vi HEXAPODA METAMORPHOSES OF INSECTS 499 



C. The Inner Processes in the Metamorphoses of Insects. 



In insects without metamorphosis or with gradual or incomplete metamorphosis 

 the organs of the larva simply pass into those of the imago. There is no breaking up 

 and disappearance of the larval organs, and no new formation of the imaginal organs, 

 if we leave out of account the throwing off of tracheal gills and the formation of 

 wings, compound eyes, and so on. 



In insects with complete metamorphosis the case is different. The larval organisa- 

 tion has here been adapted, independently of the imago, to special conditions of 

 existence. A gradual continuous transformation of the larval organs into the often 

 quite differently constructed organs of the imago, during which the different stages 

 would feed independently, is inconceivable, since organs undergoing such complete 

 transformations could hardly be capable of functioning. There are, further, numerous 

 phenomena in the most various divisions of the animal kingdom which prove that 

 organs which have functioned actively during larval life are only slightly capable 

 of development and metamorphosis. They are more often reabsorbed or thrown off 

 in the further course of development. We thus see why in insects with complete 

 metamorphosis the transition from the larva to the imago almost necessarily takes 

 place during a pupal stage, the pupa changing into the imago partly at the expense 

 of the reserve nourishment accumulated by the larva, it being unable to acquire 

 food for itself. 



So as to understand the inner processes in the metamorphosis of the holometabo- 

 listic insect, we will take as an example the larva of Corethra plumicornis (Diptera, 

 Tipularia). Most of the larval organs here simply pass 

 during metamorphosis into those of the pupa and of the 

 imago. The larva, however, is footless and wingless. 

 The rudiments of feet and wings form shortly before the 

 pupal stage. Three pairs of ventral and 3 pairs of dorsal 

 invaginations of the hypodermis appear and are called 

 imaginal discs. In the bases of these invaginations out- 

 growths appear and grow continuously longer, while the 

 invagination in which they lie deepens (Fig. 354). The 

 outgrowths in the 6 ventral invaginations are the rudi- 

 ments of the thoracic limbs, the outgrowths of the 2 

 posterior dorsal pairs of imaginal discs are the rudiments 

 of the wings and halteres which thus lie hidden within Fig. 354. Rudiments 01 

 the body, till they are protruded and attain development. * n e imaginal discs in the 



The muscles of the wings are already rudimentarily larva of Corethra , diagram- 



, ,, i i - .LI i matte. Invaginations (fe and 



present m the embryo as cell strands, but they only ^ of the la rval hyp j de rmis 



begin to differentiate at the end of larval life. (ihy), in whose bases the rudi- 



The complete metamorphosis of some insects, especi- ments of wings (fa) and legs 

 ally of the Muscidce (e.g. Musca vomitoria), is accom- < fca ) arise 5 ^, chitinous in- 

 panied by far more thorough transformations. 



It must, first of all, be pointed out that the distinction between larval, pupal, and 

 imaginal stages rests upon external phenomena. In the inner organisation the series 

 of alterations is continuous ; the larva on the one hand already possesses the rudi- 

 ments of the imaginal organs, and in the pupa on the other, the larval organs only 

 gradually disappear. Speaking generally the inner metamorphosis is such that the 

 imaginal organs proceed out of parts of corresponding larval organs, which remain 

 during larval life in an undifferentiated embryonic condition (formative centres) ; the 

 portions which function during larval life gradually disappear during metamorphosis in 

 proportion as the imaginal parts attain development. The amoeboid blood corpuscles 



