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Carbonated
sodas come in all flavors, but across all cultures, it is
the
carbonation that is the same. Mauby is a tree bark flavored
soda
sold in the West Indies islands of Bermuda, Trinidad and the
like.
On the left is a bird’s nest/white fungus soda sold in Vietnam.
I don’t think it includes the bird.
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Many insects will tell you that it’s the CO2 that
makes the difference. Fruit flies, mosquitoes, ticks and other insects can
taste CO2 on surfaces and in the air. For mosquitoes and ticks,
tasting CO2 helps them find food. These are hematophagous
(blood-eating) organisms, and they find their victims by flying upstream along
their exhaled CO2 and the CO2 that is exuded from their
skin.
Even more amazing, fruit flies and other insects taste the
increased CO2 that stressed (injured, diseased) flies emit. They may
avoid other insects that are dying so they won’t be near disease or danger. In
other insects, they may follow it to animal carcasses - their buffets. In either
case, the insects can actually taste death.
But are they tasting CO2? It’s a gas, and we have
said that gases are detected and perceived by smell, not taste (except for us and DMS). It turns out that CO2 sensation is really an exception. A 2007 paper from John Carlson’s group showed that the receptor heterodimer (hetero = different, and dimer = two different proteins) is made
of GR21a and GR63a, two gustatory proteins (hence the GR in the name).
However, the two taste receptors are located on olfactory
neurons. The signal is detected by taste signaling on a smell neuron, and the
signals are then sent to the smell portion of the brain! This may be one of the
biggest exceptions in all of taste science, and it’s the insects that have it
and use it.
For insects to accomplish many different tasks with taste,
it helps to have the taste receptors in specific places. Catfish had them all over their body, but that’s not very specific. In insects they are found in
distinct places, and may have
distinct functions.
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The
shrimp is a good example to show metamerism in arthropods.
All
the parts are just reiterations of the same subunit. Some kept
their
appendages, and some changed them into something else.
Tagmentization
is the result of modifications so that some of the
somites
act together as the cephalothorax, and others
from
the abdomen.
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Metamerism (meta = subsequent, and mer = unit) is a biology concept for
efficient addition of complexity in an animal. Over time and evolution, certain
specific structures and functions may develop in response to pressures. It is much more
efficient to just create another unit using the same blue prints instead of
creating a new part from scratch. The repeat is metamerism; the specialization over time
of the different mers is called tagmatization.
You can see metamerism and tagmentization at work in
arthropods and annelids (worms) by looking for repeating units. Millipedes and centipedes are great examples.
Their bodies are made from many copies of the same basic unit. In many animals,
repetition of units allows for drift over time and slow changes in structure
and function, even grouping of different mers together for special function
(tagmatization).
Mers (or somites)
in insects include appendages like legs. But over time, many of the appendages
evolved into other structures, like mouthparts, antennae, and egg-laying
apparatus. Some characteristics are retained, others are dropped or altered,
and some new characteristics appear.
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Feel
like your being stared at? The left picture is good for showing
the
mouthparts of a grasshopper. Every one is a remnant of an
appendage.
The mostly come in pairs, one from each modified
appendage.
On the right, the cartoon shows the different
mouthparts,
the labrum (lr) and hypopharynx/labium (hp/lb)
have fused to form just one piece. md = mandible, mx = maxilla
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A run down of tasting anatomy is hard for insects as a
whole, because different arthropods taste with different parts, but some structures
are more common. Mouthparts seem to be a favorite, and that makes sense.
Flies taste with their probsocises (am I making up the plural?), but they also
taste with the ends of their legs. Arthropod legs come in segments, and the last
segments are called the tarsi.
Flies can taste food with their tarsi just by landing on it,
but the also have taste receptors higher on their legs as well. Honeybees taste
primarily with their antennae, but other flying insects can actually taste things
with their wings! Wing tasters include fruit flies and mosquitoes, and they are
more of an exception than you might think. We talked above about how tasting
with different parts isn’t so crazy, since metamerism is just the modification
of similar starting parts. But wings are
not modified appendages.
Wings actually evolved from abdominal gills, and most
insects have either given up these early structures and those that have them
don’t taste with them. It may be that taste receptors on wings developed on
their own, or that taste is older than metamerism. We don’t know their function
yet – you work on that one.
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Drosophila
is the quintessential research model. The left
cartoon
shows the olfactory and gustatory receptors. Notice
how
many taste receptors are around the proboscis. On the
right,
the red dots show all the different places gustatory
receptors
are found. Wing margins, legs, tarsi, and mouthparts
all
have taste receptors.
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Citronellal is only smelled by mosquitoes; the active
molecule triggers only olfactory receptors. But DEET triggers both olfactory
and gustatory receptors, it is smelled and tasted. Both senses stimulate
avoidance responses in insects, so even if a mosquito lands on you, the DEET
you put on will be tasted and may keep it from biting.
So some insects taste with wings - is that as weird as it
gets? Nope, some females taste with their ovipositors
(ovi = egg, and posit = laying). Ovipositors are a result of metamerism, they
are modified appendages. The females of many species can taste the plants or
places they land to determine if they are a suitable place to lay eggs.
The ovipositors most likely have rare taste receptors, applied
to only to this one specific task. For example, there are two subspecies of a
particular fruit fly called a goldenrod gall fly (Eurosta solidaginis). The females look for specific plants, and
then for buds of the right age in which to insert their eggs. The growing larvae
then feed on the bud, and cause a tumor (gall) to form.
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The
ovipositor of a female wasp or fruit fly is also a modified
appendage.
In the wasp on top has a rigid ovipositor that may
be
used to inject eggs into a caterpillar larva. On the fruit fly
ovipositor
below, you can almost see the sensillae that
contain
the taste receptors.
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Obviously, some insects pick their plants very carefully.
Let me give you an example that really knocks this point home. Tiger moth (Grammia incorrupta) caterpillars are sometimes parasitized by flies or wasps
that lay their eggs inside the wooly bear (tiger moth caterpillar). A 2009 paper shows
that when this occurs, the caterpillars switch the kind of plant food they eat,
opting for poisonous plants that contain pyrrolizidine alkaloids (PA).
The PA-rich food is much less nutritious than the
caterpillar’s regular food, so it definitely costs the caterpillar in terms of
grown and health, but the PA is toxic to the parasites. The food choice
sometimes depends on the number of parasitic eggs laid in one individual
caterpillar. Just one egg – a caterpillar may eat some PA-rich plant material
and let its immune system do the rest of the work. But with more eggs, the woolly
bear will consume PA-rich plants exclusively – hoping to kill off all the eggs.
The caterpillars are self-medicating, tasting their way back to health.
Turnabout is fair play – we haven’t discussed the plants
that are being eaten by all these insects. In some cases, it turns out that the
plants are tasting them right back, and even tasting each others' messages.
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You
can see the parasitic wasp injecting eggs into the
caterpillar.
When the eggs hatch, they will feed on the
caterpillar
through their larval stage. Two things may
happen.
The caterpillar may switch plants (based on taste)
to
try and poison the parasites. Second, the plant they are on
now
may have called in the wasps to kill the caterpillar using
volatile
chemicals, and the toxic plant that the caterpillar
switches
to may do it again.
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The volatile chemical that maize uses comes in two forms;
it’s the switch from primarily one form to the other that attracts the wasps.
But even before the plant starts to produce the attractive form, the
caterpillar’s saliva converts the inactive form to the attractive form. The
attractive message starts about a day before
the plant starts to make the attractive form. The maize molecule has evolved to
make the caterpillar call the cops on itself.
What is more, plants can send taste messages to nearby
plants through the dirt. In a 2011 study, researchers induced drought like
conditions on one row of plants. In less than an hour, plants five rows away
started to close their stomata (pores in leaves) to conserve water for an impending
drought. Plants that were just as close, but planted in a different container
did not prepare for drought, so the message had to be traveling through the
soil. I leave it to you to decide if this is really a taste sense.
So - if you’re a raw food enthusiast, you might be being
tasted back. And maybe your food is spreading the word about you to his neighbors.
Next week – why do we call spicy food "hot?"
For
more information or classroom activities, see:
Carbon
dioxide taste in insects –
Parasitic
wasps –
DEET/citronella
–
Plant
volatile defense chemicals -
