244 



AN INTRODUCTION TO ZOOLOGY 



abdomen. These are supposed to be of service to the bee during 

 flight, since their size can be increased at will, and the specific 

 gravity of the insect, therefore, decreased. Air is drawn into 

 and expelled from the tracheae by alternate expansions and con- 

 tractions of the abdomen. Each spiracle contains a valve, which 

 may be closed or opened. Dust is prevented from entering by 

 hairs which surround the opening. 



The rate of respiration depends upon the activity of the indi- 

 vidual. Normally there are about forty inspirations per minute, 

 but in fatigued bees the number reaches as high as one hundred 

 and sixty per minute. The air brought into the bee's body is 

 carried by the tracheae directly to the tissues, no circulatory sys- 

 tem being necessary for the distribution of oxygen. 



THE NERVOUS SYSTEM (Fig. 137, Op. L.-j Gng.). The nervous 

 system of the bee consists of a large chain of paired ganglia and 

 two groups of smaller ganglia, called the stomatogastric, and the 

 sympathetic, respectively. The large ganglionic chain is by far 

 the most important. It is formed of seven masses of nervous 

 tissue, two in the head, two in the thorax, and live in the abdo- 

 men. Each mass is composed of two ganglia lying side by side, 

 and connected with the mass in front and behind by two nerve 

 cords. All, except the foremost ganglionic mass, are situated 

 near the center of the ventral body wall. The ganglia at the 

 extreme anterior end occupy a cavity in the dorsal part of the 

 head; they are known as the brain, or supraoesophageal ganglia. 

 Nerves connect the brain with the compound eyes (Fig. 137, Op. L.), 

 the ocelli, the antennae, and the labrum. Beneath the oesopha- 

 gus in the head lies the subcesophageal ganglion which innervates 

 the mandibles, labium, and other mouth parts. The anterior 



FIG. 137. Longitudinal, median, vertical section of entire body of worker 

 bee, showing nervous system (Op. L.-?Gng.), tracheal system (Tra. Sc. 

 i-io), dorsal and ventral diaphragms of abdomen (D. Dph. and V. Dph.}, 

 and dorsal vessel consisting of heart (Ht.} and aorta (Ao.}. (From 

 Snodgrass, Technical Series 18, Bureau of Entomology, United States 

 Department of Agriculture.) 



