174 ROUND THE YEAR 



parts. Short and powerful mandibles will be dis- 

 carded, and replaced by a suctorial proboscis. Change 

 of mouth-parts inevitably means cessation from 

 feeding, and almost inevitably cessation from work 

 and travel. Then we get a resting-stage. When 

 once established, the resting-stage may be turned to 

 good account in refitting the internal organs, and 

 indirectly in promoting specialisation of the earlier 

 and later stages. Where a pupa-stage is provided, 

 the larva may be yet heavier and slower, the fly yet 

 more swift and light. If the flying adult is specialised, 

 and the female capable of flying far and scenting 

 food at a distance, the better will be the provision 

 made for the young larva and the less the exertion 

 demanded of it. But the more inert the larva, and 

 the greater the interval between it and the active, 

 quick-witted Fly or Moth, the more complete will be 

 the change to be undergone in the resting-stage. 



Though some Insects and not others are described 

 as undergoing transformation, the essential and prim- 

 itive feature is the periodical change of skin which 

 occurs in all Insects. In many cases advantage is 

 taken of the change of skin to secure a change of 

 form. The interval between the last moult and the 

 last but one, when passed in outward inactivity for 

 the purpose of effecting conspicuous change of form, 

 is what we call the pupa-stage. The more the larva 

 resembles the imago, the less the need of a true 

 pupa-stage. Difference of food in the early and 

 final stages, scattered food in the larval stage, are 

 among the reasons for conspicuous difference be- 

 tween the larva and the winged Insect, and indirectly 



