Warty Leaf Beetle
Exema sp.
Photo taken 15 July 2012 at Rackensack Wash, Maricopa Co., Arizona. The beetle is about 2 mm in length and was found on foliage of Burrobrush (Hymenoclea salsola). The insect is lying on its right side. The head is at lower left where the eyes can be seen.
When disturbed or threatened these small beeltes pull in the their legs, letting go of whatever they were crawling on, and often drop to the ground. There they remain motionless looking like a speck of dirt or else a caterpillar dropping. The point of course is to avoid being eaten by an insectivore like a small bird. Once the threat is gone and the beetle feels secure it will unfold a pair of wings from beneath the elytra and fly back up to the plant. In the vulgar language of coleopterists they're sometimes known as turd beetles. Taking the disguise further, the immature grub lives within a casing made in part from its own excrement.
Both these two specimens were found on members of the sunflower family, Asteraceae. Follow the thumb image links above for more information about Burrobrush and Canyon Ragweed.
Photo taken 26 May 2013 at Organ Pipe Cactus National Monument, Pima Co., Arizona. The beetle, possibly Neochlamisus, is about 2 mm in length and was found on foliage of Canyon Ragweed. In this close view the abundant leaf trichomes (hairs) are visible. This pubescence plays complex rolls in the interactions between the plant, herbivores like this beetle, hymenopterous parasitoids and the ambient air flows.