ALIMENTARY CANAL 329 



not between the palp and the chelicera, but between the palp and 

 the first leg. 



Various suggestions have been hazarded as to the use of these 

 organs, but they partake largely of the nature of conjecture, 

 especially in connexion with the doubt as to the possession of a 

 true auditory organ by the Araneae. They may be summarised 

 as follows. The Theridiid spiders are among those which show 

 most indication of auditory powers, and the stridulating organs, 

 being practically confined to the male, may have a sexual signifi- 

 cance. Chiloirachys stridulates when attacked, assuming at the 

 same time a " terrifying attitude,'' and its stridulating organ may 

 serve the purpose attributed to the rattle of the rattlesnake, and 

 warn its enemies that it is best let alone. If this be the case, 

 there is no need that it should itself hear the sound, and, indeed, 

 there is no evidence that the Aviculariidae possess the power of 

 hearing. In the inoffensive stridulating Sicariid spiders the sounds 

 could hardly serve this purpose, and the presence of the organ in 

 both sexes, and in immature examples, precludes the idea that its 

 function is to utter a sexual call. Instead of trying to escape 

 when disturbed, the spider starts stridulating, and Pocock suggests 

 that the similarity of the sound produced to the buzzing of a bee 

 may be calculated to induce its enemies to leave it in peace. 



Internal Anatomy. 



Alimentary System. — The alimentary canal of the Spider is 

 divided into three regions, the " stomodaeum," the mid-gut or 

 " mesenteron," and the hind-gut or " proctodaeum." 



The Stomodaeum consists of the pharynx, the oesophagus, and 

 the sucking stomach. As we have said, the mouth is to be 

 found between the rostrum and the labium. It opens into the 

 pharynx, the anterior wall of which is formed by a chitinous plate 

 on the inner surface of the rostrum, sometimes called the palate. 

 As the inner surfaces of the rostrum and labium are practically 

 flat, the cavity of the pharynx would be obliterated when they 

 are pressed together, were it not for a groove running down the 

 centre of the palate, which the apposed labium converts into a 

 tube, up which the fluids of the prey are sucked. In the Thera- 

 phosidae there is a corresponding groove on the inner surface of 

 the labium. 



