1920] Dii Porte: Muscular System of Gryllus 47 



The Occlusor of the Abdominal Spiracles (Figs. 40 and 41, oc. 

 sp.) is a short muscle which takes its origin from the dorso- 

 posterior edge of the triangular posterior valve. It is inserted 

 into the process of the bow and closes the spiracle by pulling 

 the bow down against the other lip. 



The Dilator Muscle (Figs. 40 and 41, d. sp., Fig. 32, 103) 

 originates from the lateral edge of the abdominal sternite and is 

 inserted into the extreme ventral end of the closing lever. 



VI. THE MUSCLES OF THE VISCERA. 



The Constrictor Muscles of the Oesophagus and Crop form a 

 muscular tunic composed of a single layer of circular muscles. 



The Constrictor Muscles of the Proventriculus (Fig. 42, c. m.) 

 form a very strong muscular coat consisting in its thickest part of 

 as many as ten layers of circular muscles. 



The Dilator Muscles of the Proventriculus (Fig. 42, d. m.) are 

 situated within the cavity of each median tooth. The edges are 

 attached at the bases of the outer barbated lobes near the 

 partitions between the epithelial folds. The muscle folds on 

 itself, the fold extending far within the cavity of the median 

 tooth. The effect of the contraction of this peculiar muscle is to 

 pull the epithelial folds outwards towards the muscular tunic, 

 thus enlarging the lumen of the proventriculus. 



The Constrictors of the Mesenteron form a single layer of 

 circular muscles. 



The Dilators of the Mesenteron are longitudinal muscles, 

 several groups of which lie along this organ outside the circular 

 muscles. 



The Constrictors of the Rectum (Figs. 43 and 44, c. m.) are 

 usually two layers thick. 



The Dilators of the Rectum (Figs. 43 and 44, d. m.) There 

 are six groups of dilator muscles in the rectum, which originate 

 from the body wall as follows : The two dorsals from the anterior 

 edge of the tenth tergite, one on each side of the median line; 

 the two laterals from the lateral side of the anterior edge of the 

 same sclerite, in front of the cerci; the two ventrals in the male 

 from the latero-anterior processes of the eighth sternite, in the 

 female from the base of the ventral gonapophyses in the angle 

 between the ventral and dorsal processes. Each of these groups 



