152 BUTTERFLIES AND MOTHS 



breathing pores are called stigmata or spiracles. Each spiracle is 

 in the form of an oval slit, and consists of two openings an inner 

 one which is closed by a valve, and an external one protected 

 by hairs. From the inner opening of the spiracles arise curious 

 tubular structures, called trachea, which function as lungs, and 

 branch in every direction through the body. The breathing 

 pores or spiracles are found in different situations on different 

 insects, though, as a rule, they are situated along the sides of 

 the abdomen and thorax. 



Although no regular system of blood-vessels is recognised in 

 insects, as in the higher animals, organs of circulation exist. A 

 large vessel, situated immediately under the dorsal integument, 

 extends from near the apex of the abdomen to the right of the 

 brain, at which end it is open. This dorsal vessel, or heart, con- 

 tains a series of cavities the ventricles having openings on each 

 side, and communicating with each other by a valve. The speed 

 of circulation and number of pulsations of the dorsal vessel depend 

 upon temperature, rising and falling with it, and almost ceasing 

 at freezing-point. The blood is colourless, and contains white 

 amoeboid corpuscles analogous to those of the vertebrates 



