'Wl 



120 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 



56. — Dorsal lateral neck nittsclc (tig. 45 A). — A band of slender 

 fibers from first phragma ( iPli), inserted on base of neck membrane. 



57. — J^cntral lateral neck muscle (fig. 45 A, B). — A short, flat 

 muscle from anterior edge of prothoracic episternum {EpSi), inserted 

 on base of neck mem]:)rane. 



VI. SPECIAL MODIFICATIONS IN THE STRUCTURE 

 OF THE HEAD 



The important structural variations in the head of biting insects 

 afifect principally the f ronto-clypeal area, and the posterior lateral and 

 ventral regions. Modifications of the facial plates are often to be 

 correlated with variations in the relative size of the buccal and pharyn- 

 geal parts of the stomodeum, or with a special development of the 

 mouth cavity. Modifications in the posterior ventral parts of the head 

 are correlated with a flattening and elongation of the cranial capsule, 

 usually resulting from an upward tilting of the head on the neck 

 by which the mouth parts become directed forward, and, in certain 

 orders, are accompanied by an elongation of the submentum an- 

 teriorly, with a dififerentiation of this plate into a posterior gular 

 sclerite and a secondary anterior submental sclerite. 



MODIFICATIONS IN THE FRONTO-CLYPEAL REGION 



The prostomial part of the insect head includes the frons, the clyp- 

 eus, and the labrum. Whether or not it comprises also the region 

 of the compound eyes may be regarded as an open question, and one 

 for the embryologists to settle. If the compound eyes belong to the 

 first true segment of the head, it is probable that the frontal sutures 

 define the posterior limit of the prostomium ; otherwise the sutures 

 must be secondary formations within the area of the prostomium. The 

 frontal sutures do not always mark the lines of cleavage in the head 

 cuticula at the time of a molt. In an odonate nymph, for example 

 (fig. 46 I), the facial clefts (t) of the molting cuticula extend from 

 the coronal suture outward and downward on each side between the 

 eyes and the bases of the antennae, far outside the possible limits of 

 the frons (Fr). 



The part of the postembryonic head that may be defined as the 

 frons is the area included between the frontal sutures, where these 

 sutures are fully developed (fig. 46 B, Fr). The frontal sutures (fs) 

 extend typically from the coronal suture (cs) to the neighborhood 

 of the anterior articulations of the mandibles (c, c). The true frontal 

 region, therefore, can not include the bases of the antennae, which 



