326 TEXT-BOOK OF ENTOMOLOGY 



the work of digestion and the secretion of the urinary or Mal- 

 pighian tubes, this secretion being purely urinary. 



These organs are exclusively depuratory and urinary, freeing the 

 body from waste products of the organic elements. The liquid they 

 secrete contains urea(?), uric acid and abundant urates, hippuric 

 acid (?), chloride of sodium, phosphates, carbonate of lime, oxalate 

 of lime in quantity, leucine, and coloring-matters. 



The products of the rectal or anal glands vary much in different 

 groups, but they take no part in digestion, nor are they depuratory 

 in their nature. 



Insects have nothing resembling chylific substances. 1 The prod- 

 ucts of digestion, dissolved salts, peptones, sugar in solution, emul- 

 sionized greasy matters, pass through the relatively delicate walls 

 of the digestive canal by osmose, and mingle outside of the canal 

 with the blood. 



Whatever substances remain undigested are expelled with the 

 excrements; such are the chitin of the integuments of insects, 

 vegetable cellulose, and chlorophyll, which is detected by the 

 microspectroscope all along the digestive canal of phytophagous 

 insects. 



In his experiments in feeding the larvae of Musca with lacmus, 

 Kowalevsky found that the oesophagus, food-reservoir, and proven- 

 triculus, with its csecal appendages, always remained blue, and had 

 an alkaline reaction ; the mid-intestine, also, in its anterior portion, 

 remained blue, but a portion of its posterior half became deep red, 

 and also exhibited a strong reaction. The hind-intestine, however, 

 always remained blue, and also had an alkaline reaction. (Biol. 

 Centralbl., ix, 1889, p. 46.) 



The mechanism of secretion. Gehuchten describes the process of 

 secretion in insects, the following extract being taken from his 

 researches on the digestive apparatus of the larva of Ptychoptera. 

 The products of secretion poured into the alimentary canal are more 

 or less fluid ; for this reason, it is impossible to say when an epi- 

 thelial cell at rest contains these products. For the secreting nature 

 of these cells is only apparent at the moment when they are ready 



1 Plateau (1877) states that the digestive fluid of insects, as well as of Arachnids, 

 Crustaceans, and Myriopods, has no analogy with the gastric juice of vertebrates; it 

 rather resembles the pancreatic sugar of the higher animals. The acidity quite often 

 observed is only very accessory in character, and not the sign of a physiological 

 property. "Farther, I have found it in insects; Hoppe-Seyler has demonstrated in 

 the Crustacea, and I have proved in the spiders, that the ferment causing the diges- 

 tion of albuminoids is evidently quite different from the gastric pepsine of verte- 

 brates ; the addition of very feeble quantities of chlorhydric acid, far from promoting 

 its action, retards or completely arrests it." (Bull. Acad. roy. Belgique, 1877, p. 27.) 



