Animal morphological traits may vary across life stages. Web‐building spiders are diverse insectivores that can display ontogenetic shifts in the design and properties of their webs. Nevertheless, we know little about how a critical component of their webs, major ampullate silk (MAS), varies in property across life stages, inferably owing to a difficulty in finding suitable model species. The Tasmanian cave spider Hickmania troglodytes presents as a good model as it is long‐lived and grows to a large body size with overlapping generations. We collected MAS from the webs of different‐sized H. troglodytes and performed tensile tests on MAS fibers collected from their webs to search for shifts in properties over life stages. We found that strength and toughness (i.e. ability to deform and absorb energy) of the MAS increased with spider carapace width and body length. We expect that such a shift in silk performance across life stages has distinctive advantages, including enhanced prey capture capabilities, an improvement in the economy of silk production and ability of the web to support the spider's larger body. Web‐building spiders are diverse insectivores that can display ontogenetic shifts in the design and properties of their webs. Throughout ontogeny, we found that the Tasmanian cave spider Hickmania troglodytes varies in the biomechanical properties of a key component of their webs: major ampullate silk. Increases in the strength and toughness of these silk fibers may have beneficial fitness effects, such as enhanced prey capture capabilities, improvement in the economy of silk production and ability of the web to support the spider's larger body.