372 THE GYPSY MOTH. 



Dorsal Vessel or Heart. 



This organ, in a caterpillar, is a simple longitudinal tube 

 (Fig. 1, dv), lying immediately beneath the dorsal line and 

 extending the whole length of the body. It is closed at the 

 posterior end, while the anterior end opens at the base of the 

 brain. Bands of muscle fibres, placed more or less oblique- 

 ly, appear in the walls of the organ, while it is supported at 

 the sides by fan-shaped muscles (alse) which are attached 

 to the upper body walls. 



In the pupal stage (Fig. 5, dv) a marked bend is found 

 near the anterior end of the dorsal vessel, while faint muscle 

 fibres can be traced, extending from the bend to the dorsal 

 wall of the thorax. 



In the imago (Fig. 6) the dorsal vessel (dv) follows 

 the contour of the upper surface of the abdomen, bends 

 abruptly downward at the junction of the abdomen and 

 thorax, enters the thoracic cavity, bends obliquely upward 

 to the dorsal wall, where it is firmly attached by muscle 

 fibres, then extends downward and slightly backward, 

 thence forward to the head, where it terminates at the base 

 of the brain. I have been unable to discover any aortal 

 chamber, such as was found by Burgess in Anosia plexippus^ 

 although the aortal arch (Fig. 10) is well defined. 



Respiratory System. 



This system is not figured in the plates, but possesses the 

 general features common to Lepidoptera. The spiracles 

 open internally by short tubes which communicate with two 

 Ions: lateral tracheae. These extend the whole length of the 

 body, and give ofi" ramifications to all the organs. Opposite 

 the spiracles are clusters of tracheae of medium size, which 

 are distributed over the surface of the alimentary canal. 

 The tracheae are composed of an outer semi-transparent 

 sheath, made up of large, thin cells, the nuclei of which are 

 readily revealed by staining with hasmatoxylin. Within 

 this sheath is a chitinous tube, bearing on its inner surface 

 a fine spiral ridge, which gives the tracheae their distinguish- 

 ing characteristic. 



