greater in Hawai'i than in all of mainland North America. Many of them are species of concern, and some recorded there are possibly extinct. For example, H. finitimus was last collected 100 years ago, and only ever seen on the island of Kauai. The Hawaiian bee population and diversity is somewhat fluid — within the past ten to twenty years several Hylaeus species have been found and identified on the islands as new to science. In October 2016, seven species were officially listed as endangered by U. S. Fish & Wildlife, the first bees to ever be placed on the endangered species list. North America America There are currently an estimated 130 species of Hylaeus in America. Several species have been accidentally introduced to the country. This includes: Hylaeus leptocephalus found throughout the country; Hylaeus hyalinatus found in urban New York, New Jersey and Pennsylvania; Hylaeus punctatus found in central California, Mid-Atlantic states, the Chicago region and Denver area; and Hylaeus (Prosopis) variegates in the greater New York city region. One masked bee of interest in the country is Hylaeus lunicraterius. This bee is only recorded at the Craters of the Moon National Monument, Idaho. Little is known about its biology, however it is believed to nest in the crevices and holes in the lava flows. Hawaiian Islands A single clade of 60 known Hylaeus spp. are native to Hawaii. Phylogenetic evidence suggests that the first Hylaeus species arrived on the island of Hawaii between 0.5 and 0.4 million years ago. From here, it is believed that descendants of the initial (presumed ground-nesting) colonist separated in order to specialize in the different ecological niches available. This included coastal and dry forest strands (H. longiceps and H. anthracinus); mid-elevation and montane dry shrubland strands (H. difficilis); cleptoparasite strands (H. inquilina); and wet and mesic forest wood-nesters. After this divergence, it is believed dispersal to other Hawaiian Islands occurred. It is likely that the rapid speciation of Hylaeus occurred due to open expansion into open niches in novel habitat. Using an equation (r=lnN/t) developed by other studies into the evolution of Hawaiian groups, it can be calculated that the minimum speciation rate for Hylaeus on the island of Hawaii is 9.23 species per million years. In terms of nesting biology, those that inhabit wet areas nest in stems or wood, while those inhabiting dry areas nest in the ground. Some ground-nesting species