THE CmCULATORY AND RESPIRATORY SYSTEMS. 3 



tlie nerves of relation, because they control the sensations and motions 

 which associate the animal with the world around it. But in addition 

 to these, there have been discovered a number of very fine nervous 

 filaments proceeding from the brain, and extending down into the body, 

 and furnished with minute ganglia of their own, which are supposed to 

 represent the sympathetic system of nerves which preside over the in- 

 ternal functions, such as thbse of digestion and secretion. 



The foregoing cut represents the nervous system of a butterfly, {Pa- 

 pilio brassicce — after Herold), A exhibiting that of the larva, B that of 

 the pupa, and that of the perfect insect ; and showing how the nerv- 

 ous system becomes shortened and consolidated in changing from the 

 lower to the higher stages. Fig. D. shows the more concentrated nerv- 

 ous system of a Coleopterous insect, as exhibited in the common English 

 Cockchafer or door-beetle, Melolontha vulgaris^ (copied from Straus.) 



THE CIRCULATORY, OR SANGUIFEROUS SYSTEM. 



The blood of insects is a colorless fluid, which does not circulate in 

 closed vessels or tubes, like that of the higher animals, but permeates 

 the tissues of the body. The only vessel that can be discovered is an 

 oblong, membraneous, pulsating sack, situated in the upjier or dorsal 

 part of the body, and which evidently represents the heart. This is 

 divided into several compartments by cross-valves, which are so ar- 

 ranged as to permit the blood to pass only in a forward direction. The 

 heart is prolonged anteriorly into a narrower tube analogous to the 

 aorta. Through this the blood flows first towards the head, and thence 

 through the body, returning to the heart, which it enters through open- 

 ings at its sides. As compared with that of the warm-blooded animals, 

 the blood of insects is not only colorless, but small in quantity, and 

 must circulate very slowly, as is i)roved by the fact that when their 

 bodies are wounded no blood escapes. 



THE RESPIRATORY SYSTEM. 



• 



Most of the organs of insects, and their functions, have an obvious 

 analogy to those of the higher animals, but their breathing apparatus 

 is constructed upon an entirely different plan. In all the vertebra ted 

 animals the blood is carried in vessels to a particular part or organ of 

 the body, for the purpose of being exposed to the life-giving influence 

 of the air. This part in terrestrial animals, is the lungs, and in aquatic 

 animals the gills. But in insects the process is reversed, and the air is 

 carried to the blood by being distributed to every part of the body in 

 very delicate pearl-white tubes or vessels, which present a beautiful ap- 

 pearance under the microscope. They are called trache;©, or air tubes. 

 They admit the air through little openings along the sides of the in- 



