464 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



For 



occ 



The parts of the postgenal sutures lying at the sides of the gula are 

 distinguished as the guJar sutures. The sutures extending forward 

 from the tentorial pits are the hypostomal sutures (/is), which 

 diverge anteriorly to the lower margins of the compound eyes (E), 

 and set off distinct hypostomal areas {Est) behind the bases of the 

 maxillae {Mx). 



IV. GENERAL STRUCTURE OF 

 ARTHROPOD APPENDAGES 



When we come to study 

 the appendicular organs of 

 the head associated with 

 the mouth, by which insects 

 obtain their food, we must 

 bear in mind that these 

 structures are legs modified 

 for purposes of feeding. 

 The primitive feeding legs, 

 or gnathopods^ were not 

 necessarily like the tho- 

 racic legs of insects, which 

 are specialized locomotory 

 organs, but it is probable 

 that they resembled the 

 locomotory appendages, or 

 pereiopods, of modern Ar- 

 thropoda more closely than 

 the members of any other 

 specialized group of ap- 

 pendages. 



An appendicular organ 

 having a locomotor func- 

 tion must be movable on 

 the body. Movement im- 

 plies the presence of muscles so attached on the base of the append- 

 age that the muscles and the appendage together will constitute a 

 definite mechanism capable of a specific kind of action. Since we do 

 not have any examples of really primitive arthropods, and could not 

 study the muscular system if we had a fossil specimen of one, we can 

 only construct an imaginary picture of the mechanism of a primitive 

 arthropod appendage from theoretical considerations. But, inas- 

 much as unknown truth in actuality must take some specific form, a 

 theory has at least a chance of being right. There can be little ques- 

 tion, now, that a primitive locomotory appendage turned forward 



FiGDRB 13. — Ventral surface of the head of a 

 blister beetle, Epicauta marginata, illustrating 

 the development of the gula (Ou) in an elon- 

 gate prognathous head. Posterior tentorial pits 

 (pt, pt) transposed forward on ventral side of 

 head ; lower ends of postoccipltal suture (pos) 

 correspondingly elongate at sides of gular region 

 (Ou) added to submentum (Smt) 



