George W. Uetz Professor Ph.D., University of Illinois Animal Behavior, Behavioral Ecology, & Arachnology

The Spider and Fly Revisited
ploy-counterploy behavior in a unique predator-prey system

Parasitoid fly Arachnidomyia lindae

            Colonial web-building M. incrassata exhibit predator-specific defenses against the predatory fly, Arachnidomyia lindae, which attacks egg sacs. Flies deposit a larva onto the egg sac surface, and the developing larvae eat the eggs inside. Observations of specialized behaviors for attack (flies) and defense (spiders) suggest that this predator-prey relationship may incorporate elements of ploy - counterploy behavior.

             M. incrassata recognizes this predatory fly through its wing-beat signature, and guards the egg sac with specialized behaviors, including: “shuttle” (the spider manuevers around the egg sac to position itself between the fly and the egg sac); and “groom” (the spider searches the surface of the egg sac with its mouthparts to remove larva deposited by the fly).

            In the photo, Dr. Craig Hieber is testing this hypothesis by comparing the responses of spiders to tethered flies. 

 Spiders clearly discriminated predatory A. lindae from the housefly, Musca domestica. These flies look alike and sound similar, but wingbeat frequencies are different enough for spiders to recognize. Spiders exhibited specialized defensive behaviors toward A. lindae (but not M. domestica). The adaptive value of these behaviors was demonstrated in experiments removing females from egg sacs – unguarded sacs had 2-4 times greater risk of predation.

Click to hear the wing beat signature of Arachnidomyia lindae and Musca domestica Can you hear the difference?

         The shuttle and groom defensive behaviors of the spider, and the attack behavior of the fly can be seen in the video clip (6.78 MB) and photos. In response to the spider’s defense, A. lindae uses a deceptive behavioral ploy for circumventing spider guarding behavior (by vibrating the web hub as in the photo, mimicking captured prey in the web). In turn, M. incrassata exhibits a counter-ploy behavior (signal thread cutting) to eliminate this potentially conflicting vibratory information.  These ploy and counter-ploy interactions suggest that the behavior of these two ecologically-linked species may have co-evolved in a stepwise fashion.  To view the full text of this article in Adobe Acrobat .pdf format click here.

M. incrassta being presented with a M. domestica on a stick	A. lindae  at hub of M. incrassata web	Dr. Craig Hieber testing spider responses to tethered fly

Our research has been reviewed in Nature's Science Update Online, on the National Science Foundation's news site, the University of Cincinnati's news section, as well as an 8 minute interview with Dr. G. W. Uetz on the Canadian Broadcasting Corporation's radio show Quirks and Quarks (the full interview is available for download on the site in .mp3 format).

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 Research: 1. Wolf Spider Research | 2. Colonial Web-Building Spiders Research | 3. El Niño Influence on Colonial Web-Building Spiders. |  4. Impact of a Ecosystem Disturbance on Spider Populations and Communities