Sunday, August 17, 2014

Predators or Prey- Turning the Tables

Spider marbler orbweaver
Some of those animals that we consider predators may sometimes become prey in the complex world of food webs.  For example the orb weaver spiders are familiar to us since they spin large webs with concentric rings of silk to catch insects.  For example the foliate orb weaver which has a web on the outside of one of our windows has here captured a hummingbird  clearwing sphinx moth which was feeding on nectar nearby.  Their habit of sitting on the edge or center of their web must make them susceptible to attack by predators such as birds although they often sit behind the web or off to one side, or hide during the daytime.  

Mud dauber organ pipe nests
next to paper wasps
An unsuspected small predator on these large spiders is the organ pipe mud dauber wasp (Trypoxylon politum, Sphecidae), a solitary species that builds nests made of mud up under the eves of houses and barns.  The beautiful marbled orb weaver is shown after capture by a mud dauber wasp on our porch.  My photograph at our cabin shows a series of the mud tubes next to a communal paper wasp nest (probably Polistes, Vespidae) , illustrating the extreme difference in materials and behavior utilized by these wasps from different families.  The paper
Wasp with spider prey
marbled orbweaver 
wasps typically eat caterpillars whereas the organ pipe mud dauber is a specialist on spiders.  Spiders are paralyzed by stinging and placed alive with an egg laid on them in the mud tubes.  This living death technique provides a nice source of fresh food for the larvae.  The female mud dauber builds the tubes and catches spiders very large in relation to her body weight.  The male guards the nest against parasites and trespassers while the female is away.  

Orbweaver foliate Larinioides
with prey hummingbird
clearwing sphinx moth
These two types of wasps are relatively peaceful and should be easily tolerated by humans.  Whether their feeding habits are beneficial to us is debatable, since spiders kill both useful as well as pest insects.  Which do you prefer having around your house- spiders or wasps?!  Why not tolerate both as remarkable examples of the wide variety of life history strategies for survival present in the natural world.

Bill Dunson 

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