Bug Spotlight!

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U.C. Riverside
Department of Entomology
Photo by: Rick Vetter © (left)  
and Ken Haynes © (right)
Bolas Spider, Mastophora species (Arachnida: Araneidae) by Rick Vetter   

The bolas spider genus Mastophora includes 15 species in the United States with only one, M. cornigera, found in California (the genus was recently re-revised).  Bolas spiders are not common or at least not commonly collected.  In part, they hide during the day and are cryptic, looking very much like a bird dropping (figure top left, spider from California) with light streaks mimicking the uric acid found in bird feces.  Most bolas spiders are found by first locating their black and white mottled egg sacs (figure bottom left).  Although bolas spiders are in the orb-weaver family Araneidae, they have evolved away from making a large orb web of silk for hunting.  Instead, they produce just a single droplet of sticky silk, which they suspend at the bottom of a dragline (figure on right, spider from Kentucky ). 

How could they possibly catch much prey by just suspending a little sticky ball of silk?  Many years ago, it was determined that the prey of bolas spiders were very unique: male noctuid moths of only a few species.  How is the spider selectively preying on only a few male noctuids?  The female bolas spider sits out at night emitting the sex pheromone of female moths.  Male moths fly up to her thinking that they might get a chance to mate; when the bolas spider senses the air vibrations of the approaching moth’s wings, she swings her sticky globule of silk around, strikes the male moth and reels him in.  Of course, baby bolas spiders are too small to catch large noctuid moths and for years it was a mystery what they ate.  Finally it was determined their prey were humpbacked flies (Phoridae), but only male phorid flies, so the babies are doing something very interesting with sex pheromones as well.  Also, male bolas spiders emerge from their egg sacs as penultimates, meaning that they only go through one molt before they mature and therefore are very small in comparison to their female counterparts.   Finally, because of the bolas spiders’ ability to twirl its silk, one species in Central America was named Mastophora dizzydeani, after the American baseball Hall of Fame pitcher from the 1930s and 1940s.


©2006  ERM-UC Riverside