Anal sacks‐an unknown organ in Poduromorpha (Collembola)

An undescribed organ in Collembola consisting of three eversible sacks in the last abdominal segment was first discovered in Hypogastrura socialis . Immediately prior to jumping the anal sacks become everted and appear as finger‐like appendages projecting out of the hind gut. The organ was found in all Hypogastura species available for investigation and in some other primitive genera of the families Hypogastruridae, Brachystomellidae and Neanuridae. In most species, however, the anal sacks were much smaller than those of H. socialis and probably not eversible. The function of the anal sacks is in some way associated with the jumping activity of the animals; possibly as a mechanism to increase jumping efficiency or, more likely, for release of pheromones. Further research is needed to uncover the function of the organ. It is suggested that Abd. VI has been formed by the fusion of the three lobes now constituting the Abd. VI segment and that this fusion resulted in the formation of three post‐rectal pockets from which the anal sacks have evolved.