214 



THE RATE OF GROWTH 



[CH. 



two groups of earwigs were of diiferent ages, or had passed through 

 one moult more or less, the phenomenon would be simple indeed, 

 and there would be no more to be said about it*. Diakonow made 

 the not unimportant observation that in earwigs living in un- 

 favourable conditions only the short-tailed type tended to appear. 

 In apparent close analogy with the case of the earwigs, and in 

 apparent corroboration of their dimorphism being due to age, 

 Fritz Werner measured large numbers of water-fleas, all apparently 

 adult, found his measurements falling into groups and so giving 

 multimodal curves. The several cusps, or modes, he interpreted 

 without difficulty as indicating diiferences of age, or the number 

 of moults which the creatures had passed through f (Fig. BO). 



9 

 /i. 60 



10 II 12 13 14 15 16 17 18 19 20 21 22 23 24 



80 100 120 140 160/1 



Length in ^ 



Fig. 60. Measurements of the dorsal edge in a population of 

 Chydorus sphaericus, a water-flea. 



From Fritz Werner. 



An apparently analogous but more difficult case is that of a 

 certain little beetle, Onthofhagus taurus, which bears two "horns" 

 on its head, of variable size or prominence. Linnaeus saw in it 

 a single species, Fabricius saw two; and the question long remained 

 an open one among the eniomologists. We now know that there 

 are two "modes," two predominant sizes in a continuous range of 



* The number of moults is known to be variable in many species of Orthoptera, 

 and even occasionally in higher insects; and how the number of moults may be 

 influenced by hunger, damp or cold is discussed by P. P. Calvert, Proc. Amer. 

 Pkilos. Soc. Lxviii, p. 246, 1929. On the number of moults in earwigs, see E. B. 

 Worthington, Entomologist, 1926, and W. K. Weyrauch, Biol. Centralbl. 1929, 

 pp. 543-5^8. 



■f Fritz Werner, Variationsanalytische Untersuchungen an Chydoren, Ztschr. f. 

 Morphologie u. Oekologie d. Tiere, n, pp. 58-188, 1924. 



