154 



HARDWICKE'S SCIENCE-GOSSIP. 



consist largely of crystals, which either occur singly 

 in the epithelial cells, or heaped up in the central 

 canal. Occasionally, they form spherical concretions 

 with a radiate arrangement. They contain uric acid, 

 and probably consist of urate of soda.* In the living 

 insect the tubules remove urates from the blood 

 which bathes the viscera ; the salts are condensed and 

 crystallised in the epithelial cells, by whose dehis- 

 cence they pass into the central canals of the tubules, 

 and thence into the intestine. 



In the present rudimentary state of invertebrate 

 physiology, little can be certainly affirmed of the 

 digestive processes of any insect. The difficulties of 

 observation and experiment are great, and the investi- 

 gators are few. Plateau's memoirs on the physiology 



by him seemed to show that the saliva, as well as 

 the contents of the oesophagus and crop, had an acid 

 reaction, while the contents of the chylific stomach 

 were neutral at the beginning of the tube and alkaline 

 further down. From this he concluded that the 

 deep-seated glands of the chylific stomach secreted 

 an alkaline fluid, which neutralised the acidity of the 

 saliva. Finding that the epithelial cells of the 

 stomach were often loaded with oil-drops, he con- 

 cluded that absorption, at least of fats, takes place 

 here. The chylific stomach, carefully emptied of its 

 contents, was found to convert starch into sugar at 

 ordinary temperatures. The saliva of the cockroach 

 gave a similar result, and when a weak solution of 

 hydrochloric acid was added, Bascli thought that the 



Fig. S4.^Salivary glands and receptacle of cockroach, right side. The arrow marks the opening of the common duct on the back 



oftheUngua. A, side view of lingua ; is, front view of hngua. 



of various Arthropoda are the most laborious and 

 comprehensive which we possess, f The observations 

 of Basch are earlier and less complete. 



Basch set out with a conviction' that where a 

 chitinous lining is present, the epithelium of the 

 alimentary canal secretes chitin only, and that proper 

 digestive juices are only elaborated in the chylific 

 stomach, or in the salivary glands. The tests applied 



* The contents of the malpighian tubules may be examined by 

 crushing the part in a drop of dilute acetic acid, or in dilute sul- 

 phuric acid (lo per cent.). In the first case a cover-slip is placed 

 on the fluid, and the crystals, which consist of oblique rhombohc- 

 drons or derived forms, are usually at once apparent. If sul- 

 phuric acid is used, the fluid must be allowed to evaporate. In 

 this case they are much more elongated, and usually clustered. 

 The murexide reaction does not give satisfactory indications 

 with the tubules of the cockroach. 



■j- See his " Rech.experimentalessur la digestion des Insectes," 

 and various memoirs published by the Royal Belgian Academy. 



mixture could digest blood-fibrin at ordinary tem- 

 peratures. 



Plateau's principal conclusions as to the digestive 

 processes of cockroaches are the following, (i) The 

 saliva changes starch into glucose, but it is, like all 

 the digestive secretions of insects, alkaline, and not 

 acid. Acidity in the contents of the crop is due 

 either to the ingestion of acid food, or to an acid 

 decomposition of the same. The supposed solvent 

 action of saliva upon fibrin is imaginary. (2) Absorp- 

 tion of glucose takes place in the crop, and glucose is 

 neither formed nor absorbed in succeeding parts of 

 the alimentary canal. (3) The function of the gizzard 

 is that of a strainer ; it has no triturating power. 

 (4) The caecal tubes pour forth a fluid which is often 

 distinctly alkaline (never acid), and which neutralises 



