54 Bhod of Insects. 



various organs, and gives and takes as the vital operations of 

 the animal require. 



The blood is light colored, and almost destitute of discs or 

 corpuscles, which are so numerous in the blood of higher ani- 

 mals, and which give our blood its red color. The function 

 of these discs is to carry oxygen, and as oxygen is carried 

 everywhere through the body by the ubiquitous air-tubes of 

 insects, we see the discs are not needed. Except these semi- 

 fluid discs, which are real organs, and nourished as are other 

 organs, the blood of higher animals is entirely fluid, in all 

 normal conditions, and contains not the organs themselves or 

 any part of them, but only the elements, which are absorbed 

 by the tissue and converted into the organs, or, to be scien- 

 tific, are assimilated. As the blood of insects is nearly des- 

 titute of these discs, it is almost wholly fluid, and is almost 

 wholly made up of nutritious matter. 



The respiratory or breathing system of insects has already 

 been referred to. Along the sides of the body are the spira- 

 cles or breathing mouths, which vary in number. These are 

 armed with a complex valvular arrangement which excludes 

 dust or other noxious particles. These spiracles are lined 

 with a delicate membrane which abounds with nerves, which 

 were referred to in speaking of them as smelling organs. 

 From these extends the labyrinth of air-tubes (Fig. 2,f,f), 

 which carries vitalizing oxygen into every part of the insect 

 organism. In the more active insects — as in bees — the main 

 trachese, one on each side of the abdomen, are expanded into 

 large air-sacks (Fig. 2, /). Insects often show a respiratory 

 motion, which in bees is often very marked. Newport has 

 shown that in bees the rapidity of the respiration gauges the 

 heat in the hive, and thus we see why bees, in times of severe 

 cold, which they essay to keep at bay by forced respiration, 

 consume much food, exhale much foul air and moisture, and 

 are liable to disease. Newport foUnd that in cases of severe 

 cold there would be quite a rise of mercury" in a thermometer 

 which he suspended in the hive amidst Ae cluster. In the 

 larval state, many insects breathe by fringe-like giUs. The 

 larval mosquito has gills in form of hairy tufts, while in the 

 larval dragon-fly the giUs are inside the rectum, or last part 

 of the intestine. This insect, by a muscular efibrt, draws the 

 water slowly in at the anus, where it bathes these singularly- 



