NO. 3 INSECT HEAD SNODGRASS 53 



The antennal muscles that take their origin on the dorsal arms of 

 the tentorium in most adult pterygote insects have evidently migrated 

 secondaril}^ to this position after the attachment of these arms to the 

 dorsal wall of the cranium. In the crustaceans, myriapods, and many 

 insect larvae, the antennal muscles have the primitive attachment on 

 the walls of the head capsule (figs. 23 B, 50 B, C, E, F, 5). 



Evidence fully supporting the above statements is easily adduced 

 from a comparative study of the head structure and the gnathal mus- 

 culature in the Myriapoda, Apterygota, Ephemerida, Odonata, and 

 orthopteroid Pterygota. Many of the facts have been described by 

 other writers, but their significance appears to have been unrecognized. 



In the Chilopoda, the hypopharyngeal apodemes are large chitinous 

 processes (fig. 21 A, B, HA) that arise from the inner ends of the 

 suspensorial plates of the hypopharynx (HS) close to the base of 

 the hypopharynx (Hphy). Each projects posteriorly at the side of 

 the pharynx, and the two are bridged below the pharynx by a sheet 

 of ligamentous tissue (A, C, ;'). Upon the arms, or on processes of 

 the arms, and on the uniting ligament arise the ventral adductor 

 muscles of the mandibles (B,KL), and of the first and second 

 maxillae. The relations here are the same as in the thorax of an insect 

 where the ventral leg muscles arise from a pair of sternal apophyses. 

 In the Diplopoda, a highly specialized condition has arisen through 

 the separation of the inner ends of the muscles from the apodemes 

 and their union across the median line by a tough transverse ligament 

 (fig. 26 A, A-). The large mandibular adductors (KL, KL) here 

 pull against each other from the two ends of the ligament. The liga- 

 mentous bridge suggests, in a way, the body of a tentorium, but as 

 will be seen it has no relation to the insect tentorium. A similar con- 

 dition of the mandibular adductors exists in many of the Crustacea 

 (fig. 2y A, B, KL), and in some of the fibers of these muscles in the 

 apterygote insects (C, D, KLk), as will be descril)e(l later in con- 

 nection with the mandibles (page 62). 



In the Apterygota, the hypopharyngeal apodemes are well developed 

 and extend far back in the head. Those of Japyx (fig. 21 D, HA) are 

 slender rods running parallel beneath the sides of the pharynx and 

 then diverging outward and posteriorly to the head wall behind the 

 cardines of the maxillae (Cd), but their ends appear to be free and 

 not attached to the cuticula of the cranium. Upon these arms arise 

 the hypopharyngeal retractor muscles, a set of mandibular adductors 

 (fig. 2'/ C, KLt), the adductors of the maxillary stipes and cardo 

 (fig. 21 D, admx), and muscles of the labium (Ibmcl). The hypo- 

 pharyngeal skeleton of Japyx was described first by Meinert (1867), 



