Subject: spiders & Hemileuca/plea for locality info

From: "Mark K. Stowe" <[email-masked]>
Date: Fri, 5 Jul 1996 23:10:28 -0400

Because they are showy moths and their mating flights take place in
daylight, Hemileuca silk moths (Saturniidae) are relatively well-studied
and many of the details of their biology are described in the excellent
book

Tuskes, Paul M., Tuttle, James P. and Collins, Michael M.  1995.  The wild
silk moths of North America : a natural history of the Saturniidae of the
United States and Canada. 250pp. Ithaca, N.Y. Comstock Pub. Associates, The
Cornell series in arthropod biology

As the authors indicate in a brief discussion, for more than 20 years a
handful of lepidopterists and arachnologists have reported observing male
moths in the vicinity of various orb-weaving spider species (especially
Argiope species) flying in a manner that suggested that they were attracted
to the spiders or their webs.  While the strong impression of attraction
has never been verified by controlled tests, there is good reason to
believe that the phenomenon is worthy of further study.  Most of these
observers were surprised by what they saw - they had set out to study just
the spiders or the moths - and most were unaware that others had come to
similar conclusions.

Also spider attraction of male moth prey is not unprecedented - there are
other spiders in the same family (e.g. the bolas spiders in the genus
Mastophora) which spin reduced webs and are completely dependent on mimicry
of moth sex pheromones to attract their strictly male moth prey (for one
species I and chemists at the USDA have shown that the spiders emit
appropriate moth sex pheromone compounds).   So while Argiope aurantia and
the other reported orbweavers make large webs and appear to catch most of
their prey by chance, it is not inconceivable that they could be attracting
some prey (see below for further thoughts).

So far the phenomenon has been observed in H. lucina in Massachusetts, H.
hera in Oregon and Nevada, and H. electra clio in Arizona. The only
published literature I know of is a note by Horton (1979) in J. Arachnol.
7:88.  Study of the phenomenon is complicated by the fact that the areas
where the moths are abundant change frequently, and observers don't see the
phenomenon taking place every year (see below for further thoughts).

My purpose for writing is twofold.  Firstly I'd like to keep interest in
the phenomenon alive and encourage others to report any similar
observations to those of us who have a long term interest in the phenomenon
- particularly

myself ([email-masked] - PLEASE NOTE that I have so many newsgroups I
belong to that I can only occasionally monitor this one - so please get in
touch with me directly)
David McCorkle ([email-masked]) who put out an appeal similar
to this one on entomo-l
James Tuttle at [email-masked] who put out an appeal on sci.bio.entomology.misc.

Second I would like to get a chance to see and photograph some of the moths
when I am out in CA, AZ and NV next month (from Aug. 1 to 10).  Because the
moths are very patchy and populations vary enormously from year to year, I
am hoping that some of you will be kind enough to contact me directly and
provide me with some current, very specific locality information for H.
hera which should be active at the right altitudes in the Sierras in early
August, and possibly H. electra clio if you think there is any chance of
seeing it at that time.  I am unlikely to study the phenomenon myself (I
have much too much to do working with the moth-attracting specialists here
in Florida), but I am very interested in the 'big picture' of chemical
mimicry in spiders (and other organisms) and it would be great to have
pictures and to know more so that I can encourage others.

Thank you in advance for your help.  An excerpt of my most recent email to
Jim Tuttle with my thoughts on the phenomenon is pasted below in case it is
of interest.

Best wishes, Mark

 --------------
Mark Stowe c/o Jon Reiskind
Department of Zoology
University of Florida
Gainesville, FL 32611 USA

E-mail: [email-masked]

phone: (note new area code) HOME: 352 373 3202 voice mail comes on after ~
4 rings if no one is home, BUT AFTER ONLY 0-1 rings if we are on another
call - not like call waiting: after the call we learn there is a message
and then may/may not retrieve it LAB: 352 392 1187 can't leave a message,
but in the evening you won't waste any money calling here - I'm usually the
only one to pick up the phone FAX: 352 392 3704 dept. office, use
discretion, label "Stowe/Reiskind"

...  It would be nice to know that in particular it is not a visual signal
the moths are responding to (will they fly from down wind around an opaque
barrier to get to the web?).  Cay Craig has some evidence of visual
attraction of various insects to spider webs, but her somewhat
'hand-waving' explanation that webs mimic light gaps and flowers seem
unlikely to fit here.

To make a long story short here is my wild guess as to what is going on.
Chemical mimicry of sex pheromones has evolved at least 3 times and
possibly as many as 6 times in the spider superfamily Araneioidea (to get 6
one has to include some very casual published observations of some obscure
tropical genera).  Let us say that there is something in the biochemistry
of the spiders in this superfamily that preadapts them to becoming Lep sex
pheromone mimics. Let's say that they have enzymes which produce certain
families of lipids (that share a certain not very taxonomically widespread
double bond position perhaps) that they happen to use in their cuticle and
in their webs.  (Not much work has been done on web lipid chemistry but
there is a definite lipid component to webs - who knows why it's there - to
make the silk more easily drawn or to keep glues from drying out, who
knows. Probably the lipids in question are not part of the spiders own
pheromone communication systems since in all the prey-attracting
specialised groups, males and immature and mature nonvirgin females all
attract prey).

Suppose that these preadaptation lipids or their break-down products are
shaped vaguely like moth sex pheromones.  In a few groups some mutants
produced something closer to a moth sex pheromone which started the ball
rolling towards specialization and produced the prey attracting
specialists.

Perhaps in the Hemileuca phenomenon we see a stalled stage of this process.
The spiders produce (let's say for simplicity) one particular lipid (it
could be a blend of more than one) in this family of lipids which is a very
weak mimic of Hemileuca pheromones which attract some moths to their deaths
in the spiders webs.  However, this mimicry only benefits the small
percentage of the all the populations of this species which happen to
overlap with the moth species, and only benefits those populations during
the relatively small percentage of the spiders lifetime that the moths are
sexually active.  There is no selection pressure to increase the amounts of
the lipids (there may even be a negative selection pressure if egg case
parasitoids and predators are able to more easily detect particularly
smelly spiders).  Perhaps there are no simple one or two-mutation steps
>from  producing this weak Hemileuca pheromone mimic to producing a stronger
pheromone mimic (the specialized prey attracting spiders attract many
families of moths but the Saturniidae is not known to be one of them).

The attraction of moths to webs appears not to be very reproducable.
Perhaps this is because the lipid really is a marginal Hemileuca
attractant.  In many moth groups a small percentage of males will fly all
the way to an experimental odor source emitting a blend of pheromone
components that is very different from the optimal blend.  Perhaps to see
attraction to webs requires a really large outbreak of moths so that the
small percentage of males who are fooled and go to webs is noticable.  It
is possible that all the different spider species over their entire ranges
are producing the same basic attractant and this one spider compound, (or
set of compounds) is a weak attractant for a number of different Hemileuca
species which from the viewpoint of normal high attraction levels seem to
have incompatible sex pheromone chemistries.

There is another possible reason for the irreproducability of the
phenomenon.  All the prey attractors who apparently emit the attractants
>from  the web make very minimal webs and either can count on attracting the
prey very soon after the web is made, or replace the web frequently - a web
that is more than 30 minutes old is apparently not attractive.  The larger
webs of Argiope might be attractive for longer, but perhaps the phenomenon
requires that (by chance) the spiders have made their web that morning
within let's say an hour of when the moths are flying.

A third remotely possible reason for irreproducability is the fact that the
moths might learn to avoid the webs.  In the Ophrys and other orchids which
mimic hymenopteran female sex pheromones, many of the male pollinators show
evidence of eventually learning to avoid the flowers.  Admittedly
hymenopterans are more learning-oriented insects, but perhaps Hemileuca
males also learn and the phenomenon is more likely to be observed very
early in cases of relatively synchronised male emergence.

So if I were to test this phenomenon I would collect lots of fresh webs
ahead of time - either collapsing and ULT freezing them, or solvent
extracting them .  Then I would do controlled tests to see if newly emerged
(and therefore naive) male moths in a wind tunnel were attracted to
visually blocked collapsed web material or solvent web extract sources to
confirm that the phenomenon is real and olfactory.  (If it didn't work with
single moths I'd fly large numbers at once in case the moths get less fussy
when the competition is keener).

It might be possible to do tests in the field (again with visually blocked
webs to make sure the attraction is not visual) but from my own experience
with prey-attracting spiders (and others' experience with the
pheromone-mimicking orchids)  I can say that even for chemical mimics which
are absolutely dependent on attraction and which presumably produce
relatively effective mimic odors, doing bioassays in the field is tricky,
labor intensive and otherwise risky.

Using the usual EAG and wind tunnel behavioral bioassay techniques one
might be able to identify attractant compounds in the webs.  If (contrary
to the scenario developed so far) the moths *are* exerting some selective
force on the spiders, you might expect to see spiders producing a greater
quantity of the compounds and/or disparate, more effective blend ratios for
the moths in the regions where the spiders and moths overlap.