From ZooTerms (Dictionary of Invertebrate Zoology)
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collum (noun; plural colla; Latin collum, neck): 1. The neck or collarlike structure.

2. (Arthropoda) The armoured tergite on the first segment behind the head in millipeds and pauropods that functions in locomotion.

3. (Arthropoda: Insecta) a. The slender connection between the head and thorax in Hymenoptera, Diptera and Hemiptera. b. The constriction of the median plate of the aedeagal apodeme prior to the fulcrum in male fleas.