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The Tsetse fly's most prominent and vital endosymbiont is the bacterium, "Wigglesworthia glossinidia", These bacteria are housed in the fly's bacteriocytes and produce B vitamins (B1, B6, and B9). The Tsetse fly itself lacks the ability to obtain these nutrients because of its hematophagous diet. "W. glossinidia" population in the maternal milk glands also help prime the immune system during the larval stage. Tsetse flies with bacteriocytes containing "W. glossinidia" are less susceptible to trypanosome infection later in life.