FAT-BODIES. 629 



ated membrane, attached to which are paired alary muscles ; 

 whose outer ends are inserted into the under side of the terga. 

 The contraction of these enlarges the pericardia] space around the 

 heart and the blood passes through the perforations of the 

 pericardium toward the ostia. The exact function of the peri- 

 cardial membrane and its muscles is still a matter of dispute. 



There are certain structures found in the body-cavity of insects 

 which play a part in the nutrition and excretory processes of 

 these animals. These are (i) the Fat-bodies. This term is applied 

 to masses of opaque, white cells crowded with minute fat globules 

 and often with urates. They arise from the mesoblast and are 

 often metamerically arranged. The fat-bodies evidently play a 

 part in the general metabolism of the insect and from their smaller 

 size in the imago than in the larva of some species it would seem 

 that they act as a food supply during pupation, (ii) The Pericar- 

 dial Cells. These occur around the heart and on the pericardial 

 membrane and are sometimes metamerically arranged. They 

 also are said to contain urates, and some authorities attribute 

 to them a phagocytic action, (iii) The Phosphorescent Organs. 

 These are most usually found in the beetles ; in the abdomen 

 in Lampyris and in the thorax in Pyrophorus. Essentially they 

 are plate-like modifications of the fat-body shining through 

 the body wall and like the fat-body are richly provided with 

 tracheae. Their light is the most economically produced we 

 know, since no energy is wasted by the production of heat, 

 and all the radiation lies between the two ends of the visible 

 spectrum. In those cases which have been investigated, the 

 spectrum of the phosphorescent light gives no dark bands but 

 is perfectly continuous. The usual view of the function of these 

 organs is that they serve to attract the other sex. Emery * how- 

 ever holds that they act as a danger signal, a warning sign that 

 their possessor is inedible, (iv) The Oenocyte cells. These are 

 clusters of very large cells, the largest in the body except the 

 ova, they are found usuaDy in the abdomen of winged insects 

 attached to the end of tufts of tracheae. Unlike the fat- 

 bodies mentioned above, they arise from the ectoderm, being 

 pushed in with the tracheae, and are segmentally arranged. 

 Although they somewhat resemble the blood corpuscles they are 



* Zeitschr. wiss. Zool, xl, 1884, p. 338, andBtill. Soc. ent. Ital, xvii, 1885, 

 p. 351. 



