ENEMIES OF APHIDES. 127 



Now, the percentage of nitrogen in beef is stated by Dr. 

 Letheby at ten, and Professor Goodfellow tells us that it is in the 

 form of albuminoids, which constitute fifteen per cent, of the 

 whole bulk. And very little difference exists in the composition 

 of albuminoids, whether they are derived from an animal or vege- 

 table source. We have seen that pollen contains some 17 or 18 

 per cent, of these compounds of nitrogen. But although nitrogen 

 is one of ihe most necessary constituents of animal food, and is 

 generally ingested in the form of albuminoids, yet as such, it is 

 incapable of assimilation, because albuminoids are incapable of 

 undergoing diffusion through animal membrane. Hence, it is 

 necessary for them to undergo some change which shall render 

 them diftusable, and in man this is accomplished by the first secre- 

 tion which the food meets in the stomach — that known as the 

 " gastric juice." Under the influence of certain constituents of 

 this secretion, the albuminoids are converted into " peptones," 

 which, while possessing a similar composition to albuminoids, are 

 highly diifusable, and are capable of readily passing through animal 

 membrane. The process is somewhat as follows : — When food 

 enters the human stomach, it sets up a nervous disturbance, which 

 causes the walls to become suffused with blood, and under the 

 influence of the stimulation thus applied, the gastric glands pour 

 out their secretion to effect the necessary change. 



In the Syrphus fly the similarly constituted food, on being 

 impelled from the extension of the oesophagus, known as the 

 sucking stomach, enters the proventriculus, which forms the 

 entrance to the proper or digestive stomach. Here it meets with 

 the secretion of the adjacent glands, shown on PI. 1V„ Vol. V., 

 at «, 71^ which appear to be brought into action by the presence of 

 hard food in the proventriculus, and following on this the changed 

 appearance of the pollen makes it clear that the digestive process 

 has begun. For comparison a gastric gland from the human 

 stomach is depicted on PI. VHI. at Fig. 4, and it will be seen that 

 it presents considerable analogy to the apparently similar gland of 

 the fly. 



On these grounds, therefore, it seems not unwarrantable to 

 assume that the secretion of the glands {n, n), which is the first 

 digestive fluid encountered, apart from the saliva, answers much 



