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Myrmecocystus, a genus of ants found in arid and semi-arid areas of the western United States and Mexico, is one of several ant genera that include species with a highly specialized caste known as "repletes". Repletes store large quantities of liquid food in their crops, causing their abdomens to become so distended they have difficulty moving and remain permanently in the nest as living storage vessels. When food is scarce, they regurgitate and share food with their nestmates. Repletes are drawn from the largest workers (Rissing 1984) and may vary in color from clear yellowish brown to amber. The darker crop contents apparently contain more dissolved solids, largely glucose and fructose, whereas the lighter fluid may include more sucrose and water.(Hölldobler and Wilson 1990; Kronauer and Gadau 2002)
A mature Myrmecocystus colony consists of around 15,000 workers and a single queen. One researcher investigating a Myrmecocystus colony enlisted the help of professional gravediggers to follow the nest galleries through 5 meters of Arizona desert soil to recover the queen from a small chamber at the very bottom. (Hölldobler and Wilson 1990 and references therein)
A nice overview of the biology of Myrmecocystus mendax, by Randy Morgan of the Cincinnati Zoo, is hosted by the Sonoran Arthropod Studies Institute here. Morgan also provides a summary of the taxonomy of the genus Myrmecocystus and, more broadly, of all the world's honeypot ants (including those in other genera) here (also see Morgan 1991). Snelling (1976, 1982) reviewed the systematics of Myrmecocystus.