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Electrophysiology – What do fire ants
feel?
Summary
We have recently developed a technique to record nervous activity
from the legs of fire ants. By utilizing dedicated equipment and
miniature electrode implants we are capable of measuring what kind
of acoustic signals fire ants can detect. Are fire ants sensitive to
substrate vibrations?
Background
Most insects possess vibration detection organs, which normally are
located in the tibia of the legs. These organs allow insects to
detect substrate vibrations from conspecifics, predators and other
sources. Ants can produce substrate vibrations/sound by stridulation
(moving one body part against another). These stridulatory
vibrations may serve several functions including communication. Our
main objective in the electrophysiology project is to characterize
the vibration sensitivity of fire ants.
Methods
Two miniature metal electrodes (made of 0.001 inches thin
Platinum-Iridium wires) are inserted into the leg of a live fire ant
(Figure 1). Then the leg is glued to a mini-shaker, which generates
vibrations to imitate substrate vibrations. The shaker is controlled
by a function generator in which we can vary the frequency and
amplitude of the vibrations. With this apparatus we can generate
precise vibration signals to examine which type of signals the fire
ant is sensitive to. If the applied vibration stimuli exceed the
threshold of the vibration detecting organ in the ant’s leg, then it
will fire electrical impulses into the nervous system towards the
central nervous system. When the bio-electrical signal passes our
implanted electrodes we can pick up the signal, amplify it and
record it on our PowerLab system. Figure 2 shows a 50 Hz signal and
the corresponding electrophysiological response to the signal. We
record the biological signals simultaneously with the vibrations of
the leg by laser Doppler vibrometry with the laser beam focused on a
small piece of reflective tape attached to the tibia. In this way we
can determine which frequencies the ants are sensitive to, and how
sensitive they are.
Figure 1. Two electrodes are implanted at the base of the hind leg.
Besides the insertion of two electrodes in the leg and one ground
electrode in the thorax the ant is left intact.
Figure 2. The blue trace is a LDV measurement from the stimulated
leg (50 Hz). The laser beam was focused on a small piece of
reflective tape attached to the tibia of the ant leg, while the leg
was vibrated by a mini-shaker. The red signal is the corresponding
nervous signal recorded by the leg implanted electrodes.
Figure 3. The blue trace shows the vibration of the leg (LDV), when
a gentle knock was applied to the vibration table. Above, in red, is
the corresponding nerve signal.
Figure 4. shows the electrophysiological determined threshold
displacement for one individual.
Figure 5. shows an ant leg attached to an adapter connected to a
mini-shaker.
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