There is some controversy among nematologists as to whether the appropriate term for immature stages is larva or juvenile. From the point of view of applied nematology, either term is acceptable. Although some adult nematodes may exhibit a variety of shapes, juveniles are typically vermiform having a long, thin, cylindrical shape.
After each juvenile stage, a molt occurs and the cuticle is shed. In this picture, a dagger nematode is molting and shedding it's cuticle. This allows the nematode to increase in size.
Many plant-parasitic nematode species pass through the first juvenile stage and molt to the second stage before hatching from the egg. In this picture, a second stage juvenile cyst nematode has just hatched from an egg.
The length of time for one generation to occur is different for different genera and species and is greatly influenced by temperature. Optimum temperatures for many nematodes range from a low of 55 to 65 F to highs in the 90's and approximately a month is required for many nematodes to go through a generation under these conditions (e.g. sugarbeet cyst and root-knot nematodes).
Other plant parasitic nematodes can pass through a generation in as little as two weeks (e.g. foliar), or may require a full year (e.g. some species of dagger nematode).
Soil moisture and availability of food from the host plant also influence generation time. Some nematodes have survival stages that allow them to tolerate wide fluctuations in temperature and other adverse environmental conditions.