Bacteria and bacterial metabolites stimulate feeding, defecation, egg laying. A pheromone regulates entry into the dauer stage (sounds a little like quorum-sensing in bacteria). Other pheromones mediate mating. Chemotaxis is and important behavior in enabling nematodes to locate food sources.

Nematodes are exposed to liquid and airborne chemicals at the air-water interface in soil. Water soluble attractants are detected by Caenorhabditis elegans at µmolar concs, volatile molecules at picomolar levels. Water-soluble signals may be used for short-range chemotaxis and volatiles for longer-range detection of food (important concept, I think**). Many attractants are metobolic products of bacteria.

C. elegans can discriminate between related chemicals. At least five classes of water-soluble attractants and seven classes of volatile attractants have been defined. Different classes of molecules are probably recognized by different receptor proteins.

Repellants include specific water-soluble molecules, acids, high osmotic compounds. All are toxic and cause paralysis or death. Other toxic chemicals are not avoided by the nematode.

from: Bargmann, C.I. and I Mori. 1997. Chemotaxis and thermotaxis. Chap 25 in D.L. Riddle, T. Blumenthal, B.J. Meyer and J.R. Priess (eds). C. elegans II.