Water Strider
Aquarius sp.
Photo taken Dec 16, 2007 in the Wickenburg Mountains of southwest Yavapai Co., Arizona.
Gerridae, or Water Striders, are insects (not spiders!) found on ponds and slow moving streams wherever present in the Sonoran Desert. They rely on the water surface tension to skate across the surface and prey on other insects that land or fall into the water. They are true bugs with a piercing-sucking type mouth structure that they use to withdraw the liquefied internal contents of prey.
Sometimes incorrectly termed water "spiders", which of course they are not. They possess wings as adults with which they can leave a water hole and attempt to colonize another water hole - a risky endeavor in the desert where open water is a rarity.