moths & light
DR. JAMES ADAMS
JADAMS at em.daltonstate.edu
Mon May 15 13:06:15 EDT 2000
Ken Phillip wrote:
> Jim Mason asked:
>
> > What is the current best guess why moths are attracted to light? Why
> > are insects attracted to light? Or is it some light (wavelengths)
> > and not all?
>
> The 'textbook' answer is that many nocturnal insects navigate by
> maintaining a constant flight direction with respect to celestial
> light sources (so-called menotaxis). This mechanism will lead to a
> logarithmic spiral in to a nearby light source.
>
> There are a number of other ways that insects orient themselves
> to light, so the picture is no doubt more complex than that.
As before, I will paste here a rather lengthy response I supplied
back in 1997 for the first time. I keep it archived for moments such
as these. Indeed, the answer *is* more complex!
Below is a summary of some of what's known, as well as some
extrapolation from what's known. Also, for anyone who is
interested, I have two long postings of references on insects
attracted to light that I can repost. Just let me know.
As for the discussion that follows, some of the names of the
people involved in this discussion will be familiar to some of you,
some may not.
. . . asked about whether stars are really
bright enough to be used as navigational cues. The answer is
"yes."
In studies done on the sensitivity of moth optical neurons to light,
they have an extremely low threshold, meaning that even very low
levels of light will allow the moths to "see". Not see sharply, but
undoubtedly enough to avoid large (dark) objects. All organisms
need to be able to orient themselves/navigate within in their
environment in relation
to other objects (foodplants, mates, finding shelter, etc.). I grant
you that finding foodplants and mates for moths flying at night is
probably done mostly using chemical cues. However, when just
"cruising" moths need cues that will allow them to maintain
appropriate orientation to surrounding objects, the ground, etc.,
and
one of the cues that is consistent for moths is that light sources
(back before the advent of human-made light sources) were
predictably
"UP" (in the sky), as Ken suggests. I grant you that the moon is
not always at a
consistent angle upward in the sky, but it *is* always up, as are
the stars. And, by the way, in response to the question "Do
moths migrate", the answer is also "yes". Not necessarily in
masses (though these *do* ocur) like the Monarch Butterfly, and
not necessarily in a particular direction
at a particular time of the year, but moths do fly long distances and
likely use the sky light cues in the sky *directionally* as well
(though little research has been done on this). For instance, there
is some documentation for the movement of moths out of the west
coast lowlands of Costa Rica during the dry season up into the
moister
mountainous areas to the east, and back down into the coastal
dry
forest during the rainy season. The observation that moths
(appear
to [see below]) fly more strongly on humid, cloudy nights is also
well documented, but to say that it doesn't make much sense
therefore
that they would use lights as a navigational cue doesn't
necessarily
follow. Most animals have *more than one* way to navigate
through
their environment, and just because they don't necessarily use one
all the time doesn't mean they *can't*. (Moths *do* have functional
eyes, after all!!). Just as an off the wall example, homing pigeons
can use sun compass and magnetic cues to migrate -- a pigeon
released
on a sunny day with a small magnet attached to its head (which
screws up their magnetic sense) can still navigate just fine.
Unencumbered pigeons do just fine on a cloudy day as well.
However, release a pigeon with a magnet on its head on a cloudy
day and they fly around aimlessly. The point? Moths undoubtedly
can use lights as a
navigational cue, and along with gravitational cues, use the light
sources from above to maintain appropriate "up-down" orientation
in their environment.
Martin Damus added another message to the
discussion, and I have included a perfectly worded piece of his
discussion here, since it seems completely appropriate to the
point I was attempting to make.
". . . noctuid moths migrate using the moon as a primary reference
point. To calibrate the location in the sky with actual geographical
direction, they periodically use an internal geomagnetic compass.
In fact, every hour, they alter their flight path by 16 degrees to
correct for travel of the moon across the sky (for purists, rotation
of the earth). On moonless nights they navigate solely with the
geomagnetic compass. I guess using the moon is 'easier', and
therefore they 'prefer' that when it is visible, hence the screwup
when bright lights are visible."
Just as an aside, Jim Hanlon mentioned fish
coming to the surface of the ocean during a full moon -- this is well
documented for many fresh water fish as well. Perhaps one of the
reasons is that insects flying through their environment on a
well moonlit night will be confused by *reflected* moonlight from
below and go cascading down into the water (more food for the
fish).
I know this has come up in discussions with other entomologists
before, but I am unaware if this is actually scientifically
documented.
Now, why do moths come to lights? Someone has mentioned
something about moths perhaps being able to hear vibrations from
the light and come to the light based on certain sound frequencies
being produced by the lights. This *might* be a
possibility, but there are three things which suggest that this is
certainly not a likely mechanism. I doubt very seriously
that white light bulbs and white light bulbs painted yellow emit
significantly different sound impulses, and so this would not
explain
why moths come in to different colored lights in significantly
different numbers. Secondly, there are some families of moths
which, in essence, have *no* hearing capabilities whatsoever (for
instance, the Saturniidae [egs., Polyphemus, Cecropia, Io,
Imperial, etc.]), so without the ability to hear sound frequencies,
saturniids
and some other moths would never end up at lights, and this is
certainly not the case. Lastly, and perhaps most importantly,
there would have to be some reason why the sound is *meaningful*
to the moths, and I doubt that this would be likely for most moth
species. Only a few moths actually use any kind of acoustic
communication, so for the majority of moths, sounds at night are
likely to indicate danger (flee!) as opposed to some sort of
attraction.
The use of moon and stars as navigational
cues can at least partly explain why moths end up at lights. Both
Martin Damus and David Britton bring up the point about moths
maintaining a certain angle between themselves and light sources,
which explains the "spiralling in" that is easily observable in many
species as they come to lights, and which Ken refers to in the first
part of this message. Interestingly, some of the species
that do this most strikingly, such as the saturniids, are also some
of the species that do not have hearing apparati. The reason why
they stay at the lights (after "hit(ting) the moon, an
accomplishment they never evolved a decent response to." [nice
quote, Martin] is likely because, now close to a the bright light
source, the artificial "moon" has become the "sun", and the moths
settle down . . . for their daytime "sleep"
However, I do doubt that these moths are then as result (at least
not the first time they come into the lights) half-starved, as
someone suggested. The reason
why some moths simply fall from the wall when tapped has to do
with thermoregulation. Cooled moths (in many cases) require a
certain warm up period before flight (accomplished by "shivering"
[muscular thermogenesis]). Being half starved is irrelevant for
some species which do not feed as adults, such as the saturniids
mentioned above.
Using nighttime celestial lights sources as navigational cues
would
also be a convenient explanation as to why it appears that fewer
moths come to lights on well moonlit nights. Full moonlight is
harder to compete with using the superstimulating electric light
sources, as well as why it therefore may appear that there is
*more activity* at artificial light sources on cloudy nights, though
cloudy nights also tend to be warmer and more humid, which may
have more of an effect (as suggested by some of the Jan. 2000
posters) . However, this is certainly not the entire story.
Many, many moths, if you watch them come to lights, fly
*directly* at the light source as they come in, with little
indication of any spiral. Many of these moth species can here, so
it is possible, and I stress *remotely possible* that these moths
are perhaps using *both* a light and sonic cue to get to the light.
This does *not*, however, explain why the moths
"like" either the light or the sound. There is not a lot of actual
collected data which refers to this phenomenon.
Hope clarifies some points (though clearly muddies some
others) for continued discussion!
P.S. As to the decline of species because of artificial lighting,
the main reason for this (assuming it does happen) is the
predation
that occurs on the placid moths sitting on walls, etc. in the
morning
(not against some background that they might be camouflaged on).
I have seen numerous birds "cleaning up" in the early morning, as
well as Bald-Face Hornets and wasps of the genus Tripoxylon.
There may indeed be some selection going on for individual moths
that do *not*respond strongly to light cues (but, of course, if the
navigational argument is sound, it would be impossible to select
this out of a moth species!!).
-------------------
If you have other questions, please feel free to ask.
James
Dr. James K. Adams
Dept. of Natural Science and Math
Dalton State College
213 N. College Drive
Dalton, GA 30720
Phone: (706)272-4427; fax: (706)272-2533
U of Michigan's President James Angell's
Secret of Success: "Grow antennae, not horns"
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