Why Are Moths Attracted To Lights?

[Stanley A. Gorodenski]

I also heard the talk at the Aug Lep Soc meeting on the effects of the
dark vs. full moon on trap catches.  Although the speaker concluded the
full moon has no effect, and had a graph to support his conclusion, I
observed that the graph did seem to support a small effect.  If I
remember correctly, there were some small differences in peak catches. 
If I also remember correctly, the effects due to temperature differences
were in the trough part of the graph.  However, my observation may be
faulty because of the short amount of time I had to take in the graph,
and the details of the graph just described may be incorrect due to my
poor recollection - it's now five months since the meeting.  In any
event, at the end of the talk I did not feel the question had been
definitively answered.  It seems that the effect of the full moon is
something that should be able to easily be resolved with additional
research.

Stan Gorodenski


Kenelm Philip wrote:
> 
>         The anonymous posting that started this thread raised enough
> questions that I hardly know where to start--but here's an attmempt:
> 
> 1) "They want to sleep." One could equally well say that moths being
> inactive during the day really want to be flying around. Give them
> sufficient darkness, and they're off.
> 
>         The implication here, if I understand the argument, is that moths
> deliberately fly towards lights in order that they may sleep. According
> to that idea, moths should fly very high on moonlit nights--yet an inter-
> esting paper at the August Lep. Soc. meeting pointed out that light
> trapping for many moths is no worse on full-moon nights (when you allow
> for temperature) than at the dark of the moon. Also, one might expect
> a mass eastward flight at sunrise, and westward at sunset--something I
> have not observed, or heard of.
> 
>         I fail to see any requirement to presuppose a 'desire' for sleep
> in moths. Daylight inactivates them, as might be expected for nocturnal
> organisms.
> 
> 2)  "Moths do NOT travel towards lights for some navigational purposes."
> 
> "Hasn't anybody ever stopped to realise that a moth travelling in a
> straight line by navigation using moon-light or star-light (the usual old
> theory) would serve ZERO purpose to a moth?"
> 
>         Insect flight poses some interesting problems involving stability
> and orientation. Moths _can_ in fact fly in straight lines--anyone who has
> kicked up an Arctiid in the tundra has one swing before it disappears
> from sight. Furthermore, the main utility of flight is for getting from
> point A to point B, which requires something approaching the ability for
> straight-line flight. Anyone who has spent much time observing flying
> insects will be impressed by their powers of straight-line flight.
> For insects to fly straight, a number of orientation mechanisms must be
> called into play (see any insect physiology text). One of these, called
> _menotaxis_, involves maintaining a constant angle to a distant light.
> This has been _demonstrated_ experimentally in a number of insects, in-
> cluding caterpillars (which have been observed to crawl to light in
> precisely the logarithmic spiral expected from menotaxis). I have not
> seen experiments on adult moths along these lines--but the vanes in most
> UV traps are there to intercept moths circling about the lamp. Anonymous
> would have to posit a _second_ orientation mechanism to change from his
> (presumed?) direct flight toward a light, to circling around it at close
> range.
> 
>         Anonymous clearly agrees that moths can fly straight--he just
> refuses to accept menotaxis as a mechanism, even though it is well con-
> firmed experimentally with a number of insects. I think some experiments
> are called for, if he wishes to change people's thinking on these matters.
> I am reminded of Callahan's idea that moths detect infrared rather than
> UV. He also reported observations that, in his mind, supported that idea--
> but I have never heard of _experiments_ confirming his theory. Observa-
> tion is indeed an important part of science--but in many cases one little
> experiment can cut like a knife through a fog of passive observational
> evidence.
> 
>         As far as the deleterious effects of light pollution go, I have no
> quarrel with Anonymous. (In fact, I can add astronomers as another class
> of organism that is affected by this problem.) Outdoor lighting's effect
> on moths has been known, and talked about, for decades. I do not, however,
> have access to any quantitative data comparing, for example, the effects
> of streetlights versus the predation of a large bat colony (some of which
> consume _tons_ of insects per night).
> 
>                                                         Ken Philip
> fnkwp at uaf.edu

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