Tuesday, October 20

Bright yellow spots help some orb weaver spiders lure their next meal

Many orb weaver spiders sport yellowish stripes or spots on
their undersides, and for a good reason. That color yellow tempts bees and
flies into a spider’s web, a new study suggests.

Orb weaver spiders get their name because they spin and sit on
circular webs (SN: 8/8/17). But
these spiders and their bright colors are a paradox. Why would a predator that relies
on stealth for its next meal look so conspicuous? Scientists have hypothesized
that bright colors on orb weaver spiders might serve to warn predators, to
blend into vegetation or to attract prey.

In the new study, researchers examined if yellow colorations
on a species of golden orb weaver spider (Nephila pilipes) attract their
flying insect prey. Found across Asia, this spider sits on its web day and
night with its underside — mottled and striped yellow on black — facing open
space. The team found more than 250 wild N. pilipes females in the wild.
They removed each female and either left its web vacant or replaced it with a cardboard
spider. These cardboard models had paper strips of yellow, blue or black color
glued onto them.

After almost 1,800 hours of video recording the faux
arachnids, the team found that during the daytime, the yellow-striped model
that resembled a real N. pilipes attracted
more than twice as many insects, including bees and flies, as any other fake
spider or empty web. What’s more, the yellow color worked just as well at night
attracting moths, the scientists report online February 11 in Functional Ecology.

The team then scoured online zoological databases for associations
between yellow markings and prey attraction in orb weaver spiders. Surveying dozens
of distantly related species revealed that yellow stripes or spots were more
likely to have evolved in orb weaver spiders that sit on their webs in open,
bright spaces, where visual baits may be more effective.

The research “reinforces that the color yellow lures insects,”
says Nathalia Ximenes, a behavioral ecologist at the University of São
Paulo in Brazil who studies coloration in orb weaver spiders but was not
involved in the work. Scientists don’t yet know why insects are attracted to
yellow on orb weaver spiders. Perhaps the prey mistake a spider for a
yellow-flecked flower, a hypothesis supported by the fact that most prey
attracted were pollinators.

Understanding the function of color patterns in animals is a
fundamental question for evolutionary biologists, says study coauthor Mark
Elgar, an evolutionary biologist at University of Melbourne in Australia. Studying
animal colorations, he says, can also inform practical applications. He cites
an example of how a team member’s interest in animal colors had led to
researching light reflectance with “interesting applied opportunities” in
energy storage.