Ants are among the best-known insects, but the morphology and development of their larvae are rarely studied in a systematic manner. Precise information on larval development is needed not only to understand ontogenetic development of caste phenotypes but also ultimately to allow a better understanding of the integrated development of entire ant colonies - superorganisms that have an inseminated founding queen as germ-line, cohorts of unmated workers as soma, and the iteroparously produced gyne and male reproductives as gamete analogues. Here, we present a survey of larval morphology of the fungus-growing ant Acromyrmex echinatior (FOREL, 1899), documenting the four instars of large and small workers and the five instars of gyne and male larvae. We used a combination of quantitative traits (body length, body curvature, hair patterning, head to body length ratio) and binary traits (presence / absence of anchor-tipped hairs, gut full / empty, head moving or not), and we document variation across the instars and sexes for 251 individuals with z-stacked images. Based on the statistical resolution of single and combined traits, we provide a key for the 3rd to 5th instar larvae, where sex and developmental stage can be unambiguously identified, and offer notes on the second instar, where identifications are statistically possible but with lower accuracy. This key is also available as an electronic resource . We discuss the challenges involved in this type of research and highlight opportunities for addressing new research questions that become accessible when sex-specific and caste-specific larval instars can be distinguished.