The body of knowledge, in my humble opinion, is morbidly obese. That said, I think I may have found something to add. I will gladly share the blame with an artistic mentor who frowns most sternly on taking shots of natural subjects with unnatural (flash) light. She is right, the colors are not true when you use a flash, but sometimes it’s either flash or fuzzy. I hate fuzzy.
I recently found a number of fascinating little green spiders. They were small, very colorful, and built their webs at an angle to the horizontal. A little research on Bug Guide turned up one of the Long-jawed Orb Weavers (Tetragnathidae): the Orchard Orbweaver (Leucauge venusta).
All the shots I took were within four feet of the ground; I did not notice the exact heights. I wish I had. In research conducted in southern Mexico, it was determined that the larger and more sexually developed a female spider becomes, the higher she spins her web, and the higher she spins her web, the larger the prey she captures. 1
Back to my story.
The first shots that I took were practically at night. No choice, I had to use a flash. The next web I found was on a partly cloudy and a little bit windy day, so the subject was bobbing up and down in the shade. No choice, I had to use flash to get enough light to freeze the action.
Then the sun came out and the wind stopped for a brief moment. Thinking of my mentor, I grabbed a couple of shots in sunlight without the flash.
About an hour later I came across another web, and I took a couple of shots just as the sun slipped behind a cloud; they were a touch underexposed, but I got natural light shots, in the shade—no flash.
So when I looked at the pictures I noticed what many of you will have already noticed. The colors on the abdomen change markedly depending on the lighting.
The bright flash shows a large portion of the abdomen as a reflective silver. In sunlight, the spider is dappled green and yellow, and in shade, well, the whole abdomen is green and black. There is a possibility that the spider in the last two photos is at an earlier stage of development–I can’t be certain.
Given these observations, my theory, which I now pass on to the great crowd of people who know more about spiders than do I, is that there is some structural element that is causing the colors in the abdomen to change in response to ambient light, a property that should have some camouflage value for the spider.
I posted a question about this phenomenon on Bug Guide’s Arachnological Forum, and I was informed in response that most members of the Tetragnathidae have silvery reflections as well, but there is, as yet, no answer as to whether this is a matter of structural refraction, pigmentation, or both.
If I get an answer, you folks will be the first to know.
- Y Henaut, J A Garcia-Ballinas, C Alauzet. “Variations in web construction in Leucauge venusta (Araneae, Tetragnathidae),” Journal of Arachnology Vol: 34, Issue: 1, (2006). Pages: 234-240. Available online at http://www.americanarachnology.org/JoA_free/JoA_v34_n1/arac-034-01-0234.pdf [Accessed 22 June 2012].