410 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



The fat tissue is derived from the mesoderm. It is present only 

 in small amount in the embryo, but increases rapidly in the young 

 insect after hatching, often forming great masses filling all avail- 

 able space in the body cavity, particularly in the larvae of those 

 insects that must prepare for a final metamorphosis into the adult. 



THE CIRCULATORY ORGANS 



Different kinds of animals do not necessarily have every system 

 of organs equally developed. The insect, for example, has an 

 elaborate respiratory system, but its circulatory equipment is com- 

 paratively meager. Its "blood" is 

 the slightly tinted liquid that fills 

 the body cavity and directly bathes 

 the tissues in the latter. This fluid is 

 kept in circulation by a long pulsating 

 tube, or heart, lying just beneath the 

 midline of the back. 



As the upper undivided edges of 

 the mesoderm la3^ers approach each 

 other from opposite sides in the dorsal 

 part of the embryo, their apposing 

 faces become hollowed lengthwise, and 

 when finally the two layers meet the 

 edges unite to form a tube about the 

 space between them. This tube is the 

 heart (figs. 24, 27, Ht). The meso- 

 derm cells extending outward from 

 the heart on each side are reduced to 

 thin sheets, together forming the diaphragm (Dph), a membrane in 

 the adult, in which are developed fan-shaped bundles of muscle 

 fibers attached at one end to the body wall and at the other to the 

 ventral wall of the heart. The diaphragm shuts off a space, the dor- 

 sal sinus (fig. 24, D/S), in the upper part of the body, which incloses 

 the heart, and the blood must, therefore, first enter the dorsal sinus be- 

 fore it enters the heart. It gains access to the former either through 

 open spaces along the edges of the diaphragm or through perfora- 

 tions of the latter, and it then enters the heart through apertures in 

 the sides of the heart w^alls. In the abdomen the heart becomes seg- 

 mentally enlarged into chambers, but in the thorax it remains tubular 

 and extends into the head where it opens beneath the brain. 



The course of the blood and the structure of the heart in the adult 

 insect are shown diagrammatically in Figure 26. The pulsations of 



Fig. 25. — Typical fat cells of an 

 insect during a stage preceding 

 metamorpliosis : The cell proto- 

 plasm filled with globules of 

 fatty oil (Ft) and smaller 

 granules of proteid material 

 (Alb) 



