44 



NATURAL HISTORY OF INSECTS. 



body, as in the higher animals, but consists of ma,sses of 

 twisted, ribbon-like tubes. 



The blood of insects is a colorless fluid, which does not cir- 

 culate in closed vessels or veins, but permeates all parts of the 

 body. The heart is represented by an elongated, pulsating 

 , vessel, situated in the upper part of the body, along the back; 

 it is furnished with small valves, which allow the blood to 

 pass only in one direction, which is toward the head. The 

 blood enters the heart through openings at the sides, and is 

 forced upward and expelled out of an opening in the anterior 

 end; from this it passes backward, through all parts of the 

 body,- and again enters the heart, as before. In many naked 

 caterpillars the pulsation of the heart is readily seen beneath 

 the skin on the back. 



Insects do not breathe through the mouth or nostrils, as the 

 higher animals do, but through small openings, called spiracles, 

 placed on each side of the body (Fig. 89); these open into 

 minute, pearly tubes (trachea), which carry the air to all 

 parts of the body, where it comes in contact with the blood. 

 In the perfect or winged insect these tubes are dilated so as to 

 form a great many air-sacs, facilitating the act of flying. 



Fig. 132. j n man y aquatic larva? such as those 



(Fig. 132) of the Mosquito these tubes 

 project from the body in the form of small 

 tufts, analogus to the gills of fishes. Those 

 insects which, in the perfect state, spend 

 much of their time in the water, are not 

 furnished with these gills, and hence are 

 compelled to rise occasionally to the surface 

 to get air 



Insects, unlike the higher animals, have 

 the skeleton external, or upon the outside, 

 and the muscles are attached to the inner 

 surface of the various parts. The muscles 

 are composed of numerous fibers, but which 



are not united in the rounded, compact form such as they 



have in the higher animals. 



