OR, MANUAL OF THB APIAHV. 



8S 



where bathes the dig-estive canal, and thus easily receives 

 nutriment, or gives waste by osmosis ; everywhere surrounds 

 the tracheae or air-tubes — the insect's lungs — and thus receives 

 that most needful of all food, oxygen, and gives the baneful 

 carbonic acid ; everywhere touches the various organs, and 

 gives and takes as the vital operations of the animal require. 



The heart, like animal vessels, generally, consists of an 

 outer serous membrane, an inner, epithelial coat, and a middle 

 muscular layer. Owing to the opaque crust, the pulsations of 

 the heart can not generally be seen ; but in some transparent 

 larvK, like many maggots, some parasites — those of our com- 

 mon cabbage butterfly show this admirably — and especially in 

 aquatic larvffi, the pulsations are plainly visible, and are most 

 interesting objects of study. 



The heart, as shown by L,yonet, is held to the dorsal wall 

 by muscles (Fig. 32, m). Beneath the heart are muscles which. 



Fig. 32. 



Fig. 33. 



Portion of Heart of an Insect, after Packard. 

 i/" Heart, m Muscles, o Openings. 



Diagram of JETeart, from Cowan. 



to quote from Girard, form a sort of horizontal diaphragm (Fig. 

 34, d), which as Graber shows contract, and thus aid circulation. 

 The blood is light colored, and entirely destitute of red 

 discs or corpuscles, which are so numerous in the blood of 

 higher animals, and which give our blood its red color. The 

 function of these red discs is to carry oxygen, and as oxygen is 

 carried everywhere through the body by the ubiquitous air- 

 tubes of insects, we see the red discs are not needed. Except 

 for these semi-fluid discs, which are real organs, and nourished 

 as are other organs, the blood of higher animals is entirely 



