Subject: Bird Poop Mimic Spider
Location: Mount Washington, Los Angeles, California
March 11, 2014 7:30 PM
We noticed this Spider under the post for the bird feeder, and we were struck by its excellent mimicry of bird droppings, but we could not turn around to take photos prior to leaving for work this morning. We remembered the spider as it was getting dark, but we decided to take a few images anyways. Tomorrow we plan to attempt to reshoot with more light, hopefully getting sharper images with better exposure.
We quickly matched this spiders interesting coloration and distinctive shape to a Bolas Spider, Mastophora cornigera, pictured on BugGuide. According to BugGuide: “Glistening appearance, like a fresh bird dropping, and pair of lumps on the dorsal surface of the abdomen seem to be genus-wide traits. The female spiders can be narrowed down by whether they have abdominal humps or not. However, this field marking does not work for males which can have humps or no humps in the same species” and “The only species in the west is M. cornigera.” BugGuide also notes that they feed on: “Flying insects; certain species specialize on particular species of moths, to the point of releasing mimics of their pheromones in order to attract prey (virtually all male moths) within capture range.” BugGuide also provides this information on the life cycle: “When egg sacs hatch they release immature females and *mature* males! Presumably an adaptation to avoid inbreeding. Males are short-lived and much smaller (obviously) than females.” This same behavior applies to a Bird Dropping Spider from a different genus found in Australia, according to the Victoria Museum Website which state: “During the day, female Bird-dropping Spiders sit motionless with their legs drawn up against their body; this behaviour combined with their humped abdomen and black and white colouring makes them look just like bird poo. This is a brilliant evolutionary strategy: no one wants to eat bird poo! Providing the spider doesn’t move and give away its cover, it will not draw the attention of predators. The male, as is often the case with spider species, is much smaller than the female. The hunting behaviour of this species is just as remarkable as its appearance: Bird-dropping Spiders releases a smell which resembles the sex pheromone that female moths use to attract males. When male moths fly in to investigate, ready to mate, they are grabbed by a Bird-dropping Spider.”