476 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



skeletal processes of the cranium, and are inserted on the cardo and 

 the stipes. The principal extrinsic muscles are the adductors (fig. 

 19, Klt)^ the fibers of which arise on the tentorium (Tiit), and are 

 inserted within the cardo and stipes. They are evidently the primi- 

 tive ventral promoters and remoters of a generalized limb (fig. 14, 

 K, L), the origins of which have been carried into the head with the 

 development and transposition of the anterior arms of the tentorium. 

 The other extrinsic muscles are usually but two in number. One 

 is an anterior rotator of the maxilla (fig. 19, rtmxa) inserted on the 

 cardo anterior to the articulation {a") of the latter with the head; 

 the other {flee) is inserted at the base of the lacinia, and functions as 

 a cranial flexor of the lacinia. These two muscles appear to repre- 

 sent the dorsal promotor of a generalized limb (fig. 14, /). A repre- 

 sentative of the dorsal remotor is generally absent from the maxilla, 

 but it is indicated in the diagram (fig. 19, J) because it is an impor- 

 tant muscle of the mandible, and is sometimes retained in the maxil- 

 lary musculature as a posterior rotator inserted on the cardo. The 

 intrinsic muscles of the maxilla include the muscles of the endite 

 lobes (lacinia and galea) and the muscles of the palpus, all of which 

 take their origins within the stipes. The muscles of the lobes {flcs^ 

 fga) never include antagonistic pairs of muscles; the palpus muscles, 

 on the other hand, nearly always consist of a levator {0) and a 

 depressor (^), corresponding with the muscles of the telopodite of 

 a leg inserted on the base of the first trochanter (fig. 15, A, B, (9, Q). 

 It will be shown in the following section how the mandibles and 

 the second maxillae (labium) conform with, and depart from, the 

 more generalized structure of a typical first maxilla. 



VII. THE BITING TYPE OF INSECT MOUTH PARTS 



When the gnathal appendages gave up their primitive function as 

 organs of locomotion, and became transferred to the head in the 

 capacity of organs accessory to ingestion, the mandibles, being closest 

 to the mouth, were undoubtedly the first to undergo structural modi- 

 fications in adaptation to their new duties. At first they probably 

 served as mere prehensile or grasping appendages for obtaining the 

 food and for passing it into the mouth; but in the Crustacea and 

 Hexapoda they eventually evolved into strong biting and chewing 

 jaws, and lost all semblance to their former leglike structure, except 

 in the retention of the palpi in some of the crustaceans. The first 

 maxillae, on the other hand, did not so completely lose their primi- 

 tive form until, in some of the piercing and sucking insects, they 

 became highly specialized as parts of an apparatus for feeding on 

 liquid food. The second maxillae have had a more eventful history 



