Why Are Moths Attracted To Lights?

[Kenelm Philip]

	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