INTRODUCTION 13 



soon dropped out leaving six, and names substituted for the numbers as 

 follows: — I. subcostal, III. radius, V. media, VII. cubitus, IX. and XI. anal 

 veins. Three of these were supposed tp be simple. III. fivei-branched 

 V. four or three-branched, and VII. two branched. The radius is the primary 

 vein, the media and all the branches independents. Cross-veins have no 

 place in the system. 



There are a number of distinct systems of nomenclature used in the 

 different orders as will be explained as each group is considered. 



The thorax consists of three segments known as the pro- meso- and meta- 

 thorax and these prefixes apply also to all the parts of which each segment is 

 composed. The parts of a segment are: a single piece beneath — the sternum; 

 two on either side — episternum in front and epimeron behind; and four 

 sclerites on the back — the praescutum, scutum, scutellum and postscutellum. 

 The theory has been suggested that they represent vestiges of four primitive 

 segments, but it seems more likely that the development of legs and wings 

 has caused the differentiation from a simple segment. The sutures seem 

 to be in the nature of infoldings to give greater strength where the insertion 

 of the leg weakened the segment and where the pull of the leg muscles 

 needed greater resistance. 



The legs may thus be held responsible for the cutting off of the sternum 

 and the dividing of the upper portion into an anterior and a posterior portion. 

 The development of the wing results in a more complete cutting off of the 

 dorsal portion of the segment. The prsescutum and postscutellum may be 

 chitinizations of the intersegmental membrane to give attachment and 

 resistance to the great muscle of flight. 



The great longitudinal wing muscle which causes the down stroke of 

 the wing by arching up the back connects the anterior ends of segments; 

 thus the hind wing muscle reaches from the metathorax to the first abdominal 

 segment. Where there is a great constriction at the base of the abdomen 

 it is necessarily between the first and second segment. The first abdominal 

 segment is therefore often counted as one of the thoracic segments. 



Very early in the history of winged insects provision was made for 

 doubling the wing back against the body when not in use. The Odonata 

 and Ephemerida are the only existing groups in which the wings are rigidly 

 attached to the wing roots. In all other cases we can notice: — • 



1. The articulation of the base of the anterior marginal, and the develop- 

 ment of a tendon for direct muscular attachment. 



2. The basal interruption of all veins except the posterior marginal and the 

 primary or their fusion with the base of the primary. 



3. The formation of at least one folding area involving all the base of the 

 wing back of the primary, including the posterior marginal vein. 



Associated with the complication of the base of the wing there is an entirely 

 new plan of musculation. In the Odonata the tendon of the elevator muscle 



