Sauvaget, M., Guérineau, C., Zimmer, C., Savarit, F. & Fénéron, R.

Morphology, but not morphometry, identifies instars in Ectatomma tuberculatum ant

Holometabolous insects have evolved larvae distinct from adult forms, which is at the root of their ecological success and lifestyle diversity. Moreover, plasticity in larval development has yielded to different adult phenotypes in caste systems and sexual morphs, especially in ants. While understanding larval development and growth is crucial for many research questions, identification of larval instars remains challenging. Indeed, it has been recently suggested that studies based on size distribution alone have misjudged instar number in ant species. Here, we identified larval instars of Ectatomma tuberculatum (Olivier, 1792) by comparing the morphology and morphometry methods. We also searched for potential differences between female larvae that developed into reproductive or non-reproductive caste, that is, gynes or workers. Our results showed that chaetotaxy clearly differentiated four larval instars, whereas only three instars were separated by classical morphometric tools (i.e., size-frequency distribution, implementation of model, and test of Dyar’s rule). This contradictory result comes from the partial overlapping of larval head sizes at first- and second-instar. Head growth rate was lower than expected at the first moult, and then not constant from instar to instar. Consequently, E. tuberculatum does not follow Dyar’s rule. Furthermore, gyne larva size increased exponentially at the end of development, probably without adding instar. We confirmed that the first-instar larvae in ants could be inconspicuous and then easily ignored using morphometry alone. We hypothesized this early stage serves hatching and social functions. Overall, we recommend focusing initially on larval morphology and combining approaches in order to correctly identify larval instars in ants.

Open access, licensed under CC BY 4.0. © 2025 The Author(s).

Development, growth, larvae, instar number, Hymenoptera, Formicidae, Ectatomminae.