178 Stri dul citing- Organ in Trechona zebrata. 



examined of the following Neotropical groups : — Avicularieae, 

 Eurypelmateae, Theraphoseae, and Homoeommatea?." 



The genus Trechona, however, belonging to a distinct sub- 

 family — namely, that of the Diplurinae, — and represented in 

 the National Collection by a single dried example, which will 

 very possibly prove to be Walckenaer's type, I did not 

 examine for this organ, for fear of causing needless damage 

 to a valuable and unique specimen. But upon reading- 

 Mr. Blackwall's article, I immediately took the necessary 

 steps to verify his statement, and was rewarded by rinding 

 the organ almost exactly as he described it. The set of notes 

 on the maxilla is of large size, and occupies nearly the whole 

 length of the inner surface of this segment. The notes com- 

 posing the distal two-thirds of the group are black and 

 thickly clustered together ; they are not, however, short and 

 spicular as Blackwall's figure represents, but long and straight. 

 Those, on the contrary, that compose the proximal third of the 

 cluster are thicker, more separated from each other, curved, 

 and arranged in a single line, the individual notes gradually 

 decreasing in length as they pass from the centre of the 

 cluster to its proximal end. The modified hairs on the op- 

 posable surface of the mandible, which Blackwall does not 

 mention, are constructed on very much the same plan as the 

 corresponding hairs that I have described in Psalmopceus 

 Cambridgii * — that is to say, they consist of a small number 

 of long, thick, but apically filiform setaa set on the posterior 

 portion of the lower edge of the jaw, and these pass into a set 

 of rigid setas, which ultimately blend with the hairs at the base 

 of the fang. 



A point to be noticed in connection with the occurrence of 

 this remarkable organ in a Neotropical genus belonging to 

 a subfamily with which the Oriental stridulating Seleno- 

 cosmiidaa seem to have no special affinity, is the unavoidable 

 conclusion that substantially the same structure has been 

 developed twice over. Moreover, as I have pointed outf, the 

 instrument is also found in another spider, Idiommata Black- 

 wallii, Camb., which belongs to yet a third subfamily. So 

 that apparently we have evidence that the organ has been 

 independently acquired three times within the limits of a 

 single group of spiders. 



This conclusion at once raises the question as to the value 



of the character in uniting the genera that I have grouped 



together as iSelenocosmiidaa. For it may well be asked what 



reasons there are for regarding it as a sign of affinity in the 



* Ann. Mag. Nat. Hist. (6) xv. p. 178, pi. x. fig. 3 a. 



f Ann. Mag. Nat. Hist. (6) xvi. p. 225. 



