Web Manipulation Explains Spider Behavior
Widow Spiders Alter Web Architecture in Response to Hunger
Oct 31, 2009
This article is the second page of a two-page article on how black widow spiders prevent overeating. Introductory information is on page 1.
In an experiment with Western widow spiders, small spiders were allowed to feed on crickets for two weeks until they had grown to about the size of initially larger spiders. The large spiders were kept hungry. The sated spiders produced tangled webs without sticky threads, while the hungry spiders produced sheet webs with sticky threads.
One half of the large spiders were placed on the webs of the now sated small spiders and one half of the sated spiders were placed on the webs of the large spiders and observed when crickets were released into the chambers. Spiders were not held in the chambers long enough to modify the webs.
- Of the hungry spiders on satiated spider webs captured and ate an average of one cricket each. Sixty percent of the spiders ate crickets.
- Of the hungry (large) spiders on their own webs captured and ate an average of two crickets each. Eighty five percent of the spiders ate crickets.
- Of the sated (small) spiders on hungry spider webs captured and ate an average of one cricket each. Fifty eight percent of the spiders ate crickets.
- Of the sated (small) spiders on satiated spider's webs captured and ate an average of less than one half a cricket each. Thirty percent of the spiders ate crickets.
Sheet webs with sticky threads constructed by hungry spiders captured more crickets than did tangle webs without sticky threads constructed by sated spiders.
The spiders that had eaten but were placed on the sheet webs of hungry spiders captured and ate many more crickets than those sated spiders kept on their own type of web. It appeared as though these widow spiders could not ignore the prey stuck in the web as they would kill and eat all crickets that struggled against the sticky strands. Even the sated spiders on their own type of web ate crickets that bumbled into the web tangle and kept hitting the non-sticky threads, but this happened much less frequently than crickets getting stuck on the sticky threads.
The researchers concluded that the modification of the web from sticky to non-sticky strands reduced the probability of entangling prey. They hypothesize that widow spiders are not able to ignore struggling prey even when sated and the modification of web architecture is a mechanism to avoid overeating.
The early ethologists Nikolaas Tinbergen, Konrad Lorenz, and Karl von Frisch suggested that many animals have hard-wired fixed action patterns (FAPs) that must be followed to completion once a stimulus has elicited the behavior. These widow spiders appear to follow an FAP that is triggered by a prey item struggling against the threads of their webs.
This type of FAP response may account for female widow spiders pouncing on and feeding on courting males. If the male does not make the proper species-specific display that cancels the FAP, the female simply “thinks” he is a prey item and consumes him. In fact, any organism that causes the web to move in a specific fashion may be considered a prey item – even a human. Both male and female widow spiders can be handled without fear of being bitten if they are removed from their web and are not injured while being handled.
Here are links to other articles describing unusual spider behaviors.
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