Pupal Stage in Insects generally 151 



and it is possible, tliough we liave not actually noted such 

 a case, that the brain may sometimes be lodged within 

 the larval head. It seems probable that Chironomus is 

 less primitive than Tipula and some other Nemocera 

 whose early stages are terrestrial. The species of Chiro- 

 nomus whose larvae dwell at the bottom of the water and 

 make tubes appear to be less primitive than the surface - 

 haunting species. 



It must not be forgotten that of the many species 

 of Chironomus only a minute proportion have had their 

 life-history in any degree elucidated. Increased know- 

 ledge will no doubt greatly add to the list of adaptive 

 modifications of the larval and pupal structures. 



The biological importance of the pupal stage is very The pupal 

 unequal in different insects. Some undergo no trans- insfcts" 

 formation at all (Thysanura). In Orthoptera the so-called s*""^''^^'-^'- 

 pupa is little more than a late larva with rudimentary 

 wings. Where the larva is aquatic and the fly aerial, as 

 in may-flies and dragon- flies, more conspicuous changes 

 are effected during transformation, especially in the mode 

 of respiration, but there is not of necessity a definite rest- 

 ing- stage. Where, as in Lepidoptera, Hymenoptera, and 

 Diptera, the imago adopts a new mode of feeding, great 

 and apparently sudden changes in the mouth-parts are 

 set up, and the jDupa ceases to feed, though it may still 

 retain, as in Chironomus, a limited power of movement. 

 In the Muscidae the divergence of the fly from the larva 

 reaches its extreme. The body is reconstructed during 

 the pupal stage, and the immobility of the pupa is ren- 

 dered complete by its enclosure within the hard and dead 

 larval skin. The gradations observed in existing insects 

 with respect to the completeness of their transformation 

 help us to understand that the elaborate metamorphosis 

 of the Muscidae was attained by many steps. The stages 

 by which holometabolous insects acquired their pupa are 



