NO. 6 INSECT ABDOMEN — SNODCRASS 33 



typically dorso-ventral, their fibers both intersegmental and intraseg- 

 niental. I\'. transverse muscles (C, t) , lying internal to the longi- 

 tudinals, including dorsal transverse inuselcs (td), and ventral trans- 

 verse muscles (tv). \. spiracular muscles {s). generally not more 

 tha.n two connected with each spiracle, one an ocelusor (os), the other 

 a dilator (dls). 



All the body muscles are bilateral in their origin, and all of them 

 except the ventral transverse muscles remain so in the definitive state. 

 The fibers of the ventral transverse muscles, which primarily Are 

 intersegmental, Heymons (1895) says are at first attached mesally on 

 a fold of the l)ody wall between the nerve cords. Later the fold is sup- 

 pressed and the nl)ers from opposite sides become continuous across 

 the sternal region. The fibers of the longitudinal dorsal and ventral 

 muscles are always separated into symmetrical lateral groups along 

 the midline of the dorsum and venter, but the lateral sets of dorsal 

 transverse muscles come together on the ventral wall of the heart. 



Each i)rimar\- grou]^ of muscles may undergo an endless diversifica- 

 tion resulting both from a multiplication of fibers in the group, and 

 from a rearrangement of the fibers brought about by changes in the 

 points of attachment. With respect to the dorsal and \entral muscles, 

 the most general departure from the sim])le plan, in which the fibers 

 all lie in a single i:)lane, consists of a difi'erentiation of the fibers 

 in each grou]) into internal inuselcs and ester nal muscles. Thus it is 

 found in nearly all insects that the dorsal and ventral muscles com- 

 prise each two layers of fibers, namely, internal dorsals (di) and 

 external dorsals (de), and internal ventrals {vi) and extenml z'cntrals 

 (ve). A second form of diversification afifecting the dorsal and ven- 

 tral muscles is a more or less distinct grouping of the fibers into 

 median and lateral muscles. In most insects, therefore, we may distin- 

 guish four sets of dorsal fibers, and correspondingly four sets of 

 ventral fibers. The several resulting muscles may then be distinguished 

 as median and lateral internal dorsals (fig. 14 A, B, dim. dil). median 

 and lateral external dorsals {dem. del), median and lateral internal 

 ventrals {vim, vii), and median and lateral external ventrals (z>em, 

 vel). 



The lateral muscles are more sul)ject to irregularities than are the 

 dorsal and ventral muscles, but they likewise are often divided into 

 internal laterals (fig. 13 B, C, //') and external laterals [le). 



Associated with the dorsoventral lateral muscles there is sometimes 

 present a longitudinal muscle or group of longitudinal fibers lying 

 external to the upper ends of the internal laterals (fig. 13, p). This 

 muscle is called the " epipleural" muscle by Ford (1923), but since 



