information:classification
Post-embryonic development

Under environmental conditions favourable for reproduction, hatched larvae develop through four larval stages - L1, L2, L3, and L4 - in just 3 days at 20 °C. When conditions are stressed, as in food insufficiency, excessive population density or high temperature, C. elegans can enter an alternative third larval stage, L2d, called the dauer stage (Dauer is German for permanent). A specific dauer pheromone regulates entry into the dauer state. This pheromone is composed of similar derivatives of the 3,6-dideoxy sugar, ascarylose. Ascarosides, named after the ascarylose base, are involved in many sex-specific and social behaviors. In this way, they constitute a chemical language that C. elegans uses to modulate various phenotypes. Dauer larvae are stress-resistant; they are thin and their mouths are sealed with a characteristic dauer cuticle and cannot take in food. They can remain in this stage for a few months. The stage ends when conditions improve favour further growth of the larva, now moulting into the L4 stage, even though the gonad development is arrested at the L2 stage.

Each stage transition is punctuated by a molt of the worm's transparent cuticle. Transitions through these stages are controlled by genes of the heterochronic pathway, an evolutionarily conserved set of regulatory factors. Many heterochronic genes code for microRNAs, which repress the expression of heterochronic transcription factors and other heterochronic miRNAs. miRNAs were originally discovered in C. elegans. Important developmental events controlled by heterochronic genes include the division and eventual syncitial fusion of the hypodermic seam cells, and their subsequent secretion of the alae in young adults. It is believed that the heterochronic pathway represents an evolutionarily conserved predecessor to circadian clocks.

Some nematodes have a fixed, genetically determined number of cells, a phenomenon known as eutely. The adult C. elegans hermaphrodite has 959 somatic cells and the male has 1033 cells, although it has been suggested that the number of their intestinal cells can increase by one to three in response to gut microbes experienced by mothers. Much of the literature describes the cell number in males as 1031, but the discovery of a pair of left and right MCM neurons increased the number by two in 2015. The number of cells does not change after cell division ceases at the end of the larval period, and subsequent growth is due solely to an increase in the size of individual cells.