INTERNAE ANATOMY OF THE IMAGO. 61 



The circnlatory system is extremely simple. It con- 

 sists chiefly of a long tube called the heart, with con- 

 tracting v.ilves that extend along the body directly un- 

 der the back. This tube conveys the blood to tlie for- 

 ward part of t'le body, and thence, as a cold, transparent 

 and nearly colorless fluid, it circulates through the body 

 and bathes the entire niusaular and respiratory systems. 



The only peculiarity in the muscular system to which 

 we would here call attention is that instead of bein^T^ at- 

 tached, as is the case among the vertibates to an inter- 

 nal skeleton, the muscles are fastened to an external 

 crust or skeleton. 



The muscular power of insects is enormous and vastly 

 superior to that of the higher animals in proportion to 

 their bulk. 



The nervous system of insects consists of two cords 

 running the entire length of the body, each with a nerve 

 bunch or ganglion for every ring of the body, thus mak- 

 ing as many pairs of ganglia as there are elemental 

 rings, and from these nerve threads pass out to the va- 

 rious parts of the muscular system, limbs, organs of 

 sense, &c. 



The ganglia of the head are joined together, giving the 

 appearance of a brain, and so also, in a measure, are those 

 of the thorax. We have already spoken in an earlier 

 part of this outline of the organs of sense. 



These few remarks barely mention the several systems 

 of the internal structure of the insect, but they are all 

 our plan will here permit of. 



We have thus reviewed the two first departments of 

 the study of the insect, i. 6 ., Life or history (as in the 

 ^gg^ larvae and pupa), and the structure, external and 

 internal of the perfect insect or the Imago, All that 



