Have you ever heard of a soap-powered boat? The idea is this: Take a small plastic toy boat, attach a piece of bar soap to the rear of the boat where you would normally have the outboard motor, place the boat in water, then sit back and watch the boat speed away just as if the soap actually were an outboard motor. What's going on here? The answer is surface tension.
Water has a fairly high surface tension. When you place the toy boat in water without a soap motor, surface tension pulls on the boat equally in all directions – surface tension pulls the boat backward just as strongly as it pulls it forward. As a result, the boat remains motionless. Now attach the soap to the rear of the boat. Soapy water has a lower surface tension than normal water, so now surface tension pulls the boat forward with a greater force than it pulls backward. The boat takes off in the forward direction, pulled along by the stronger surface tension there. Very nice.
What does this have to do with dragonflying? Well, today Betsy and I were watching American Emeralds at Cranberry Lake. Suddenly, a yellowish "caterpillar" fell out of a tree and landed in the water. I think it may have been the larva of a sawfly, but I'll have to look into this in more detail. In any case, after the larva landed in the water it began to move rapidly toward the shore. It moved so fast, in fact, that it left a wake. I looked at it through my binoculars, because I couldn't imagine how a caterpillar with its tiny legs could swim so rapidly. What I saw was that the water at the rear of the larva looked a bit oily. I think the larva secreted an oily substance, probably related to the web it spins to make a cocoon, and that this reduced the surface tension – just like a bar soap at the back of a toy boat. The larva seemed to be using surface tension to speed it across the water, with no paddling required. So far I haven't found anything about this online, but I'll look into it some more and let you know if I can confirm or debunk this notion.
Ahhh, you never know what you're going to see when you're out dragonflying.
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