5 Apterygote Hexapods 1. Introduction Traditionally, the groups included in the term “apterygote hexapods,” namely, the Collembola, Protura, Diplura, and Thysanura (including Microcoryphia and Zygentoma), were considered orders of primitively wingless insects and placed in the subclass Apterygota (Ametabola). | 5 Apterygote Hexapods 1. Introduction Traditionally the groups included in the term apterygote hexapods namely the Collembola Protura Diplura and Thysanura including Microcoryphia and Zygentoma were considered orders of primitively wingless insects and placed in the subclass Apterygota Ametabola . They show the following common features lack of wings lack of a pleural sulcus on the thoracic segments presence of pregenital abdominal appendages slight or absent metamorphosis and indirect sperm transfer. As more information on their structure and habits has become available it has become apparent that 1 their status as insects except for Thysanura is doubtful and 2 the relationship of the groups with each other is more distant than originally believed. Several authors have therefore recommended that the insectan subclass Apterygota be reserved solely for the Thysanura and that the Collembola Protura and Diplura each be given the rank of class with the Collembola and Protura considered as sister groups within the Ellipura see Figure . These three groups differ fundamentally from insects in several features for example they are entognathous have intrinsic musculature in the antennae and lack compound eyes which are characteristic of most insects at least in the adult stage. Thus the Ellipura and Diplura are sometimes considered sister groups within the Entognatha. However other analyses indicate that the Entognatha is a paraphyletic assemblage see Chapter 1 Section . The Collembola are probably furthest removed from the winged insects. They possess only six abdominal segments a postantennal sensory organ similar to the organ of Tomosvary found in myriapods gonads with lateral rather than apical germaria and eggs in which there is total cleavage. Non-insectan features of the Protura are the absence of antennae perhaps a secondary condition associated with their soil-dwelling habit the occurrence of anamorphosis and a genital aperture that opens behind the 11th
