270 



to the arrangement of the „hairs'' on caterpillars. It is true that 

 DE Réaumur had pointed out in 1736 the peculiar warts of different 

 caterpillars and the plumed hairs they bear, also that Millière 

 added a drawing of the back part of the body with the hairs on 

 the last thr(!e segraents (1858 I, PI. I, fig. 4) to his description 

 of Coccyx jiinipera Mil., bnt no systematic investigation of the 

 arrangement of the hairs had, as far as I know, taken place before 

 W. Muller. 



Before going further I wish to observe that the „hairs" of the 

 insects are niostly developed as offshoots of a hypodermis-cell. 

 They are absolutely different in construction from the hairs of 

 mammals and therefore the word setae has been introduced for 

 them by Lankester. Consequently I shall only use the word „hairs" 

 in this study, in cases where the writers quoted do not mention 

 the word setae for some reason or other. Fracker (1915, p. 38) 

 thinks that the setae are sensory in function. As with all kinds 

 of other organs the form of the setae of the caterpillars often 

 gets more intricate after each moult, so that it is of great impor- 

 tance to examine all the succeeding stages, and thus to get a good 

 insight into the covering of the skin of the full-grown cater- 

 pillars. To avoid confusion between the two meanings of the 

 word stage (the Caterpillar or larval stage, the pupal stage, and 

 the period between two moults of caterpillars), Fischer has intro- 

 duced the term instar for the last mentioned meaning. The larval 

 stage therefore consists of soveral instars. 



Muller began to pay attention to the first instar of the Nymphalid- 

 larvae. In them he discovered a constant arrangement of the so-called 

 „primary bristles" which he gave the numbers 1 — 6 (see Nomen- 

 clature). Bristle 6 oïily occurs on the segments 2 — 5 and 10 — 12. 



The segments 1 , 2 and 3 (the thoracic rings) are different and 

 so is segment 12. A comparison of the bristles proves that 

 there is a special segment 12», but this only exists during the 

 first instar. It is very easy to homologize the bristles on the 

 segments 4—12 (the abdominal ones) but the mesothorax and meta- 

 thorax are widely different, a shifting may perhaps have taken 



