The Plecoptera and Orthoptera agree in their numerous Malpighian tubes and in the development of a folding anal area in the hind-wing.
As shown by the number and variety of species, the Orthoptera are the most dominant order of this group. Eminently terrestrial in habit, the differentiation of their fore-wings and hindwings can be traced from Carboniferous, isopteroid ancestors through intermediate Mesozoic forms. The Plecoptera resemble the Ephemeroptera and Odonata in the aquatic habits of their larvae, and by the occasional presence of tufted thoracic gills in the imago exhibit an aquatic character unknown in any other winged insects.
The Odonata are in many imaginal and larval characters highly specialized; yet they probably arose with the Ephemeroptera as a divergent offshoot of the same primitive isopteroid stock which developed more directly into the living Isoptera, Plecoptera, Dermaptera and Orthoptera.
Orders: Dermaptera, Orthoptera, Plecoptera, Isoptera, Corrodentia, Ephemoptera, Odonata, Thysanoptera, Hemiptera, Anoplura.
A remarkable point in the Plecoptera is the presence in some forms (Pteronarcys) of small branching gills on the three thoracic and the front abdominal segments.
The Plecoptera are world-wide in their range and fossils referable to them have been described from rocks of Eocene, Miocene and Jurassic age, while C. Brongniart states that allied forms lived in the Carboniferous Period.
The two families included in this order agree with the Plecoptera in the young insect resembling the parent, but they are all terrestrial After C. L.
In the absence of cerci the Corrodentia are more specialized than the Isoptera and Plecoptera, but some of them show a more primitive character in the retention of vestigial maxillulae - the minute pair of jaws that are found behind the mandibles in the Aptera.
Noteworthy writings on the special orders are: PLECOPTERA': F.