276 



METAZOAN PHYLA 



of apparatus used in copulation, egg laying, or stinging. This results in a 

 lessening of the apparent number of metameres. 



311. Internal Structures. — The body cavities of an insect are not 

 truly coelomic but are parts of a hemocoel. A heart lies under the dorsal 

 wall of the abdomen and blood circulates through these hemocoelic 

 spaces. The circulation is not so important as in most animals, however, 

 since it plays practically no part in respiration. When the body of an 



Base of 

 antenna 



Subesophageal^ 

 ganglia 



Supra esophageal 

 ganglia or brain 



Compounal eye 



■Pharynx 



Honey sac 



Thoracic 

 ganglia 



Digestive 

 stomach 



Ma/pighran 

 tubules 



Ventral 

 nerve cord 



Rectal glands 

 Hind gut 



Fig. 175. — A honeybee dissected to show the digestive, nervous, and tracheal systems. 

 (From Leuckart wall chart.) Illustrates the digestive system of a suctorial insect. 



insect is opened, many white glistening tubes are seen. These are the 

 tracheae. They are held open by rings of chitin, branch repeatedly, and 

 the finer branches reach all parts of the body. In insects of active 

 flight the tracheae are dilated in certain places and form air sacs (Fig. 

 175). By means of this system of tubes oxygen is conveyed directly 

 to the tissues of the body and carbon dioxide is carried away. Although 

 the blood contains both oxygen and carbon dioxide it is only in the 

 amount that any tissue would have. In some cases aquatic forms do 

 not possess gills, but water is taken into the posterior end of the alimen- 

 tary canal, the wall of which is lined with papillae supplied with tracheal 

 tubes. 



