ALIMENTARY SYSTEM I 27 



swallowed in the same manner as the food, not improbably along 

 with it. The silk so copiously produced by some larvae comes 

 from very long tubes similar in form and situation to the simple 

 tubes of the salivary glands. 



The Malpighian tubules are present in most Insects, though 

 they are considered on good authority to be absent in many 

 CoUembola and in some Thysanura. They are placed near the 

 posterior part of the body, usually opening into the alimentary 

 canal just at the junction of the stomach and the intestine, at a 

 spot called the pylorus. They vary excessively in length and in 

 number,^ being sometimes only two, while in other cases there 

 may be a hundred or even more of them. In some cases they 

 are budded off from the hind-gut of the embryo when this is 

 still very small ; in other cases they appear later ; frequently 

 their number is greater in the adult than it is in the young. 

 In Gryllotal-pa there is one tube or duct with a considerable 

 number of finer tubes at the end of it. There is no muscular 

 layer in the Malpighian tubes, they being lined with cells which 

 leave a free canal in the centre. The tubes are now thought, on 

 considerable evidence, to be organs for the excretion of uric acid 

 or urates, but it is not known how they are emptied. Marchal 

 has stated ^ that he has seen the Malpighian tubes, on extrac- 

 tion from the body, undergo worm-like movements ; he suggests 

 that their contents may be expelled by similar movements when 

 they are in the body. 



The functions of the different portions of the alimentary 

 canal, and the extent to which the ingested food is acted on by 

 their mechanical structures or their products is very obscure, and 

 different opinions prevail on important points. It would appear 

 that the saliva exercises a preparatory action on the food, and 

 that the absorption of the nutritive matter into the body cavity 

 takes place chiefly from the true stomach, while the Malpighian 

 tubes perform an excretory function. Beyond these elementary, 

 though but vaguely ascertained facts, little is known, though 

 Plateau's ^ and Jousset's researches on the digestion of Insects 

 throw some light on the subject. 



^ For a review of their number see Wheeler, Psyelie, vi. 1893, pp. 457, etc. 

 ^ Ann. Soc. Ent. France, Ixi. 1892, Bull. p. colvi. 



2 Mem. Ac. BeUjigue (2), xli. 1875, and Bull. Ac. Bchjique (2), xliv. 1877, 

 p. 710. 



