Lambir spider project

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The Predatory Success of Nephila pilipes

Kazemde George, Pagi Toko and Zachary Herring

Contents

Abstract

Nephila pilipes (Tetragnatha) is a species of spider that is widely distributed across South East Asia. Like other spiders, this species hunts by constructing webs, specifically orb webs that are built over or near water, and are built solely by the females of the species. Like other Tetragnathas, Nephila pilipes mostly feed on small insects and their success depends on their web structure and location. Within this study we sought to understand the correlation between spider size, web size and predatory success. After analyzing our data no relationship was found between these variables, although a weak correlation between web size and mean size of prey larger than 1mm (p-value = .14) was found. And although little correlation was found, we believe if this experiment was conducted over a longer period of time, and were to include distance from water as another variable the results may differ.

Large specimen of Nephila pilipes located adjacent to Latak Waterfall

Introduction

Spiders have evolved numerous methods of catching prey, but are most noted for their abilities to build webs of silk. These structures take on differing forms, but are all extremely strong tools that directly impact the success of spiders. Nephila pilipes is a common spider widely distributed throughout Asia, Southeast Asia, and Australia, and is characterized by its long legs and large orb webs (Murphy, 2000). This species like many spiders is sexually dimorphic and has thus attracted attention in the scientific community because females may be nearly ten times larger than their male counterparts. And while sexual dimorphism within this species has been studied extensively (Gillespie, 2002), there has been little study in regards to the relationship between spider size, web size, and predatory success – success being measured by the amount and size of prey caught. In this study we seek to examine this relationship, and see whether there is in fact a direct correlation between spider size, web size, and predatory success in regards to Nephila pilipes in the primary forests of Lambir Hills National Park, located in Sarawak, Malaysia.

Methods

Field methods

Samples were taken along the main trail to Latak Waterfall in Lambir Hills National Park. Data was taken from 11 female spiders, due to the fact that males of Nephila pilipes do not construct webs. Measurements of web height and width were taken with a tape-measure and the size of both spiders and prey were measured with a caliper. We counted and measured the insects caught in the web in intervals of about 3 hours throughout the day in order to observe accumulation of prey over the course of a day.

Analysis

After collecting data over a three day period we created a table using the statistical analysis program R. We then proceeded to conduct a series of correlation tests to assess the statistical connection between spider size, web size, number of small prey, number of large prey, total number of prey, mean size of large prey, and total mean prey size. Web size was calculated by multiplying width by height, and the number of planes were ignored. The number of prey was calculated as an average per collection bout. And the total mean prey size was calculated by averaging the averages of both large prey and small prey, assuming that small prey averaged around .05 cm. Scatter plots were also created in order to assess the correlation between these variables in regards to Nephila pilipes.

Results

Our statistical analysis revealed no significant correlation between spider size, web size, and predatory success. The resulting p values of the correlation tests are as follows : spider size and web size = .87 , spider size and number of small prey = .67, spider size and number of large prey = .81, web size and number of small prey = .53, web size and number of large prey = .74, spider size and mean size of large prey = .97, web size and mean size of large prey = .14, spider size and mean prey size = .97, web size and mean prey size = .31, spider size and total prey = .93, web size and total prey = .54, spider size and total catch (total catch being number of prey multiplied by the average of total prey size) = .99, and web size and total catch = .40.


Nephila pilipes feeding on a wasp

Discussion

After conducting our experiment, the results we obtained suggest that there is little or no connection between web size, spider size, and predatory success. The lowest p-value we found (p = .14) was in our correlation test between web size and mean size of prey larger than 1mm. From this we could potentially infer a correlation between the two variables. However, this connection is weak, and our data suggests that factors other than web and spider size influence successful predation. Our observations in the field have led us to believe that web location is more closely related to the predatory success of Nephila pilipes; spiders on bridges and near to water seemed to consistently catch more and larger prey. This observation would require more data collection to test.

Acknowledgements

Thanks to Cam Webb, Kinari Webb, Frank Reindt, Chuck Davis, Rod Eastwood, Dave Lohman, Henry and all others who helped throughout the process.

References

  • Frances Murphy, J. M. An Introduction to the Spiders of South East Asia, Malaysian Nature Society, Malayasia, 2000, 382-383
  • Gillespie, R. G. Biogeographay of spider on remote oceanic island of the Pacific: archipelagoes as stepping stones? Journal of biogeography, 2002, 29, 655-662