Transcriptional changes in the life cycle of Haemonchus contortus. In a 3-week life cycle of the parasite, eggs (E) are excreted in host feces; the first-stage larva (L1) develops inside the egg to hatch and molt through to the second-stage (L2) and third-stage (L3) larval stages within a week. The infective L3s are then ingested by the small ruminant host, where they exsheath and, after a short tissue phase, develop through the fourth-stage larval (L4) stage to dioecious adults; both of these stages feed on host blood from capillaries in the internal wall of the stomach. Disease in the host relates to this blood-feeding activity. In this figure, changes in transcription in the transition from stage to stage are summarized and interpreted in the context of the biology of the parasite. Information is given on key genes differentially transcribed between adult female (Af) and male (Am) H. contortus, and involved in reproductive and other biological processes; gene codes follow those of Caenorhabditis elegans orthologs.