27 



CHAPTER IV. 

 On the number of the seqments and on the abdominal legs. 



According to Kowalewsky'b observations on the development 

 of Stnerinthm populi (1871) the abdomen of insects originates from 

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

 legs. (p. 53, PI. XII fig. 8 and 10). He was followed by TiCHO- 

 MIROFF (1879) who counted 11 abdominal souuU^s \n lionibyx nwn, 

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

 dominal legs also occur in other orders of insects. Ratiikk showed 

 them in 1846 for Melolontha, Heidkr (1889) for HydrophUus. 

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

 Wheeler observed in 1893 also 11 abdominal somites in Xiphi- 

 dium etuiferum. 



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 1 2 abdominal ganglions and 3 proctentrical ones in accor- 

 dance with the 15 metameres. 



When W. Miller occupied himself in 1886 with the setal 

 pattern, he clearly saw that the 12th segment (= the 9th 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 



