Published data for spiders were analyzed to find out if the increment in size at molt (Dyar's constant) of spiders approached 28% as if each instar acts as an ecologically isolated form. Size increments of roughly 20% were found in small, actively hunting spiders, 25% in sallying predators which use no web or use only the upper surface of a web, and 30% for araneid and theridiid species which use the lower surface of a web; the highest value found for Dyar's constant was in an ambushing, thomisid species. Larger species of spiders have more instars, have larger values for Dyar's constant, and also seem to be less active foragers than smaller species. Published data for insects revealed that grazing holometabolous insects have the largest growth increments, and insects which hunt for food as immatures have lower values, the range from ca. 60% to ca. 20%.Lower “freight” costs and more constant agility (power/wt ratio) are mechanical advantages associated with low Dyar's constant–the implications of this for animals with continuous growth are discussed. Data on growth increments of both spiders and insects may be better explained by convergent evolution resulting from the need for agility during hunting than by Juberthie's (1955) idea of phylogenetically significant growth increments. Finally, the coexistence of species of lycosid-like spiders is discussed in relation to phenology, size and habitat; anr-adapted species of the genusPardosa(Lycosidae) has the same increment at the molt as a relatedK-selected species, but uses fewer instars and smaller size to achieve rapid population growth in its distinct habitat.