536 INSECTA. 



The so-called fat bodies stand in the closest relation to respiration 

 and the nutritive processes. They are fat-like shining and usually 

 coloured, lobed and globular bodies, which are distributed beneath 

 the skin and between the organs, and are especially abundant during 

 larval life. The chief importance of these organs depends on the part 

 they play with regard to metabolism. They consist essentially of an 

 accumulation of superfluous nutritive material, and seem to be used 

 for nourishment and for the production of heat, and especially during 

 the development into the perfect insect for the formation of new 

 parts of the body and for the growth of the generative organs. The 

 rich distribution of the tracheae to the fat cells points to the con- 

 sumption of a large amount of oxygen, and consequently to an 

 active metabolism, which is further demonstrated by the frequent 

 deposition of nitrogenous waste material, especially of uric acid. 



The phosphorescent organs of the Lampyridce and various 

 Elateridce show a certain resemblance to fat bodies. These organs 

 are delicate plates, which in Lampyris are present on the ventral 

 surface of several of the abdominal segments and consist partly of 

 pale albuminous cells, and partly of granular cells, containing uric 

 acid ; richly branched trachea and nerves are distributed amongst 

 these cells. The pale cells compose the lower ventral layer of the 

 plates, and it is this layer alone which is phosphorescent. These cells, 

 together with the terminal cells of the tracheae, which are always very 

 numerous, are to be regarded as the active elements, the chemical 

 changes of which, under the influence of oxygen, and to a certain 

 extent of the nervous system, give rise to the phenomenon of 

 phosphorescence. The cells of the upper non-luminous layer of the 

 plates contain a great number of refractile granules, which, accord- 

 ing to Kolliker, consist of uric acid compounds, the final products of 

 the metabolism which causes the phenomenon of phosphorescence. 



The nervous system of insects presents a very high development, 

 and a great amount of variation in arrangement; all transitions 

 between a long ventral ganglionic chain, consisting of about twelve 

 pairs of ganglia, and a common thoracic ganglionic mass are found 

 (figs. 77 and 78). The brain (supra-o3sophageal ganglion), which is 

 placed in the head, attains a considerable size. It presents several 

 groups of swellings ; these are especially marked in the Jlymenoptera, 

 which have the highest psychical development. It gives origin to the 

 sense nerves, and seems to be the seat of the will and of the psychical 

 activity. The small suboesophageal ganglion supplies the mouth 

 parts, and corresponds to several pairs of ganglia fused together. 



