457 



circulates. The fat-body is confined almost entirely to the 

 thoracic and abdominal segments. 



Shortly after the feeding has commenced, the cells of the 

 fat-body begin to accumulate within the cytoplasm globules 

 of fat (fig. 10), and at the end of the first larval instar a 

 number of these, often quite large, are present, and the cell 

 has increased considerably in size, measuring now about 25^ 

 in diameter. In almost all the fat cells examined at this 

 stage a great space was observed around the nucleus; this is 

 probably an artefax^t. 



During larval life a great growth takes place in the size 

 of these cells, till at the end they may be as large as 92^ in 

 diameter. This generally results in a partial crushing 

 together of cells; but the increase in size of the haemocoele 

 has been so great, that in places, even now, they lie loose 

 within it (fig. 3). But after defaecation, when the space 

 occupied by the intestine is so greatly diminished, the cells 

 again separate from one another. 



In the second larval instar the accumulation of a second 

 type of reserve substance becomes manifest within the fat cells 

 as a heavily staining, apparently structureless, mass around 

 the nucleus. But a little later this mass breaks up into 

 numerous minute granules, which move partly outwards, but 

 are most concentrated in the more central part of the cell. 

 Other granules are formed in the more peripheral regions, 

 and these are much larger, often irregular in shape, being 

 sometimes even angular, and stain heavily with haematoxylin. 

 The fat cells have grown greatly in size, and accumulate 

 mostly just below the cell membrane. All these storage sub- 

 stances, gathered up from the surrounding blood, lie sus- 

 pended within the delicate, often exceedingly delicate, cyto- 

 plasmic meshwork of the fat cell. 



In the larva some hours after defaecation has com- 

 menced, when the imaginal discs of the integument have 

 begun to grow, at the same time considerably constricting the 

 body volume, the pressure exerted upon the fat cells as they 

 float loosely in the blood forces numbers of these cells into 

 the cavities of the outgrowing appendages — wings, legs, 

 antennae — while the cavity of the head, which in the feeding 

 larva was not well provided with fat cells, now becomes 

 crowded with these, as the contracting abdomen presses its 

 contents forwards (cf. fig. 154). 



As the abdomen contracts more and more during its 

 transformation into the adult abdomen, the fat cells which 

 remain within it become very tightly packed together, and 

 it is only with the greatest difiiculty that cell boundaries can 



