94 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



The posterior articulation is the primitive one, the anterior a secon- 

 dary one. By this change in the articulation and movement of the 

 jaw, the primitive tergal promotor muscle becomes an abductor, and 

 the primitive tergal remotor becomes an adductor. The base line of 

 the mandible slopes downward and forward in Lepisina and in a few 

 of the lower pterygotes ; in all others its slope is reversed, allowing 

 the tip of the jaw to swing inward and posteriorly during adduction. 

 A similar evolution of the mandible has taken place in the Crustacea. 

 22. The ridge of the base of the insect cranium, on which the pro- 

 thoracic and neck muscles are inserted, is probably a chitinization of 

 the intersegmental fold between the maxillary and labial segments. 

 The posterior tentorial arms arise from its ventro-lateral ends by in- 

 vaginations in the external suture. The neck of the insect, therefore, 

 may be unchitinized parts of both the labial and the prothoracic 

 segments. 



V. THE HEAD OF A GRASSHOPPER 



After laying down the general principles worked out in the pre- 

 ceding sections, it will be well to test them with a few specific examples. 

 The head of a grasshopper is a good subject for an elementary study 

 of the structure of the pterygote insect head, because it preserves the 

 generalized orientation in having the face directed forward and the 

 mouth appendages hanging downward. Terms of direction, therefore, 

 do not have to be qualified — ventral is downward, dorsal is upward, 

 and anterior is forward. The descriptions here given are based on 

 the Carolina locust (Dissosteira Carolina), a fairly large grasshopper 

 to be obtained in almost any part of the United States. 



The muscles are designated numerically for convenience of refer- 

 ence only, and the same numbers do not refer to corresponding muscles 

 in the grasshopper and in the caterpillar (Section VII) . The myology 

 of insects is as yet too little advanced to furnish a satisfactory general 

 nomenclature for insect muscles, and no attempt is made here to use 

 a set of names for the muscles of the grasshopper that could in all 

 cases be applied to the muscles of other insects. The usual method of 

 naming muscles according to their function, or their supposed func- 

 tion, gives terms fitting for the species described; but in many cases, 

 by a change in the articulation between the skeletal parts involved, 

 muscles that are clearly homologous have their functions completely 

 altered. Again, it is impossible to name muscles consistently according 

 to their points of origin and insertion, for either end of a muscle may 

 shift and may migrate into a territory quite foreign to its original 



