NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS 3 



fine the somites as the coelomic sacs and then contend that segmenta- 

 tion begins in the mesoderm. This usage is confusing because the 

 true mechanical segmentation of the body results from muscle attach- 

 ments to the body wall. The muscles themselves, however, are de- 

 rived from the walls of the mesodermal coelomic sacs. Since the 

 coelomic sacs are typically connected with the exterior by coelomic 

 ducts, their primary function was probably the collection of waste 

 products to be excreted through these ducts. 



The primary segments of the body are established by the attach- 

 ment of the longitudinal muscles to the cuticle. The lines of muscle 

 attachment, as seen on the abdomen, are marked externally by trans- 

 verse grooves which form internally submarginal ridges, the ante- 

 costae, near the anterior edges of the terga and sterna. In a soft- 

 bodied worm or insect larva the musculature, attached at the true seg- 

 mental lines, brings about a shortening of the body and allows squirm- 

 ing or flexing movements. In an animal with a fully sclerotized 

 integument, however, such movements would be impossible. To give 

 freedom of intersegmental movement, the posterior part of eacli 

 segment remains membranous. The functional segments thus become 

 the sclerotized annuli, and the connecting membranes are known as 

 the intersegmental membranes. The definitive mechanism is thus a 

 secondary segmentation. 



Segments (L. segmentum, from secare, sectiim, cut off) : The 

 term applies to body segments or somites and also to leg segments or 

 podites. 



The functional body segments are the sclerotized rings of the 

 integument separated by flexible unsclerotized areas and movable on 

 each other by intersegmental muscles. 



The true body segments are limited by the lines of attachment of 

 the longitudinal muscles, marked externally by grooves of the cuticle 

 forming anterior submarginal ridges or antecostae of the segmental 

 plates on which the muscles are attached. This is the primary body 

 segmentation which corresponds with the musculature. The func- 

 tional segments represent a secondary segmentation since the so- 

 called intersegmental membranes are the posterior of the primary 

 segments. This secondary segmentation allows the consecutive seg- 

 ments to be movable on each other because the connecting membranes 

 can be infolded or extended according to the tension of the muscles. 

 Where segments are united, as in the thorax, the membranes are 

 either eliminated or themselves sclerotized as postnotal plates. 



The leg segments are movable by muscles arising in the proximal 



