their little behaviors.
And I'm just going to show a couple of pictures.
This is my former student, Mike, and I
watching some of these caterpillars
in the field in the Santa Rita mountains.
In fact, here's one of the caterpillars.
And we could usually watch two at once
and record all the things they did
because we had these little handheld computers.
We had various people help us.
So this was a Brazilian student, Danny,
who worked with us for six months.
And we had a variety of people come.
They liked to come for a little while.
They don't want to do it as long as we do.
[LAUGHTER]
And Thomas Hartmann, who's my collaborator from Germany,
came out one summer and spent the summer here
watching caterpillars.
He's the biochemist in the team.
And he does all the chemistry and the molecular biology.
So he's really a lab person, but he had fun.
But after six hours of watching, he was very pleased to get up.
[LAUGHTER]
Anyway, just some basic information
to give you an idea of what these caterpillars are like.
Number of plant species eaten in six hours on average, five.
Number of individual plants eaten in six hours, nine.
Because they move around a lot, so they go from place to place.
They walk on things.
Number of plant parts and species in six hours.
So we're talking leaves, flowers, fruit stems,
that kind of thing, 21.
The number of plant species recorded as host, 79.
The percent of plant species rejected
in six observations, 49.
So half the plants they come across they eat, and half they
reject.
But you can see they're real generalists.
They eat loads and loads of different plants.
But what they like are plants containing
toxins called cool pyrrolizidine alkaloids, otherwise known
as PAs.
I'm going to talk about PAs.
Here's a molecule of a PA.
It's a complicated molecule.
It's a poisonous chemical.
Poisonous to pretty well everything except these kinds
of caterpillars.
I've put part of it in red because they don't just
use these things for defense.
They take a part of this molecule
and make it into a courtship pheromone.
So the males produce this thing, and it
makes the females want to mate with them.
So they have a lot of uses.
Now, again, we go back to behavior.
Caterpillars wander around.
On first contact with the plant, 100% acceptance
of plants containing PAs, but less
acceptance on other nutritious plants.
So here are two PA plants.
100% likelihood of feeding if they encounter them,
but somewhat less on all the other plants.
So we know that they like these plants containing
PAs, which makes sense because they want those chemicals.
And this is a summary slide of a large amount of work,
as you might imagine.
But the PAs sequestered from plants
provide protection from a lot of natural enemies.
And here are different things.
The sucking bugs, the mantids, the lacewings, the wasps,
spiders, ants.
You name it.
These chemicals protect the caterpillars
from a lot of different predators,
and they're very effective.
So it's a good strategy.
So Estigmene mainly sequester pyrrolizidine alkaloids
efficiently, concentrating trace amounts in foods
to levels that are biologically useful.
Now here's where Thomas came in.
Thomas Hartmann in Germany, of course,
did all the analysis of the things
to show the plants didn't have much,
but the caterpillars had huge amounts.
So they're very effective.
Males and females equally accomplished.
PA is found throughout body, but high concentrations
in the integument.
Makes sense.
If you're a predator, you want to know the bad taste quickly.
Males donate PAs to females during copulation,
and the females endow their eggs with these chemicals.
So the eggs are protected.
And the males produce the courtship pheromones.
So basically, PAs matter.
That's the whole point.
They like the plants.
They like the PAs.
The PAs are terribly important to them biologically.
So the interesting thing, then, was
to look at how they taste these things.
And so I've put a very simple schema here of a taste
bud of a caterpillar.
[LAUGHTER]
They have taste buds like we do.
And each taste bud has four taste cells in it,
and I've shown each one as a separate color.
Four taste cells.
And these things join up to the brain.
So it's a regular taste bud.
And then you can record from them.
And what I do is I have a glass electrode here
and a silver wire in the middle, too.
So if a chemical that tastes on one of these cells
causes a firing of the cell, you get electrical impulses.
It's transmitted through the material
here, through a salt solution, onto the silver wire.
And the silver wire's connected to all kinds of amplifiers
and filters to a computer.
So what we can do is we have a salt solution in here
to carry the electrical impulse, and then it
goes onto the silver wire.
But we can also put in this salt solution
various chemicals to see which ones these cells respond to.
Now there are four cells, and we're recording all of them
together.
But each cell produces a very different pattern of firing,
so we can identify the different cells.
And you'll see it's pretty small.
So here's the scale.
0.1 of a millimeter is this length.
So we're talking pretty small stuff here.
So I have to pull these little glass electrodes out
to make a very small end.
Here are the two main taste buds of the caterpillars.
So there's a lateral one.
This is right out here.
And a medial one, which is sort of more in the middle.
And each of them has these four cells.
And usually, there are two that say yes, and two that say no.
Just like for us, sweet says yes, and bitter says no.
We have the same kind of situation in caterpillars.
So here, the salt and the bitter chemicals
will be going to a part of the brain that says no,
and the cells responding to PAs and the cells
responding to sugars says yes.
So the balance of these things will tell it
whether to eat or not.
There's a little bit of difference in the two taste
buds.
But basically, there's two yeses and two nos.
And the yeses in each case have a cell
that is specific to the PAs.
So we don't have such thing, but these guys do.
And the sensitivity is incredible.
We're talking 100,000 times more sensitive to PAs
than we are to sugar.
So it's the most sensitive taste cells
that have ever been found.
So anyway, I record from these.
And this is the sort of result I get.
So in half a second, I get these records.
And each one of these little spikes
means that there's a nerve impulse.
And so if you look at the salt, the salt cell
is responding here to the presence of the salt.
And here's the response to PA, a different cell,
a different shaped spike.
But each one of these little spikes
represents a nerve impulse on their taste cell.
So we can determine which cell's which
by the shape and size of these spikes,
and eventually work out which cells
are responding to which things, and how sensitive, or specific
they are.
Anyway, it's rather handy that the PA cell
has these great, big spikes because it makes
it nice and easy to identify.
And as I said, extremely sensitive.
Now going back to the behavior.
Although the PA plants are always accepted initially,
repeated encounters lead to rejection.
So this is what you saw before.
The PA plant's first encounter, always accepted 100%.
The other plants varied.
But when you look at later encounters,
this starts dropping.
They stop wanting it anymore, whereas the pattern is rather
variable with other plants.
But the PA plants you'd think, jeez, they want all this stuff.
It's so important for them.
They should stay there, but they don't.
And after a while, they leave those plants.
So one of the things that interested us was,
why did they leave these plants when
they need this stuff so much?
It's so important in their lives.
That was one of the reasons why we studied it.
Well, we fed them on the PA plants
for different periods of time, and looked at the response
of those PA cells.
And you see, here you are at the beginning.
We're getting 40 spikes per half second.
But if they fed on these PA plants for a while,
it goes way down.
And some insects, there's nothing left at all.
Insects, they just stop responding.
We don't get that kind of thing in humans.
Your taste cells are more or less constant.
They do what they're supposed to do whatever you eat.
But in these caterpillars, if they eat a lot of PA plants,
the PA cell stops responding.
With Lavender.
Isn't there a smell difference there?
Well, you do get used to smells and stop responding to smells.
[INAUDIBLE] smell.
Yeah, in a lot of cases, that's true.
But it usually doesn't happen with your taste buds.
It happens with your smell that you get used to a smell
and stop responding.
And anyway, when you stop responding to a smell,
it's a short-lived thing.
You get fresh air again, and you immediately smell it again.
Of course, this lasts quite a long time.
It lasts for hours.
What we did was we injected PAs into the caterpillar's
bloodstream, and found that it shed off those taste
cells in the same way.
Here we are.
Over 40 spikes per half second.
And if you inject them with just a plain saline,
it doesn't change over time.
But if you inject them with the PAs, the response falls off,
and it lasts for a couple of hours.
What's interesting about this is that when they feed on the PA
plants, these PAs are in the bloodstream
and clearly has some kind of feedback on the taste cells
that stops them responding.
We injected the amounts of PAs that
reflected exactly what happened naturally.
So you analyze the blood, find out the amount of PA
in the blood, and then you inject that amount of PA
in the blood for these experiments.
So it required quite a lot of preliminary work
to actually get it into a biologically reasonable amount.