Cat-claw Scale
These insects were photographed in the New River Mountains, Maricopa Co., Arizona, USA. 28 May 2011. The host is cat-claw acacia, Acacia greggii.
Coccoidae -- Scales and Meally Bug Family
Scale insects are among the most peculiar insects, because they just don't look much like insects. In fact they may not appear do be alive at all. These bugs were immobile and concealed beneath a blanket of fine white threads made of wax. The wax offers protection from the sun and also possibly predators. It is made by the bugs from excess sugar in their phloem sap diet. These bugs clasp the twig so closely that the tiny legs, located beneath are hidden.
Scales are true bugs and lead a largely sedentary life drawing sap from their host plant, in this case is Cat-claw Acacia, Acacia greggii. When the eggs first hatch they give rise to minute legged versions called crawlers that can get knocked or blown from plant to plant. Once they settle down onto a new host their legs become pretty much useless.