S8 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



The epipleurites are derived from the episternum and epimeron. In 

 a young nymph of Dissostcira or Gryllns, the basalar and subalar 

 muscles (fig. 26 B, £, F,) are attached directly to the upper edges of 

 the pleuron, one before the pleural ridge, the other behind it, and it 

 is only in the adult stage that the areas of attachment are separated 

 as the basalar and subalar plates. In some adult insects, however, 

 the basalare is not distinct from the episternum, or it appears as a 

 lobe of the latter (fig, 14, Ba). The subalare is always an independent 

 plate in adult insects, but it may be reduced to a small chitinous disc 

 in the membrane beneath the wing. 



In nymphal insects having the epipleural muscles attached above to 

 the pleuron and below to the coxa, the muscles evidently function as 



PIS 



D PIR 



Acx 



Fig. 26. — Mesopleuron and coxa of young nymph of Gryllus assimilis. 



A, external view of left pleuron; B, internal view of right pleuron. D, base 

 of muscle of third axillary; E, basalar muscle of coxa; E' , sternal branch of E; 

 F, subalar muscle of coxa ; M, M', abductors of coxa ; P, episternal branch of 

 depressor muscle of trochanter. 



leg muscles, though there is usually a branch of the basalar muscle 

 to the sternum (fig. 26 B, E'). In the adult, the basalare is attached 

 to the humeral angle of the wing base by a tendinous thickening of 

 the cuticula (fig. 30 A, a), and the subalare is similarly connected 

 (b) with the second axillary sclerite. In the adult winged insect, 

 therefore, the epipleural muscles become muscles of the wing. In 

 some of the higher insects, the coxal branch of the basalar muscle 

 is lost, and the anterior branch alone remains, attached below to the 

 sternum, or to the episternum. When a second subalar sclerite is 

 present, it sometimes bears a small muscle arising upon the epimeron. 



IV. THE WINGS AND THE MECHANISM OF FLIGHT 



Morphologically, the insect wing is simply a flat, hollow outgrowth 

 of the lateral marginal area of the dorsum that has become a motile 



