NO. 6 



INSECT ABDOMEN — SNODGRASS 



51 



to overlap internally the internal dorsals. The ventral muscles include 

 internal oblique (vi) and external oblique muscles (vc) , all of seg- 

 mental length. The lateral muscles comprise dorsoventral and oblique 

 external laterals (le), and a strong, oblique, internal stenio-tergal 

 muscle (//) attached on the consecutive intersegmental folds. It may 

 be questioned whether this last muscle represents the internal laterals 

 of the Dytiscus larva (fig. 18, //), l)ut the only difference between 

 the two is that the upper end of the muscle in the bee larva is attached 

 on the intersegmental fold following that of its ventral attachment, 

 a change that might have come about by a posterior migration of its 

 dorsal end. 



Fig. 19. — ^Musculature of right half of two consecutive abdominal segments 

 of honeybee larva. (Figure from Nelson, 1924, but relettered in accord with 

 muscle nomenclature adopted in this paper.) 



Con, ganglionic connectives; de, external dorsal muscles, some of them secon- 

 darily internal at posterior ends ; (/;", internal dorsals ; Cng, segmental ganglion : 

 Hf. heart ; Ic, external lateral muscles ; //, internal lateral muscle ; Sp, spiracle ; 

 ve, external ventral muscles; I'i. internal ventrals. 



Lepidoptera. — The larvae of Lepidoptera have long been noted for 

 the great number of muscles that lie against the body wall, and for the 

 extreme comj^lexity in the arrangement of the fibers. Fully 150 mus- 

 cles, mostly individual fibers, may be counted in a typical abdominal 

 segment of any caterpillar (figs. 20, 21). The principal muscles of 

 the innermost layer (figs. 20, 21 A) are definite bands of parallel 

 longitudinal fibers having segmental lengths and attaclied on the inter- 

 segmental folds. In the ventral region there are also strong external 

 muscles of segmental length having an oblique position. Most of 

 the external fibers, however, are of various lengths and are disposed 

 in all directions against the body wall (fig. 21 B). On each side of 

 the body, between the principal dorsal and ventral groups of muscles 

 4 



