18 ELEMENTARY LESSONS ON INSECTS 



few at the rear end), extended lengthwise beneath the food 

 tube on the ventral side. In the top of the head there is a 

 brain from wh,ich two nerve bundles descend, surrounding 

 the front end of the food tube, to join the foremost of the 

 ventral ganglia. So much of structure insects have in com- 

 mon with other Arthropods. 



The food tube or alimentary canal begins at the mouth. 

 Here there are minute paired salivary glands opening into 

 it. It soon narrows down to a slender tubular esophagus or 



Head 



Fig. 6. — Diagram of the internal plan of structure of a stonefly. Body wall marked as before 

 Head, thorax (I, II, III) and abdomen (1 to 10), with antennae (a) at the front and tails or cerci — 

 (/) at the rear — the inner food tube or alimentary canal begins at the mouth (m) and extends to 

 rearward in esophagus (e), crop (c), proventriculus or gizzard (p), stomach (s) with a whorl of 

 renal or Malpighian tubules at its rear end), intestine, (i) and rectum (r) to end at the anus (an). 

 Underneath the intestine lie the paired reproductive organs (ovaries or spermaries, according to 

 sex) opening to rearward at gn. Opening into the mouth are paired salivary glands (gl). jAbove the 

 food tube lies the dorsal vessel or heart (d.v.). The central parts of the nervous system are shown 

 in solid black; brain or cerebral ganglion (br) above the esophagus, and chain of ventral ganglia 

 (g.g.g.) on the floor of the body cavity. 



gullet. It widens again sometimes into a thin-walled food 

 receptacle called a crop, or into a thick-walled chewing 

 stomach called a gizzard, or both. So far it is lined with 

 chitin, mostly thin, but in the gizzard the chitin often forms 

 teeth or ridges that are useful for grinding and mixing the 

 food. 



Then comes the stomach proper, whose naked walls are 



