For purposes of comparison the classification by Hyatt and Arms, which is substantially that of Brauer, may be introduced. In linear arrangement it is as follows: I. Thysanura (_Spring-tails_, etc.). II. Ephemeroptera (_Ephemeridae_; May-flies). (=_Plectoptera_ Pack.) III. Odonata (_Libellulidae_; Dragon-flies). IV. Plecoptera (_Perlidae_; Stone-flies). V. Platyptera (_Termites_, _Mallophaga_, etc.). VI. Dermaptera (_Forficulidae_; Earwigs). VII. Orthoptera (Locusts, Grasshoppers, etc.). VIII. Thysanoptera (_Thripidae_; Fringe-wings). IX. Hemiptera (Bugs). X. Coleoptera (Beetles). XI. Neuroptera (_Sialidae_, _Hemerobiidae_; Lace-wings, etc.). XII. Mecoptera (_Panorpidae_; Scorpion-flies). XIII. Trichoptera (_Phryganeidae_; Caddis-flies). XIV. Lepidoptera (Butterflies and Moths). XV. Hymenoptera (Bees, Wasps, etc.). XVI. Diptera (Two-winged flies). The relationship of these orders cannot be indicated in a linear arrangement, and is admirably shown by Hyatt and Arms by means of diagrams which I reproduce (Figs. 2, 3.) [Illustration: FIG. 2.--Scheme illustrating origin and relationship of Orders. (After Hyatt.)] The relation of these sixteen orders to the older, septenary scheme is shown by the following arrangement: 1. Hymenoptera Hymenoptera XV. 2. Coleoptera Coleoptera X. 3. Lepidoptera Lepidoptera XIV. {Homoptera. 4. Hemiptera {Hemiptera IX. {Heteroptera. {Thysanoptera VIII. 5. Diptera {Diptera XVI. {Including Aphaniptera or Siphonaptera {of some authors. 6. Orthoptera {Orthoptera VII. {Dermaptera VI. {Trichoptera XIII } {Mecoptera XII }Neuroptera. {Neuroptera XI } 7. Neuroptera {Platyptera V } {Plecoptera IV } {Odonata III }Pseudo-neuroptera. {Ephemeroptera II } {Thysanura I } It will be seen that the changes are not so great as would at first appear. The three more important orders, namely, the Hymenoptera, Coleoptera, and Lepidoptera, remain substantially the same in all classifications, and so with the three orders next in importance--the Hemiptera, Diptera, and Orthoptera. All that has been done with these three has been to rank as separate orders what by former authors were preferably considered as either families or suborders. The principal change is in the Neuroptera, of which no less than eight orders have been made. This is not to be wondered at, because the order, as formerly construed, was conceded to be that which represents the lowest forms and more synthetic types of insects, and as such necessarily contained forms which it is difficult to classify definitely. [Illustration: FIG. 3.--Cross section of Fig. 2.]