SEXUAL SELECTION 361 



males of several species of Theridion" have the power of 

 making a stridulating sound, while the females are mute. 

 The apparatus consists of a serrated ridge at the base of the 

 abdomen, against which the hard hinder part of the thorax 

 is rubbed ; and of this structure not a trace can be detected 

 in the females. It deserves notice that several writers, 

 including the well-known arachnologist, "Walckenaer, have 

 declared that spiders are attracted by music.'* From the 

 analogy of the Orthoptera and Homoptera, to be described 

 in the next chapter, we may feel almost sure that the stridu- 

 lation serves, as Westring also believes, to call or to excite 

 the female ; and 'this is the first case known to me in the 

 ascending scale of the animal kingdom of sounds emitted 

 for this purpose." 



Glass, Myriapoda. — In neither of the two orders in this 

 class, the millipeds and centipeds, can I find any well- 

 marked instances of such sexual differences as more par- 

 ticularly concern us. In Glomeris limhata, however, and 

 perhaps in some few other species, the males differ slightly 

 in color from the females; but this Glomeris is a highly 

 variable species. In the males of the Diplopoda, the legs 

 belonging either to one of the anterior or of the posterior 

 segments of the body are modified into prehensile hooks 

 which serve to secure the female. In some species of lulus 

 the tarsi of the male are furnished with membranous suckers 

 for the same purpose. As we shall see when we treat of 

 Insects, it is a much more unusual circumstance that it is the 

 female in Lithobius which is furnished with prehensile ap- 

 pendages at the extremity of her body for holding the male." 



^ Theridion {Asagena, Sund.) serratipes, 4-pimciatv,m et guUatum; see 

 Westring, in.Kroyer, "Natnrhist. Tidskrift," vol iv., 1842-43, p. 349; and 

 vol. ii., 1846-49, p. 342. See also, for other species, "Aranese Siiecicse," 

 p. 184. 



*» Dr. H. H. van Zouteveen, in his Dutch translation of this work (vol. L 

 p. 444), has collected several cases. 



" Hilgendorf, however, has lately called attention to an analogous strao- 

 ture in some of the higher crustaceans, which seems adapted to produce sound; 

 see "Zoological Eecord," 1869, p. 603. 



" Walckenaer et P. Gervais, "Hist Nat. des Insectes: Apteres," torn. It., 

 1847, pp. 17, 19, 68. 

 Descent — ^Vol. I. — 16 



