Fig. [184].—Chilobrachys stridulans in stridulating attitude. After Wood-Mason. Natural size.

Meanwhile the noise produced by a large Theraphosid spider in Assam (Chilobrachys stridulans) had attracted attention, and its stridulating apparatus was described in 1875 by Wood-Mason.[[262]] The sound resembled that obtained by “drawing the back of a knife along the edge of a strong comb.”

Subsequently certain Sicariid spiders of a genus confined to the southern hemisphere were heard to produce a sound like the buzzing of a bee by the agitation of their palps, and both sexes were found to possess a very perfect stridulating organ, consisting of a row of short teeth on the femur of the pedipalp, and a striated area on the paturon of the chelicera.

Pocock has recently discovered that all the large kinds of Theraphosidae in the countries between India and New Zealand are, like Chilobrachys, provided with a stridulating organ. In these spiders also it is between the palp and the chelicera, and consists of a row of teeth or spines constituting a “pecten,” and a series of vibratile spines or “lyra,” but whereas in Chilobrachys and its near relations the lyra is on the palp and the pecten on the paturon, in other spiders the positions are reversed. The lyra is a very remarkable organ, consisting of club-shaped, often feathery bristles or spines, which lie parallel to the surface to which they are attached, and which is slightly excavated for their reception.

Lastly, many African Theraphosids possess a similar organ, 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 significance. 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 Theraphosidae there is a corresponding groove on the inner surface of the labium.

At the top of the pharynx, which is nearly perpendicular, the canal continues backwards and upwards as a narrow tube, the oesophagus, passing right through the nerve-mass, which embraces it closely on all sides, to the sucking stomach. At the commencement of the oesophagus is the opening of a gland, probably salivary, which is situated in the rostrum.