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