A class of hexapod (six-legged) arthropods consisting of the orders Collembola and Protura. Class Parainsecta (also known as Ellipura) is one of several classifications of those hexapods, which, while similar to insects, are sufficiently different that most researchers believe they deserve separate class status.
Collembola and Protura have frequently been lumped with the Diplura (Entotrophi) into one class (Entognatha) on the basis of their mouthparts, which are inside the head capsule rather than exposed as in the Insecta. However, some recent morphological and paleontological studies support a closer link between insects and Diplura. In addition, there are a number of features found in Protura and Collembola that are not found in Diplura. These include the developmental process: Both Collembola and Protura have epimorphic development (that is, development of the young is completed in the egg). Two unique features of the Parainsecta are the presence of a ventral midline or groove and coxal vesicles (fluid-filled sacs, generally associated with leg bases) that are limited to the first abdominal segment. In the Collembola these vesicles are fused into a single ventral tube. The digestive tract lacks anterior or posterior enlargements, which are often present in Diplura and primitive insects. Other distinguishing features of the Parainsecta are the absence of styli and unpaired pretarsal claws.
Both Collembola and Protura contain very small animals, generally less than 4 mm long. Protura are exclusively, and Collembola primarily, soil and litter inhabitants. Genetic evidence concerning relationships is at present confusing. In addition, differences of interpretation concerning the phylogenetic importance of various morphological features leave the status of the Parainsecta ambiguous. See also Arthropoda; Collembola; Diplura; Insecta; Protura.
