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Friday, May 4, 2001


River damselfly

*Japanese name: Kawatombo *Scientific name: Mnais costalis *Description: Damselflies perch with their wings folded shut (their bigger cousins, dragonflies, perch with their wings held open). There are many damselflies that live around rivers, but some male river damselflies have orange wings, making them easy to identify. Another, smaller type of male has clear wings. Both types of male have metallic-blue bodies. Females have clear wings and green bodies. Adults are 49-63 mm long. *Where to find them: Flying above stony mountain streams, from May to July. Males with orange wings will often perch on roots and bits of wood in the river, males with clear wings stick to vegetation on the riverbank. You may see two orange-winged males flying in an ever-higher spiral above a log in the river -- they are fighting over ownership of the log. If you look carefully, you might be able to see females laying their eggs into the log. *Food: Damselflies (and dragonflies) have hairs on their legs that help form a basket when the legs are held together; this is used as a kind of net to catch other flying insects. Despite what you may have heard, damselflies don't bite humans. As aerial hunters, they have excellent vision of almost 360 degrees, which means they can see above, below and behind themselves at the same time. *Special features: Logs in the river are valuable because females visit them to lay their eggs. The last male to mate with a female will be the father of the eggs. So males will guard females when they are laying their eggs (see photo). Orange-winged males are built for fighting, with big muscles in their thorax; their orange wings advertise that they are fighters. But there is another way to get females. Instead of fighting, clear-winged males sneak onto the territory of orange-winged males and try to steal their females. Fighting all the time gets orange-winged males lots of females, but it takes up lots of energy. Clear-winged males are weaker, but live longer than orange-winged ones. In the end, clear-winged and orange-winged males generally father the same number of offspring.

Territory: Animals often try to stop other animals from entering an area containing something valuable. This space can be called their territory.

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