We describe a possible new defensive behaviour of larval stages of mantis shrimps (Stomatopoda). Mantis shrimp larvae are rarely observed in nature, thus the study is based on postures of museum material and functional morphological aspects. Specimens described here are tightly enrolled, their pleon is bent forward, and the telson is locked into the frontal margin of the shield. This margin has two lobes into which the two posterolateral spines of the telson fit. The shield shows further adaptions to enrolment; e.g., the ventral gape of the shield perfectly matches the width of the pleon and leaves no major gaps when the pleon is bent forward. Based on these observations, we briefly discuss the possibilities to infer behavioural aspects from functional morphological aspects. Enrolment in modern day organisms is primarily known from terrestrial arthropods, e.g., pill bugs and pill millipedes, but in the Palaeozoic it was mainly performed by marine organisms such as trilobites, agnostines and their relatives. Stomatopod larvae that appear to be able to perform enrolling in a marine environment are therefore a potential functional equivalent for better understanding the functional aspects of enrolment in extinct marine arthropods.
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