The orb-weaver spiders (family Araneidae) are the builders of spiral wheel-shaped webs often found in gardens, fields and forests. The family is a large one, including
over 2800 species in over 160 genera worldwide, making it the third largest family
of spiders known (behind Salticidae and Linyphiidae). The oldest known orb-weaving
spider is Mesozygiella dunlopi, with specimens in amber dating from the
Early Cretaceous.
Generally, orb-weaving spiders are three-clawed
builders of flat webs with sticky spiral capture silk. The building of a web is
an engineering feat, begun when the spider floats a line on the wind to another
surface. The spider secures the line and then drops another line from the center,
making a "Y". The rest of the scaffolding follows with many radii of non-sticky
silk being constructed before a final spiral of sticky capture silk. The
third claw is used to walk on the non-sticky part of the web. Characteristically,
the prey insect that blunders into the sticky lines is stunned by a quick bite
and then wrapped in silk. If the prey is a venomous insect, such as a wasp,
wrapping may precede biting.
A female marbled orb weaver (Araneus
marmoreus)Some "orb-weavers" do not build webs at all. Members of the genera
Mastophora in the Americas and Dicrostichus in Australia produce sticky globules,
which contain a pheromone analog. The globule is hung from a silken thread
dangled by the spider from its front legs. The pheromone analog attracts male
moths of only a few species. These get stuck on the globule and are reeled in
to be eaten. Interestingly, both types of "bolas" spiders are highly camouflaged
and difficult to locate.
The spiny orb-weaving spiders in the genera
Gasteracantha and Micrathena look like plant seeds or thorns hanging in their
orb-webs. Some species of Gasteracantha have very long horn-like spines protruding
from their abdomens.
One feature of the webs of some orb-weavers
is the stabilimentum, a crisscross band of silk through the center of the web.
It is found in a number of genera, but Argiope, which includes the common garden
spider of Europe as well as the yellow and banded garden spiders of North
America, is a prime example. The band has been hypothesized to be a lure for prey,
a marker to warn birds away from the web and a camouflage for the spider when
it sits in the center of the web.
Most arachnid webs are vertical
and the spiders usually hang with their head downward. A few webs, such as those
of orb-weaver in the genus Metepiera have the orb hidden within a tangled space
of web. Some Metepiera are semi-social and live in communal webs. In Mexico
such communal webs have been cut out of trees or bushes and used for living fly
paper.
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