NO. 6 INSECT ABDOMEN — SNODGRASS 37 



median and lateral muscles is sometimes marked by the points of 

 origin of the dorsal transverse muscles on the tergal wall (figs. 13 C, 

 15 B, td). 



The paratergal muscle of the dorsum (figs. 13 B, C, 14 B, p) is not 

 commonly present in adult insects, or, at least, its fibers are not 

 generally separated from those of the other lateral dorsal muscles. It 

 is well developed in the Acrididae (fig. 15 B, i6g), where it is repre- 

 sented in each of the pregenital segments but the first (fig. 16) by 

 a band of intertergal fibers lying above the line of the spiracles ex- 

 ternal to the upper ends of the internal lateral muscles (fig. 15 B, 

 775, 176). According to Ford (1923) it is also present in the 

 Plecoptera (" epipleural muscle"). The paratergal muscle occurs 

 more commonly in larval insects. In the abdomen of the larva of 

 Dytiscus it is represented by a lateral band of fibers (fig. iS, p) cut 

 ofif from the other dorsal muscles (dil, dill) by the upper ends of the 

 internal lateral muscles (lill). An apparently corresponding mus- 

 cle, consisting of a pair of parallel fibers extending lengthwise on 

 each side of the body above the line of the spiracles, is characteristic 

 of the caterpillars (figs. 20, 21, p). In the larva of Tipula there is a 

 broad band of paratergal fibers on each side of the body attached on 

 the middle of the laterodorsal areas of successive segments (fig. 

 22, />). 



The ventral muscles. — The ventral abdominal muscles undergo an 

 evolution parallel in most respects with that of the dorsal muscles. 

 Their fibers are nearly always dififerentiated into internal and external 

 layers, and those of both groups are commonly separated into 

 median and lateral groups. The fibers of the internal layer are typically 

 intersegmental wherever complete sternal plates are developed, and 

 serve as retractors of the ventral arcs of the segments. The external 

 ventrals are usually short and take their origins on the posterior parts 

 of the sterna. Frequently they become sternal protractors by a re- 

 versal of their position, owing to the carrying forward of their points 

 of insertion on apodemal arms of the anterior margins of the sterna 

 until their morphologically posterior ends lie anterior to their points 

 of origin on the preceding sterna. The length of the sternal apodemes 

 commonly gives to the ventral protractor mechanism of the abdomen 

 a greater effectiveness than has that of the dorsum. 



The lateral muscles. — It is diffictilt to make satisfactory general- 

 izations concerning the lateral muscles of the abdomen, because these 

 muscles are subject to more variations in position and attachments 

 than are either the dorsals or the ventrals. Most commonly the lateral 

 muscles are tergo-sternal in their attachments (fig. 13 C. //, le), and 



