404 



than twice as broad as long in jacobsoni (text-fig. 12). The 

 general shape and structure are the same in the two species, 

 the anterior edge appearing slightly incurved on account of its 

 central portion being bent downward. There are a greater 

 number of moderately small bristles in between the larger ones 

 in jacobsoni than in esau, which is also the case on other parts 

 of these insects, but the anterior edge of the labrum bears 

 more bristles in esau than in jacobsoni. 



In his account of the morphology of A. esau Jordan laid 

 special stress on the peculiar development of the mandibles, 

 which differ very remarkably from those of other Dermaptera. 

 The new Arixenia proves these organs to be strongly susceptible 

 to modification. This fact is well known in many other groups 

 of insects ; but the mandibles of the Dermaptera have not yet 

 been studied comparatively. They are concealed by the upper- 

 lip and can only be seen without much trouble in specimens 

 preserved in alcohol, which allow of the upperlip being lifted 

 up. The series of dermapterous mandibles which our figures 

 15-19 represent is of some interest from the view-point of com- 

 parative morphology. The usual type as represented by the 

 common Earwig {Forfícula auricularia) is triangular and more 

 or less strongly flattened in a dorso-ventral direction, with 

 the tip pointed (text-fig. 15). The tip is divided into two 

 teeth, which appears to be characteristic of all Dermaptera 

 inclusive of Hemimerus. The inner or masticating edge of the 

 mandible is widened in Forfícula (text-fig. 15) into a low and long 

 double ridge (= dorsal and ventral edges) and bears proximally 

 to this ridge a few bristles. The mandible of Hemimerus 

 (text-fig. 16) is similar, but the ridge terminates proximally in the 

 shape of a tooth, and the bristles are slightly more numerous. 

 In Arixenia jacobsoni (text-figs. 17 and 18) the mandibles are 

 very large and strong, the two apical teeth are long and sharp, 

 and the median tooth is large and curved in the right mandible 

 (text-fig. 18) and shorter and more regularly triangular in 

 the left one (text-fig. 17). The tooth of the left mandible is 

 divided by a notch, the two tips corresponding to the double 

 tooth occurring in Earwigs. Moreover, the setiferous portion 

 of the inner edge is much more extended than in Forfícula and 

 Hemimcrtis. 



