420 TEXT-BOOK OF ENTOMOLOGY 



The fat-cells are distinct, spherical, and as a rule possess only one nucleus, 

 though in those of Apis and Melophagus there are two nuclei, and in Musea 

 several. Sometimes the cells contain a substance like the white of an egg, and 

 concretions of uric acid, or these take the place of the fat-drops. The presence 

 of uric acid shows that a very active metabolism goes on in the fat-body. " In 

 some cases it has been proved that the fat-body in the larva is rich in fat and 

 poor in concretions of uric acid, while in the imago it is poor in fat and rich in 

 concretions of uric acid " (Lang). 



Leydig, in 1857 (Lehrbuch der Histiologie), spoke of the presence of dark 

 concretions in the fat-body, and afterwards (1864) showed that there was a 

 wide distribution of uric acid salts and concretions. Witlaczil, also, has de- 

 tected concretions in the fat-body of the Psyllidae, in larval Cecidomyiidse, in 

 the larvae and pupse of ants, and in the pupa of Musca. 



The physiological processes which take place in the fat-bodies are obscure. 

 Graber regarded the whole system of the fat-bodies as "a single, many-lobed 

 lung," while before him Landois, taking into account the intimate relation 

 existing between the finer tracheal branches and the fat-body, considered that 

 the latter was concerned in respiration. Marchal thinks that the fat-body is a 

 urinary organ, as the urates are formed within the cells of this body. 



Moreover, Schaffer maintains that a special kind of fat-body cell has the 

 important function of taking up and giving out nutritious matters during the 

 internal processes of metamorphosis, while he also believes that there is a gen- 

 etic connection between the fat-body and the blood corpuscles a view com- 

 bated by Wheeler. 



Kowalevsky finds that the fat-body remains absolutely insensible to the action 

 of the substances which stained the Malpighian tubes (p. 352). So long as the 

 cells are healthy and living they are not stained and do not absorb the colors in 

 question ; and this insensibility persists, even when the cells are of a different 

 nature, as those of the fly (adipose and " intercalary " cells). 



b. The pericardial fat-body or pericardial cells 



We have already, on p. 405, called attention to these organs, but 

 they also have an intimate relation to the fat-body. 



Kowalevsky (1892) remarks that the disposition of these cells 

 varies much in different insects and even in the same animal. Thus, 

 in the Diptera and the ordinary flies there are found around the lower 

 part of the dorsal vessel 13 pairs of large pericardial cells which lie 

 next to a crowded bed of small cells forming a compact mass around 

 the anterior part of the dorsal vessel. In caterpillars, notably silk- 

 worms, from the compact layer of pericardial cells which surround 

 the heart, pass off trunks which are directed towards the lateral 

 walls of the body, also forming close networks around the tracheae 

 and then passing down into the abdominal cavity of the body of the 

 larva. 



In the larvae of certain Hymenoptera, the trunks which pass off 

 from the pericardial region form a loose cord, a sort of fatty tissue 

 covering the entire body cavity. 



