Moths coming to lights at night

[Dr. James Adams]

To interested parties,

    I have been following the discussion about moths coming to lights 
with some interest, and several points have arisen in the discussion 
(involving Liz Day, Jim Hanlon, Ian W., David Britton, and GSlS dog 
[the initiator of this discussion]) that I would like to address.

First things first.  GSlS dog 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, as suggested by 
Ian.  Liz asked some related questions, namely "1) Why would moths 
need to navigate (using light)?"  Well, if you don't like the word 
navigate, perhaps orient would be a better answer.  All organisms 
need to be able to orient themselves 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 
almost exclusively done 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).  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 your question "Do moths 
migrate", the answer is also "yes".  Not necessarily in masses 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?  They 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.  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?  To address your other point 
Liz, you mentioned something about moths perhaps being able to hear 
the light and come to the light based on certain sound frequencies 
being produced by the lights.  I agree that this may be a 
possibility, but there are two things which suggest that this is 
certainly not the only (main?) 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.  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.  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, I like 
it!]), is likely because, now close to a the bright light source, the 
artificial "moon" has become the "sun", and the moths settle down.  
However, I do doubt that these moths are then as result (at least not 
the first time they come into the lights) half-starved.  The reason 
why some moths simply fall from the wall when tap 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 (thanks for the work, David), as well as why it therefore 
appears that there is *more activity* at artificial light sources on 
cloudy nights.  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.  This may get back to Liz's point about 
potential use of sonic cues.  Many moths can hear, and it is these 
moths which appear to come more directly at the light.  My bet is 
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.  And I certainly agree again 
with you, Liz, in that this would make an excellent research project.

    Hope clarifies some points, as well as muddies some others for 
continued discussion!!!

    James Adams