Petrobius is a genus of jumping bristletails (family Machilidae). However, apterygotes (bristletails and silverfish) and many immature aquatic insects have abdominal appendages. They are sometimes known as rock bristletails. Heteropsontus is a genus of jumping bristletails in the family Machilidae. Promesomachilis is a genus of jumping bristletails in the family Machilidae. Praemachilis is a genus of jumping bristletails in the family Machilidae. Machilis is a genus of jumping bristletails in the family Machilidae. Machilinus is a genus of rock bristletails in the family Meinertellidae. Machiloides is a genus of rock bristletails in the family Meinertellidae. Helladocampa is a genus of two-pronged bristletails in the family Campodeidae. Dicellurata is a suborder of two-pronged bristletails in the order Diplura. Some early ametabolous "true insects" are still present today, such as bristletails and silverfish. Together with jumping bristletails, the predecessors of silverfish are considered the earliest, most primitive insects.