XIII 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. Chilobrachys 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. 
