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Talking Caterpillars
Let's revisit some of the hypotheses for the function of the TOs:
Direct defense: TOs deter natural enemies such
as parasitic wasps
Weeks used videotaped encounters between lycaenids and parasitic wasps.
If the TOs functioned to repel the wasps, then there would be a negative
correlation between TO extrusion and enemy attacks.
Instead, Weeks found a positive relationship
between the amount of time the TOs were extruded and wasp attacks, whether
the attacks were measured as contacts, mounts, or ovipositions.
Thus, her experimental evidence does not
support the hypothesis that the TOs function as a direct defense by deterring
parasitic wasps.
Indirect defense: TOs attract or retain ants or
more ants
To test this hypothesis, Weeks transformed some of the larvae by gluing
their TOs shut ("occluded"). Control larvae had a spot of glue
placed near the TOs, to control for the effects of the glue.
She found no differences between the occulded larvae and the control larvae
in the time it took before ants began tending the larvae. There was also
no difference between occluded and control larvae in the number of ants
tending.
Furthermore, within the control group, the number of times the TOs were
extruded had no effect on the number of ants tending. This was also true
for the number of times the ants were contacted by the TO.
Thus, the hypothesis that TOs act as an indirect defense by attracting
or retaining ants or more ants was not supported
by Weeks' experimental evidence.
Indirect defense: TOs make ants more aggressive
or more vigilant
To test this hypothesis, Weeks recorded ant behavior following the extrusion
of the TOs. For both species of ants, ants became aggressive (running
agitatedly, opening mandibles, lunging, snapping mandibles) 95% of the
time when the TO physically touched the ant. When the TO did not touch
the ant, ants displayed aggressive behavior less than 1% of the time.
Thus the experimental evidence supports the hypothesis that TOs act as an indirect defense by making ants more aggressive or more vigilant.
The evidence also supports the hypothesis that the TOs produce a non-volatile or low-volatile stimulus to the ants. More on this on the next page.
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