Tune, igij.l WeBSTER : MoULTS OF THE PeaR-SlUG. 129 



II, .35; III, .55; IV, .8; V, I.I and VI, i.i mm. Comparing these 

 with the average measurements just given it is evident that Dyar 

 missed one stage, stage III. while the figure suggested for stage I 

 is much too low. The figures for stages II and III are equivalent 

 to those for stages I and II in our averages. Stage III then is miss- 

 ing in Dyar's figures. The measurements given for stages IV, V 

 and VI are approximately equivalent to our averages. 



Taking up the matter of increase in size of the head widths, it 

 is seen that such an increase is by no means constant. The old idea 

 of the moulting process as one merely to allow for the growth of the 

 lar\''a will certainly not hold here, for in many cases little or no 

 growth is recorded after a moult. Dr. O. W. Oestland, of the Uni- 

 versity of Minnesota, suggested to me that the idea of moulting as 

 an excretory process would fit just such a case as this. Folsom 

 (1906), and probably others also, have suggested an excetory func- 

 tion of the moulting process. 



x-\n explanation of the large number of moults, based on the idea 

 suggested in the preceding paragraph, may be found in the somewhat 

 artificial conditions of the insectary cages. Inside the glass battery 

 jars it frequently becomes very hot. The water in the glass bottles 

 which held the cherry twigs probably caused a high humidity also. 

 Such conditions are very different from those outside on a cherry tree, 

 where abundant shade and air currents would keep the temperature 

 and moisture at lower points. It seems quite likely that these arti- 

 ficial conditions had an ill effect on the larv.x, and these, in the 

 endeaA'or to throw off this effect, moulted more than the normal num- 

 ber of times. 



Saunders (1870) found that slugs would moult easily when sand 

 or ashes were thrown over them. He applied sand to some slugs 

 several times, but instead of being harmed by it, they simply moulted, 

 leaving the sand with the old cast skin. Apparently the application 

 of the sand was conducive to the moulting; Saunders said that it 

 occurred several times. 



While the growth of the larva is without doubt an important factor 

 in the ecdysis of the pear-slug, it apparently is not the only factor 

 concerned. The fact that there may be moulting without perceptible 

 growth indicates that other reasons have an important bearing in the 

 explanation of moulting. In fact, growth may not even be a primary 

 factor in the true explanation of the moulting process. 



