Those structures may be what are called "Gin Traps". They are used as an antipredetor defense and when hairs are trigeered, the pupae will twitch to pinch the attacker. You can sort of see the poin on one is looke darker and more heavily sclerotized. Occasionally you can find pupae with ant legs or other predetor parts stuck on them. If you want to learn more, there is a discussion in Bouchard and Steiner 2004 "First Description of Coelometopini Pupae from Australia, Southeast Asia..." Just google search and you should be able to get a PDF. In the bibliography, the paper, Steiner 1995 goes more in depth on the topic, but unfortunately I don't think it is online.