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Can Monarchs Detect Sound or Do They Live in a Silent World?
Dr. Orley R. "Chip" Taylor, Professor of Entomology at the University of Kansas,
www.MonarchWatch.org
Photo by Paul B. Southerland
There is a well known "Far Side" cartoon by Gary Larson in which a person is giving his dog commands with a balloon over the dog's head indicating what the dog is actually hearing: "blah, blah, blah, blah". As someone who has attempted to train a dog or two, this cartoon is right on. What we say and what is understood are often distinctly different. In fact, what we say to some of the creatures we talk to may not be heard at all.
 Have
you ever talked to your butterflies or caterpillars? Do you think
they can hear you? Ok, so you're too embarrassed to admit talking
to some of the lesser critters you've encountered; nevertheless, it
is of interest to know if we are heard, even if we don't expect to
be understood. Well, I'll admit it. I've talked to caterpillars and
I know they can hear me.
Often, when rearing large numbers of monarch larvae, I've demonstrated to visitors that they can perceive sound. The next time you have an open cage with a large number of fifth instar larvae, walk up to the cage and say "Hello" in strong bass tones. The larvae, often in unison, will rear back and swing the fore part of their body from side to side. If you say "Hello" sharply enough, about three times, some of the larvae will drop from the plants. High frequency sound seems to be less effective in eliciting this response. The literature suggests that in many species caterpillars have hairs that are sensitive to low frequency sounds. Why they respond to sound in this manner is a matter of conjecture but one possibility is that the rearing back and head swinging is a defensive behavior designed to drive off predators or parasites.
What about adult monarch butterflies? Well, I've talked to them too
but I've never been able to detect a response — nor did I expect
to. Actually, some butterflies have hearing organs (Vogel's organ)
at the base of the underside of the wings, e.g., longwings (Heliconius),
crackers (Hamadryas) and several satyrids (Cercyonis, Erebia, Maniola,
etc.). These structures are analogous to the human eardrum in that
there is a membrane (tympanum) that receives the sound. Sound induced
vibrations in the tympanum stimulate nerves in a chordodontal organ
attached to its inner surface. One group of primitive butterflies
with hearing organs on the wings, Hedyloidea or hedylids, has been
shown to respond to bat sonar. This is the only group of butterflies
known to fly at night. The hedylids seem to be a transitional group
between butterflies and moths, with immatures similar to those of
butterflies but with adult features that are more moth-like. Auditory
organs are used to detect and avoid bats in several families of moths.
Many butterfly web sites declare that butterflies hear with their wings. I know of no evidence supporting such a statement for species that lack distinct hearing organs. Airborne sounds, as opposed to substrate vibrations, strong enough to vibrate wings are likely to be detected on sensory hairs on the legs or mouthparts before they are "detected" by wings. If the hedylids with hearing organs are ancestral to modern butterflies, these organs were lost in most species as the diurnal habit was acquired. In other words, if butterflies are derived from night flying insects with auditory organs, most species have lost this function and live in a silent world unless they have hearing organs (e.g., specialized labial palps such as those found on the mouthparts of some hawkmoths) that have yet to be detected.
Highly developed structures such as Vogel's organs of some butterflies and the hearing organs of moths are associated with specific functions in which detection of sound facilitates avoidance of predators, species recognition, or courtship. In cracker butterflies (Hamadryas), males perch on tree trunks and produce sound as they fly in pursuit of passing butterflies. This sound is evidently detected by the Vogel's organ on the wings of their own and related species but is probably unheard by unrelated species. It seems likely that other butterfly species (Heliconius, Maniola, etc.) with hearing organs also employ sound in recognition or courtship although this has yet to be established. At present, there is no indication that adult monarchs, and the majority of butterflies, produce sounds or use them to communicate and therefore have no refined sense of hearing or hearing organs.
For more information, visit www.MonarchWatch.org.
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