Samuel Zschokke (1), Stefanie Countryman (2) & Paula E. Cushing (3)
(1) Section of Conservation Biology, University of Basel, Basel, Switzerland
(2) BioServe Space Technologies, University of Colorado, Boulder, CO, USA
(3) Denver Museum of Nature & Science, Denver, CO, USA
Gravity affects all organisms that do not spend their entire life on a horizontal plane. Since understanding the relevance of gravity for organisms requires studying them in a zerogravity environment, NASA (among others) have brought different organisms into their space laboratories for observations and experiments. In my presentation, I describe the results of such an experiment where two juvenile Nephila clavipes spiders were brought to the International Space Station and observed over a period of two months. During that time, a camera took pictures of the spiders and the webs they built every five minutes. Spiders were kept in a cubic habitat with an artificial day-night cycle (12:12) and fed with a continuous supply of fruit flies. Under natural conditions, Nephila spiders build webs with the hub near the upper edge of the web and always orient themselves downwards when sitting on the hub whilst waiting for prey. As these asymmetries are considered to be linked to gravity, we expected the spiders experiencing no gravity to either build webs with the hub in the centre (as 1st instar Nephila do) or to place the hub at a random edge of the web. Similarly, we expected the spiders to orient themselves either towards the larger part of the web when sitting on the hub, or to show an inconsistent or random orientation. The results of our study only partially matched our expectations. While most webs built without gravity were indeed much more symmetric than the control webs built under normal gravity, some webs still had a rather pronounced vertical asymmetry. In addition, spiders showed a random orientation only during the time when the lights were turned off, whereas the spiders quite consistently faced away from the lights, which were all placed along one side of the habitat, when the lights were on. Closer analysis furthermore revealed, that the webs, whose building had started after the lights had been turned on were more asymmetric (with the hub being nearer the lights) than those whose building had started before the lights had been turned on. However, there was no relationship between web asymmetry and the orientation of the spider. We conclude that the behaviour of Nephila spiders is greatly influenced by gravity. We also conclude that in the absence of gravity, the direction of light can serve as an orientation guide during web building and when sitting on the hub in a similar way that gravity serves as an orientation guide in the absence of light.
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Samuel Zschokke, Stefanie Countryman & Paula E. Cushing