6 Paleoptera 1. Introduction In the infraclass Paleoptera are the orders Ephemeroptera (mayflies) and Odonata (dragonflies and damselflies), the living species of which represent the few remains of two formerly very extensive groups. Although both are placed in the Paleoptera, authorities disagree on whether the two orders are monophyletic. | 6 Paleoptera 1. Introduction In the infraclass Paleoptera are the orders Ephemeroptera mayflies and Odonata dragonflies and damselflies the living species of which represent the few remains of two formerly very extensive groups. Although both are placed in the Paleoptera authorities disagree on whether the two orders are monophyletic or have separate origins see Chapter 2 Section . Even if monophyletic the Ephemeroptera and Odonata are two very different groups that must have diverged at a very early stage in the evolution of winged insects. They possess the following common features that unite them as Paleoptera wings that cannot be folded back against the body when not in use retention of the anterior median wing vein netlike arrangement of wing veins many crossveins aquatic juvenile stage and considerable change from juvenile to adult form. In members of both orders wing development is external though this feature is not of course restricted to Paleoptera. 2. Ephemeroptera Synonyms Plectoptera Ephemerida Common names mayflies shadflies Adults small- to medium-sized elongate fragile insects antennae short and setaceous mouthparts vestigial compound eyes large three ocelli present generally two pairs of membranous wings though hind pair greatly reduced held vertically over body when at rest with many crossveins abdomen terminated with two very long cerci and frequently a median caudal filament with subimaginal and imaginal winged stages. Larvae aquatic body campodeiform antennae short compound eyes well-developed biting mouthparts abdomen usually with long cerci and a median caudal filament and four to seven pairs of segmental tracheal gills. Approximately 2100 species of this widely distributed order have been described though this may represent only about one-third of the extant species. Of the described species about 675 occur in North America 84 in Australia and about 50 in Britain. 127 128 Structure chapter 6 Adult. The head is triangular in shape when .
