The female nematode lays eggs inside the host, which are then most often passed out in the host's feces. At this point, the eggs hatch in the external environment. Cues such as moisture levels or temperature trigger the larva inside the egg to begin producing enzymes that will dissolve the membrane of the egg. In response to the right conditions, the larva then also pushes on the weakened membrane to break through.
Once hatched, the nematode larvae begin to eat bacteria and grow. Some types of nematode larvae that didn't leave the initial host as an egg, such as viviparous filarial worms, are transferred to an intermediate host at this time, usually through the bite of an fly or other arthropod. Or, as in the case of Strongyloides stercoralis, they might be passed out in the feces as stage 1 larvae. When the larvae cannot grow anymore because of the size of their cuticle (skin), they must molt. To molt, they develop a new cuticle under the old one and then shed the former cuticle. The first molt marks the transition of a larva from stage 1 (L1) to stage 2 (L2).
With the notable exceptions of ascarids and pinworms, most nematodes become infective during the third larval stage (L3) after the second molt. While it is during this period that they infect the host where they will eventually reproduce, different nematodes go about it in different ways. In some instances, the host may become infected by accidentally swallowing the larvae. Other stage 3 larvae, like those of the hookworm, directly invade the host. Filarial worms, on the other hand, are again transferred by fly bite from the intermediate host to the final host. Finally, Trichinela spiralis larvae remain in their initial host, go into dormancy in the muscles during stage 3, and then infect a new host when it eats the contaminated meat.
Once inside the final, or definitive, host, nematode larvae molt another two times until they develop into immature adults after four larval stages. They also migrate through the host's body, using the bloodstream or lymphatic system. The larvae travel from the heart to the lungs, trachea and intestine.
Nemotodes usually require two different sexes to reproduce and, therefore, develop into both males and females as adults. The males produce sperm in the testis and deposit them into receptacles in the female. The female holds onto the sperm until she needs to use them for fertilization. After she has finished fertilization, the eggs develop in her uterus. When they are ready, she then lays them in the host, using her muscular ovijector. At this point, the life cycle of nematodes is complete.