We believe this may be Tibellus oblongus, one of seven species of Tibellus in North America north of Mexico. Slender crab spiders are ambush hunters, almost always found lying in wait on stems, stalks or leaves. On a different backdrop you can imagine how it would almost disappear. Female slender crab spiders are dutiful mothers, spinning a flattened egg case and guarding it until the spiderlings emerge. 

Size: About 10 mm

Photo by: Morgan McNeill on 7/6/22 in Lolo, MT