What is it you ask ? No need to be so impatient I will tell you. The big brown bug is a "Dog-day cicada nymph" or Tibicen spp. for the entomologists among you.
According to Garden Insects of North America by Whitney Cranshaw (which arrived on my doorstep today), the dog-day cicada is the largest cicada in North America. They are commonly heard but less commonly seen, which I take to mean I was lucky that day in the garden.
The nymphs need 2 to 5 years to complete development. Once hatched from the egg the new nymph burrows underground near tree roots ( they sip on the sap from the roots) for several years while it develops and molts. Once it is fully developed the nymph returns topside and finds a place to sit (like a Culver's root plant). Then it undergoes its final molt and the adult cicada leaves the brown body casing through a slit in the back.
Male cicadas rest on tree trunks and branches and "sing" to attract females, by means of two special vibrating membranes in the sides of the abdomen. Females do not sing. Adult cicadas do not feed on leaves, and may suck juices from tender twigs. They are not considered garden pests, unless you don't care for the singing.
Adult Dog-day cicada - Photo from Hilton Pond Center