Squatting spiders
In summer 2005 while I was
walking into the Maremma Natural Park (in Tuscany) looking for interesting
arthropods, my attention was drawn by a Cyrtophora citricola web. This conspicuous
spider (more then 1 cm long) builds an interesting and particularly efficient
web, which is a modification of the original orb web design of her relatives
(Araneidae like the garden spider). Apparently a mess, the web is an intricate
thread of silk with a collecting layer. Cyrtophora is so efficient that is
going to become a pest in countries where it was not present before, like in
the Caribean area.
However, this time was not the
Cyrtophora herself to attract me, rather a group of smaller and strange
inhabitants of that web. They were microscopic spiders (2-3 mm long) with a
silvery and conical abdomen, and some of them (the males) with a strange tower
like protrusion of the cephalothorax that ended with two knobs, on top of which
there were the eyes. Hanged up side down, and consequently moving on the host
web facing the ground, they were not eaten by the larger web owner. On the
contrary they consume the small preys trapped into the Cyrtophora web. I
immediately recognize them as Argyrodes. These spiders belong to the same
family of the famous black widow (Therididae). Argyrodes (celebrated also in
the new BBC documentary "Life in the undergrowth" presented by Sir
David Attenborough) are effective squatters and kleptoparasites. They live
close or in the web of a bigger hosts and take advantage of the larger
dimension of those webs to collect food.
Almost Cosmopolitan, they live
all-around the world and squat Araneids and other web builders. In Italy I sow
them only in the center and south and not in the north where I live.
My observed micro-thieves spent
much of their time still in two different positions: with the legs relaxed
around the body, or with the pair of the first two legs stretched forward.
Sometimes they moved very slowly in the host web, but even in this case it
seems that they were not detected by the Cyrtophora. However, they are not slow
at all. One day while I was peering in a Cyrtophora web, and the big owner
where wrapping a large prey with her silk, an Argyrodes moved with an unusual
speed toward the action point, and started biting and sucking the prey from the
Cyrtofora opposite (and hidden) side. It seems to me that Cyrtophora did not
pay much attention to that kind of commensality.
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Argyrodes female with young of Cyrtophora
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However, other Argyrodes in New
Zeland, were reported not only to steal food but also attacking in pack and
eating the large web owners, such as Nephila spiders. In my experience, no more
than five individuals were present in a host web, and I have not ever noticed
any kind of attack to the host from Argyrodes. Nonetheless, I am not really
sure about safety of the Cyrtophora babies when these tiny thieves are around.
One hot afternoon, I came across a Cyrtophora web that contained newly hatched
spiderlings under the guard of the powerful mother. In spite of this, a pair of
Argyrodes were freely moving in the middle of the spiderling group and seemed
really interested of the new event.
The year after I returned to
Maremma Natural Park with the aim of dedicating more time looking for
Argyrodes. Since they are so tiny I brought with me not only my preferred Kiron
105/2.8 macro 1:1 lens, but also the additional lens Rynox 6+ diopters, to be
able to capture higher reproduction ratio. During my field searches I found
that this tiny squatters not only inhabit the web of Cyrtophora, but I found
them also in those of Argiope bruennichi (the so called orb-web wasp spider),
and the in those of the grass funnel-weaver (Agelena labirynthica). In this
latter web, Argyrodes were less silvery and more reddish (maybe they are
another species).
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Argyrodes with a Argiope female
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Argyrodes with a Argiope female
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Differently from the Cyrtophora
aplomb, Argiope was really upset of having those thieves around. Actually she
chased them when they approached the wrapped preys, or just when she came
across one of them. In some cases the thieves were forced to drop out of the
Argiope web and they remained attached to that with their safety dragline. Like
expert climbers they recovered and returned to the host web when sure to be out
of danger. Despite of the Argiope behavior, Argyroredes were usually able to
feed of the preys just captured and wrapped by the web owner. Usually by
cutting them from the web and running away on the web borders, or even eating
together with Argiope. In a particular case an Argyrodes ate so much of the
Argiope prey that her conical abdomen became almost a small tiny ball.
On the contrary of what I
noticed for the two araneids, in the Agelena funnel web I have not ever seen
any thieving activities. All the Argyrodes there ate the preys that got trapped
and were too small to be of interest for the funnel weaver. I also counted more
Argyrodes in these webs than in those of the araneids.
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Argyrodes just after
mating
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Argyrodes mating: approaching
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One particular day, I acted as
voyeur observing two of them mating in a Agelena web. The male that was no more
than 2mm long approached a resting female (of almost the same size) with a
clearly exited walking, since differently from the other times he oscillated
very much up and down. When he met the female I thought that the poor chap
would have became an unexpected meal. Actually, she grabbed the male head and
when he ran away she chased him for some centimeters. The fellow was not
frightened at all and when she stop chasing, he triumphantly returned toward
her with his cowboy like walk and bounced her head. She then repeated the
aggressive action again, but with less impetus. This dramatic dance continued
for several times, and any time with less vigor. Until she grabbed the male
head and mouthed it! Yes the female jaws got locked into the male head. The two
male knobs have two grooves which are perfect to accommodate the female jaws.
This gives a simple reason for the strange male head shape. It seems that also
attractive substances are secreted from the male head in this phase (probably
pheromones). Then, the male poked his palps one at a time into the female
sexual organ (epigynus). During their mating sometimes both relaxed and the
female sometimes freed the male head and mouthed it again. After fifteen
minutes they stopped, moved a pair of centimeter apart and relaxed for few
seconds, then while the male groomed his palps the female moved again toward
him and they rejoined. This lasted for about two hours and they mated for six
times. Not a bad performance for a 2mm long spiders. Isn’t it?
When the copulation ended, I
looked around and I stood up, then I noticed that I was not alone, since the
noise I produced made a pair of fallow-deer ran away. Then I realized that I
was one of the few people in the world who had spent two incredible hours
observing a pair of 2 millimeters long creatures mating. I breathed deeply and
I felt stupidly happy.