THE FAT-BODY 419 



THE BLOOD TISSUE 



Under this name Wielowiejski has included several important 

 tissues or cellular bodies intimately concerned with the nutrition of 

 the insect. These are : 



1. The blood corpxiscles. (See p. 407, leucocytes and phagocytes.) 



2. The fat-body proper (Corpus adiposum). 



3. The pericardial fat-body (pericardial cells). 



4. The cenocytes. 



5. The garland-shaped cord of muscid larvae. 



6. The subcesophageal body, a peculiar organ found by Wheeler 

 in the embryos and young larvae of Blatta and Xiphidium. 



7. The phosphorescent organs. 



. The fat-body 



In the body cavity of winged insects and of their larvae occur 

 yellowish masses of large cells filled with small drops of fat, and 

 forming the " fat-body." It is of various shapes, more or less lobu- 

 lated or net-like, and covers or envelops parts of the viscera, also 

 forming a layer under the integument (Fig. 143). The tracheal 

 endings are usually enveloped by the fat-body. It is larger in the 

 larvae than in the adults, especially in Lepidoptera, in them forming 

 a reserve of nutrition, used during metamorphosis and during the 

 formation and ripening of the eggs and male cells. 



Wielowiejski has shown that there is a regular arrangement of the fat-body in 

 the general cavity of the body. For example, in the larva of Chironomus 

 occur the following forms of this tissue. Around the periphery, on each side of 

 the body cavity, is a loose network of lobes with large meshes constituting the 

 peripheral layer or external lobular fat-body ; these lobular masses are seg- 

 mentally arranged. 



Within these segmental lobes, on each side of and along the digestive tract, 

 extending along through almost the entire body, is an unbroken strand of this 

 tissue, forming the internal fat-body cords. From the first larval stage, and 

 even before hatching, its cells are so unusually large, being filled with large, 

 clear, mostly colorless fat-drops, that their limits cannot be defined, and their 

 nuclei can only with great difficulty be detected. Only in some large larvse of 

 Chironomus has Wielowiejski found clearly defined cells ; the protoplasm of 

 these cells contain almost no fat-drops. 



The fat-body is of mesodermal origin, and as Wheeler insists, is not derived 

 from the oenocytes, as supposed by Graber. Formed from the mesoderm, it is a 

 differentiation of portions of the coelomic walls, and therefore metameric in 

 origin. That the fat-body gives origin to the blood corpuscles Wheeler is doubt- 

 ful. 



