287 



CHAPTER IV. 



On tiie number of the segments and on the abdominal legs. 



According to Kowalewsky's observations on the development 

 of Smerinthus poimJi (1871) the abdomen of insects originates from 

 10 somites, all of which possess a tendeiicy to form abdominal 

 legs. (p. 53, PI. XTI fig. 8 and 10). He was folio wed by TiCHO- 

 MiROFF (1879) who counted 11 abdominal somites in Boitibyx iiiori, 

 likewise provided with pedes spurii, except the first. These ab- 

 dominal legs also occur in othcr orders of insects. Rathke showed 

 them in 1846 for MelolontJia^ Heider (1889) for Hydrophilus. 

 The first abdominal legs are remarkably large (Ziegler's Model). 

 Wheeler observed in 1893 also 11 abdominal somites in Xlph'i- 

 dium ensiferum. 



Janet in 1909 comes to a total of 27 metameres of which 9 

 pass into the head and 3 into the thorax, the other 15 form the 

 abdomen. The three posterior ones which appear immediately 

 after the first three head-metameres constitute the proctenteron. 

 The metameres appear in triads, the first and last member of 

 each triad always showing themselves before the middle one. 

 The last triad is formed after the first, the others are regularly 

 developed from the oral to the caudal side. Janet also distin- 

 guishes 12 abdominal ganglions and 3 proctentrical ones in accor- 

 dance with the 15 metameres. 



When W. Muller occupied himself in 1886 with the setal 

 pattern, he clearly saw that the 12tli segment (= the 9tïi abdo- 

 minal) consisted of two parts which were separated by a furrow 

 (1. c. p. 106 — 107). The first part develops into a nearly complete 

 normal segment, the second part, though in fact also a special 

 segment, is called in his description 12a. As he adds „accord- 

 ing to tradition and owing to the circumstance that the 

 value of 12a as an independent ring can only be proved during 

 the first stage". 



As we are obliged to agree with Henneguy's contention 



