14 



The organ of circulation is the heart or dorsal vessel. As 

 has been said, it is called the dorsal vessel because of its 

 position in the body, lying as it docs along the median line 

 of the back, just under the body wall. The heart is a 

 straight, unbranched, tubular organ provided with muscles, 

 extending from the hinder end of the body to the head : it is 

 generally closed at the posterior end and open anteriorly. 

 The portion lying within the abdomen is constricted at 

 intervals, and at each constriction there is a valve. These 

 valves divide the dorsal vessel into compartments or 

 chambers. By means of a rhythmic contraction and expan- 

 sion of the walls of the dorsal vessel, accompanied by a 

 regular opening and closing of the valves, the blood is forced 

 from the rear toward the head, to the long aorta-like 

 portion of the dorsal vessel lying within the thorax. The 

 chambers are provided with small openings which communi- 

 cate directly with the body-cavity, and through which the 

 blood in the body-cavity is taken into the heart and put into 

 circulation. 



The pulsations of the dorsal vessel and the movement of 

 the blood in the body-cavity can easily be observed in 

 certain caterpillars. The arrowroot worm, or canna leaf- 

 roller (Ctilpodei ethlvus), is the best of our common West 

 Indian insects for the purpose. 



Respiration. 



Insects breathe by means of air-tubes (tracheae) which 

 begin at openings in the body wall, and extend throughout 

 all parts of the insect structure. The apertures, which are 

 called spiracles, or stigmata, are valvular, and are capable of 

 being opened and closed. On the outside, they are also 

 generally protected by hairs. There are usually ten stigmata 

 on each side of the insect body; but there are sometimes less, 

 and they also vary in their position, according to the life- 

 habits of the insect. They may be distinctly seen with the 

 naked eye in many insects. In certain large caterpillars, 

 they are surrounded by fine lines, or are enclosed in spots of 

 colour, which make them prominent. In the grasshopper, 

 they may be easily seen two on the sides of the thorax, and 

 eight on the abdomen. The tracheae are composed of chitin, 

 and are continuous with the body wall. The smaller air- 

 tubes are simple tubular structures, but the larger are 

 strengthened on the inside by ridges, arranged spirally. 



