Life cycles

Nematodes all have a similar developmental pattern. Eggs are produced, within which a first larval, or juvenile, stage develops (= L1 or J1). There are four larval stages, separated from one another by a molt, or ecdysis.

The most common life cycle pattern among parasitic nematodes is the direct one. However, several taxa use an indirect cycle with 2 hosts. Three host cycles are rare. Usually the first, but in some species the second, juvenile stage hatches from the egg. In direct life cycles, it is usually the L3, but occasionally the L2, that is the infective stage to the host. In indirect cycles, generally it is the L1 or L2 which is infective to the intermediate host (if present), and the L3 which is infective to the final host. There is a molt to the L4 stage and then the adult stage occurs in the final host.

Nematodes exploit a variety of invertebrates and vertebrates as both intermediate and definitive hosts. They are common in terrestrial and aquatic hosts.