Good design in miniature
Published: 30 October 2007 (GMT+10)
When Smithsonian Tropical Research Institute researcher William Eberhard put miniaturized orb-weaving spiders to the test, he got a big surprise. He had expected that the bigger spiders (weighing 50–100 mg) in his study would substantially outperform the most tiny ones (less than 0.005 mg).1
His expectation that ‘the brains of very small species should be functionally inferior’ had a reasonable basis, as he explained:
‘Neuron size appears to reach a minimum (approx. 2 μm in diameter) near the lower end of the range of insects body sizes (approx. 0.9 mm) and then does not decrease further in smaller species; so very small species probably have reduced numbers of neurons.’
It was also known that ‘the internal substructures of the brains of very small insects may also be simpler or fewer in number, and smaller insects tend to have reduced numbers of sensory organs, such as chemosensory and tactile setae, and ommatidia in their compound eyes.’ Thus, given the apparently reduced nervous system capabilities in very tiny species, Eberhard had expected to see ‘increased imprecision’ in the spacing of successive loops of sticky spiral in orb webs, and that the smaller spiders would move more slowly.
As it turned out, he saw no evidence of any ‘handicaps of miniaturization’ whatsoever, with the tiniest spiders not only matching the web-building precision of their large counterparts, but also the speed of construction. In fact, the smallest spiders ‘were thus moving much more rapidly in terms of their body size than were the largest species’.
‘All creatures great and small,and:
‘He gave us eyes to see them
- Eberhard, William, G., Miniaturized orb-weaving spiders: behavioural precision is not limited by small size, Proceedings of the Royal Society B 274(1622):2203–2210, 7 September 2007. Return to text.
- From the hymn ‘All things bright and beautiful’, www.hymnsite.com/lyrics/umh147.sht, acc. 25 September 2007. Return to text.