Butterfly Evolution Can Also Be Driven By Spiders
March 13, 2013

Jumping Spiders Miss The Mark When Chasing Hairstreak Butterflies

April Flowers for redOrbit.com - Your Universe Online

Butterflies are vibrant and colorful insects, with colorations designed to deflect predators. A new study from the University of Florida reveals that some of these predator driven defenses may be caused by enemies one-tenth the size of the butterfly.

Since Darwin sailed on the Beagle over 150 years ago, scientists have theorized that the main influence on the evolution of coloration in butterflies came from large predators such as birds. UF lepidopterist Andrei Sourakov conducted the first behavioral study to directly test the defense mechanism of hairstreak butterflies. Sourakov found that the appearance of a false head — a wing pattern found on hundreds of hairstreak butterflies worldwide — was 100 percent effective against jumping spider attacks. These findings suggest that small arthropods, rather than larger vertebrate predators, influence butterfly evolution.

“Everything we observe out there has been blamed on birds: aposematic coloration, mimicry and various defensive patterns like eyespots,” said Sourakov, a collection coordinator at the Florida Museum of Natural History´s McGuire Center for Lepidoptera and Biodiversity on the UF campus. “It´s a big step in general and a big leap of faith to realize that a creature as tiny as a jumping spider, whose brain and life span are really small compared to birds, can actually be partially responsible for the great diversity of patterns that evolved out there among Lepidoptera and other insects.”

In laboratory experiments, the Red-banded Hairstreak butterfly (Calycopis cecrops) successfully escaped all 16 attacks from the jumping spider Phidippus pulcherrimus. Eleven other butterfly and moth species from seven different families were exposed to the jumping spider as well, and not one was able to escape attack.

“From the video, you can see the spider is always very precise,” Sourakov said. “In one video, the spider sees a moth that looks like a leaf and it walks very carefully around to the head and then jumps at the head region. The spider has an innate or acquired ability to distinguish the head region very well and it always attacks there to deliver its venom to the vital center to instantly paralyze the prey. Most importantly, the spider is very small, so sometimes its prey is 10 times larger.”

Both the Hairstreak butterfly species and the jumping spider species are common in the southeastern US, with similar relatives distributed globally. In their natural habitats, the butterfly and the spider come into contact when the butterfly lands on leaves or flowers to rest and feed. Leaf litter, where female Red-banded Hairstreak butterflies lay their eggs, are often crawling with spiders.

The research shows that scientists need to rethink what drives adaptive coloration patterns, said David Wagner, a professor of ecology and evolutionary biology at the University of Connecticut who was not involved with the study, because “birds are only part of the story.”

“I´m just so impressed with Andrei´s experimental protocol and the fact that the jumping spider could not catch the hairstreak butterflies,” Wagner said. “His empirical study will do much to cause us to rethink the vision and the visual acuity that certain invertebrate predators have when hunting their prey and how this has really molded how some organisms not only look like, but perhaps how they act, as well.”

Hairstreak butterflies, unlike other species, constantly move the hind wings that carry the false head pattern. This behavior seems to increase in the presence of the spider, as if the butterfly is attracting attention to itself. Hairstreak specimens in museum collections are frequently found with the false-head portion of the wings missing. In the experimental exposures, the spider always attacked the false head, avoiding its vital organs.

“The false head hypothesis in hairstreaks has been in circulation for a long time because people always speculated that their tails move around in order to fake out the predators, but there was little experimental evidence,” Sourakov said.

Sourakov would like his results to encourage behavioral ecologists to further test the idea that spiders may drive butterfly evolution.

“This clearly shows it´s possible that many spectacular patterns that we find in smaller insects may be due to spider pressure rather than bird pressure,” Sourakov said. “The butterfly escapes from the spider — it´s a fairytale story.”

The results of this study were published in a recent issue of the Journal of Natural History.