From: gamckeon@unix1.tcd.ie (The Blob)
Subject: Re: questions about spiders
Organization: Trinity College, Dublin
Distribution: sci
Date: Fri, 2 Oct 1992 14:00:06 GMT

In <ARA.92Sep27232431@camelot.ai.mit.edu> ara@zurich.ai.mit.edu (Allan Adler) writes:

>After stringing the web from the outside in, using the old foundations and
>building on them, she stopped at one point and gathered up the center of
>the web and ate it. As soon as she had done this, about half a dozen strands
>came out of her abdomen at once, instead of just one. These she spread
>thickly all over the center and finished by spreading it thickly in one
>narrow direction downward from the center. She just sits on the center
>and waits. 

I don't claim to be an expert on this but I have read something about
spiders from various sources and I do know some of the answers to your
questions. 

>Anyway, I have a lot of questions about this. First, is the old center that
>she ate immediately recycled as the new stuff that came out of her abdomen?

No it isn't. The reason she eats the web is partly to do with economics;
the old web contains ingredients which can be reused to make a new one. It
is therefore more logical to eat the old web, since by doing this she can
reduce the chore of having to search for more of these ingredients.
Since it takes time for the old web to be digested this means that a certain
time must elapse before the fresh silk which would be made from the old
web can be produced. As a result, the old centre is not immediately
recycled as the new silk that emerged from her abdomen.
 
>Why did she eat it instead of just spinning more? 

I have partly explained the reason for this above, but as far as I know
(and I'm not completely certain of this) eating the web also stimulates the
production of more silk which she then uses to make a new web.

>Is the web in the center of different composition than the rest of the web?

Yes, to a minor extent it is. For some species of spider the centre of
the web is designed to be an area of hiding. This means that in order to
prevent her getting stuck in *her* own web the centre is kept free of the
'glue' which she spreads on the rest of the web to help trap insects. 
This is the only difference that I know of between the composition of the
centre and the rest of the web.

>Is it sticky or soft and comfortable (if you are a spider).

The centre isn't sticky. It isn't soft either since the silken threads
consist of the same building structure as those in the rest of the web.

>Whereas tiny insects that alight on the rest of the web get stuck, I saw a 
>tiny fly walking on the center pad without getting caught, which suggests
>that the center part is not sticky. What purpose does the center part serve? 

As was mentioned above, the centre is used as an area of hiding for the
spider while she is waiting for her dinner. The reason why she chooses it is
because the centre of the web has the characteristic of being the region
where _all_ vibrations that the web picks up are always transmitted to
this area. Since she depends on these vibrations to tell her when
'dinner' has been caught it would seem to be the best place to go when
hiding. Therefore the centre serves as two things; an effective area for
reception of vibrations, and a ideal place of concealment where she can
reach all parts of the web as soon as possible thus reducing the chances
of the escape of her prey. 

>There were some fairly strong winds and it doesn't seem like there is much
>holding the spider to the web. So how does the spider manage to have such
>a good grip on the web? 

I'm not entirely sure but two methods come to mind, the second of which
I think is the real method. Have you ever wondered how a common housefly
can manage to walk on the ceiling? It does this through secretions of a
sticky mucous on the bottom of its feet. These secretions are strong
enough to hold the fly on the ceiling or wall. The spider's excellent
grip may come about via the same principle, ie; it's feet may also secrete
such a mucous thus enabling it to stick firmly to the stands of the web.

The second method (which as far as I remember is the actual method)
involves a special adaptation at the bottom of the spider's foot in the
form of a claw (not unlike the biting mouthpiece of an ant). By closing
these claws around the threads of the web the spider would be able to
grip the web very stongly and this would allow her to remain on the web
during strong winds.

>Is there some reason why she likes my window as opposed to some other
>place? For example, at night the light in my apartment attracts bugs to the
>window and I am sure that is good for business.
 
The likelihood is that your windowsil has ideal anchorage possibilities
for a web. Therefore the spider would be attracted to it. Other than
that there isn't any other reason. The example that you gave above would
be an added bonus but if you consider that a spider will build a web
between two branches of a tree as readily as in your window it would
seem that light wouldn't have much to do with it. 

>Is the pattern in the center supposed to look like a leaf or is the
>downward direction of the thickly layered strands supposed to be the
>entrance to her boudoir?

I don't know. Webs can vary from species to species. Since I haven't
seem the web I can't really say. (I still probably couldn't say even if
I did)

>Also, which parts of the web are sticky and how can one tell by looking
>at them? It did seem like some of the strands were like strings of tiny
>pearls, but that might have been moisture.

There are two parts of the web that aren't sticky; the centre is one
of them. There are also regions which have been left bare by the spider
so that it can walk around the web without becoming entangled itself in
it. These are a type of 'safe passageway' or secret route on the web.
 
>Pointers to any good books or papers where I could get answers to all the above
>would also be welcome.

David Attenborrough (I hope I spelt that correctly) has written a few
excellent books, not dedicated to the spider, but which give some
interesting bits of information about spiders. Try his 'Life on Earth'
trilogy that came onto the market one or two years ago. He also produced
an excellent wildlife series for television which also at one stage or
another deals with spiders.
If I can answer any more questions for you then just mail me.


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