moths & light

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  
 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., 
 one of the cues that is consistent for moths is that light sources 
 (back before the advent of human-made light sources) were  
"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  
 mountainous areas to the east, and back down into the coastal 
 forest during the rainy season.  The observation that moths 
 to [see below]) fly more strongly on humid, cloudy nights is also 
 well documented, but to say that it doesn't make much sense  
 that they would use lights as a navigational cue doesn't 
 follow.  Most animals have *more than one* way to navigate  
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  
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 
I know this has come up in discussions with other entomologists 
before, but I am unaware if this is actually scientifically  

    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 
 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,  
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  
   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 
 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 
that occurs on the placid moths sitting on walls, etc. in the 
(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.


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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