NORTH AMERICAN ODONATA. 161 



lying between the right and left latero-ventral raetathoracie carinte, 

 and extending from the first abdominal sternnm to between the third 

 coxfe. 



Internally, at each of the sutures separating the pleural selerites, 

 is a chitinous ridge (apodeme), resulting from an infolding of the 

 integument, to serve for muscular attachment. A development of 

 apodemic processes and the fusion thereof, forms, on the floor of the 

 mesothorax and of the anterior part of the metathorax, a tunnel 

 (^neural cayial) for the ventral nerve-cord. 



The pattern of the external coloring of the thorax conforms in 

 the main to the pleural sutures, and by reference to those sutures, 

 the precise location of stripes may be indicated. Thus, very fi-e- 

 quently a stripe covers the humeral suture, and is consequently re- 

 ferred to as a " humeral stripe;" an " ante-humeral stripe" is ap- 

 proximately parallel to, but to the inner (mid-dorsal) side of the 

 humeral suture. 



Two pairs of spiracles, or stigmata, exist in the thorax. The 

 mesosUgmata are situated on the lower part of each mesepisternum, 

 close to and somewhat above the margins of the anterior mesotho- 

 racic opening ; they are often partly concealed by the overhanging 

 of the prothorax. The metasfigmatu lie, one in the lower part of each 

 metepisternum, and are very distinct in all the Odonata. 



The legs do not present any wide range of variation. Their leno-th 

 successively increases from the first to the third. The second and 

 third are placed closer together than are the first and the second. 

 If the legs be extended in the position natural to the insect when at 

 rest, four faces may be distinguished in the limb — viz. : superior, in- 

 ferior, anterior and posterior. The terms exterior and interior, as 

 applied by some authors, correspond to the superior and inferior 

 faces, respectively. The coxa is fairly robust and short. The tro- 

 chanter is longer, constricted near the middle so as to frequently 

 present the appearance of a suture, and its articulation with the 

 femur is oblique. While usually smooth, the trochanter may become 

 spinous (GoDiphus). The femur and the tibia constitute by far the 

 greater part of the leg. The former is the stouter and usually the 

 shorter. Both are characterized by the possession of two principal 

 longitudinal rows of downwardly directed, divergent spines, of which 

 one divides the anterior face from the inferior, the other the po.<terior 

 face from the inferior. The relative size of these spines may be 

 nearly the same on femur and tibia (Calopteryx), or those on the 



TRANS. AM. ENT. SCO. XX. (21) JULY. 1893. 



