SECT. VIII.] 



INTRODUCTION. 



39 



of very great size, as in the well-known Hercules beetle (Dynastes 

 hercules) and others. The females of these forms are usually 

 without horns. In such genera it is commonly found that the 

 males are not all alike, but some are of about the size of the 

 females and have little or no development of horns, while others 

 are more than twice the size of the females and have enormous 

 horns. These two forms of male are called " low " and " high " 

 males respectively. 



In many places in the Tropics such beetles abound, both 

 " high " and " low " males occurring in the same locality. An 

 admirable example of this phenomenon is seen in Xylotmvpes 

 gideon, of which a " high," " low," and medium male are shewn 

 in profile in Fig. 1. Of this insect a very large number were 

 kindly given to me by Baron Anatole von Hugel, who collected 

 them at one time, in one locality, in Java. In this species there is 

 one cephalic and one thoracic horn, placed in the positions shewn 

 in the figure. Fig. 1, I shews a "high" male, II is a medium, 

 and ill a " low " male. In the gathering received there were 

 342 males. My friend, Mr H. H. Brindley, has made careful 

 measurements of the lengths of the horns of these specimens and 

 has constructed the diagram, Fig. 2. In this each clot represents 

 an individual, and the abscissae shew the measurements of the 

 length of the cephalic horn. For clearness these measurements 

 are represented as of twice the natural size. So far as the 

 numbers go the result shews that the most frequent forms are 



M' 



Fig. 2. Diagram representing the frequency of the lengths of cephalic horn in 

 male Xylotnqies gideon. M, the mean case ; M' the mean value. The abscissae 

 give lengths of cephalic horn in lines. 



