Instars
Although it is unclear how Daphnia itself perceives own body size, the body size is an important factor in determining the extent to which inducible defense should be expressed (Tollrian 1995; Hart and Bychek 2010). This is because predation sensitivity changes with age/instar changes in body size. It is important to be able to identify the type of predator, i.e. gape-limited or visual predator, by primary factors at first. Chaoborus larvae prefer a narrow range of small-sized prey (Swift and Fedorenko 1975; Pastrok 1981), whereas fish prefer larger-sized prey, because they are readily visible (Brooks and Dodson 1965; Nunn et al. 2012). Hence inducible defense varies among instars. For example, neckteeth induction is stronger at the 2nd and 3rd instars of Daphnia than at other stages (Tollrian 1993; Tollrian 1995a,b; Imai et al. 2009), because the former are the most vulnerable to Chaoborus larva predation. Therefore, depending on the trait, the degree of expression plasticity can be vary large within instar. The presence of fish chemicals decreases Daphnia body size (Brett 1992; Weber and Declerck 1997; Boersma et al. 1998; Fisk et al. 2007; Carter et al. 2013). Daphnia expresses inducible defense throughout its entire lifespan in the presence of predators capable of ingesting prey of any size (Laforsch and Tollrian 2004; Rabus et al. 2011).