Page:The Zoologist, 4th series, vol 2 (1898).djvu/42

16 therefore are areas favourable for the development of stridulating organs; for in the great majority of cases—the Cicada, by the way, being a notable exception—stridulation in the articulated animals results from the friction of two mutually roughened adjacent chitinous areas.

Strictly speaking, however, this is not the case with the stridulating organs that have been found in the Spiders now under discussion; for in all cases these organs consist of modified bristles. In the Oriental members of the family two such organs exist, namely, the one discovered by Wood-Mason, and another discovered by myself in several more genera. In both cases the organ lies between the outer surface of the mandible and the inner surface of the maxilla—the basal segment of the palp; and each consists of a set of vibratile bristles, which are set a-twanging by a series of spines. But whereas in Wood-Mason's instrument the vibratile bristles or notes are placed on the maxilla, and the spines or scraper on the mandible, exactly the opposite obtains in the other instrument, the notes being on the mandible and the scraper on the maxilla.

In some of the African Theraphosidæ I have also had the good fortune to discover two stridulating organs, which are not only quite different from each other, but also quite different from those possessed by the genera inhabiting Tropical Asia. One of these organs occurs in the genus Harpactira, the common "Mygale" of Cape Colony. It occupies the same position as the analogous organs existing in the Oriental species, being situated between the mandible and the maxilla. A glance at Fig. 1 will show the construction of the organ. The outer surface of the mandible (A) is furnished with a large pad of feathery hairs (b), and on the area between this pad and the oral fringe (c) are two sets of bristles, both of which, judging from their colour and structure, originally formed part of the oral fringe, and have been derived from it. Those of the upper series are long, and have their free ends bent over and more or less interlacing with each other. Those of the lower series are less regularly arranged. In the species figured they are short and spiniform; but in some allied forms they are much less distinctly differentiated from the