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
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
©2006 ERM-UC Riverside