TIIE CIRCULATORY AND RESPIRATORY SYSTEMS. 
3 
the 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 those of digestion and secretion. 
The foregoing cut represents the nervous System of a butterfly, (Pa- 
jtilio brassiccv — after Herold), A exhibiting that of the larva, B that of 
the pupa, and C 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, Melolontlia 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 upper 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 proved by the fact that when their 
bodies are wounded no blood escapes. 
THE RESPIRATORY SYSTEM. 
Most of the organs of insects, aud 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 vertebrated 
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 iu terrestrial animals, is the lungs, and in aquatic 
animals the gills. But in insects the process is reversed, aud 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 tracheae, or air tubes. 
They admit the air through little openings along the sides of the iu- 
