Figure 1. Schematic of the life cycle of Strongyloides spp For the genus Strongyloides, only parasitic females have been observed, and these reproduce by mitotic parthenogenesis. Depending on the species, either eggs or first stage larvae (L1), hatched within the host intestine, are passed in the host feces. Resulting post parasitic larvae are male or female. Female larvae have alternative developmental pathways leading either directly to infective third-stage larvae (L3i) or to a generation of free-living females. All male L1 develop to free-living males. Free-living adults mate and produce a generation of progeny, which in S. stercoralis and S. ratti are uniformly fated to develop to L3i. L3i invade the host by skin penetration and develop via the L4 to parasitic females. Strongyloides stercoralis is unique in its ability to develop precociously to the L3i within the primary host and thereby initiate repetitive autoinfective cycles that lead to geometrically expanding parasite burdens with serious, potentially fatal consequences for the host. Because of their anatomical similarity to hermaphrodites of Caenorhabditis elegans, free-living females of Strongyloides are readily transfected with DNA constructs by standard techniques for gonadal microinjection.