THE METAMORPHOSIS OF INSECTS 201 



parts of the larva are small and those of the adult are large, only the 

 tips of the developing adult organs are within those of the larva at the 

 close of the larval period, a considerable part of the adult organs being 

 embedded in the head of the old larva. 



In a few Coleoptera and Neuroptera (the Dytiscidae, Myrme- 

 leonidae, and Hemerobiidae) the larvae, although mandibulate, have 

 the mouth-parts fitted for sucking. In these cases the form o the 

 mouth-parts have been modified to fit them for a peculiar metho d of 

 taking nourishment during the larval life. The mouth-parts of the 

 adults are of the form characteristic of the orders to which these 

 insects belong. 



In those insects in which the larvae have biting mouth-parts and 

 the adults those fitted for sucking, the development is less direct. In 

 the Lepidoptera, for example, to take an extreme case, there are great 

 differences in the development ot thf. different organs; within the 

 mandibles of the old larvae there are no developing mandibles, these 

 organs being atrophied in the adult; but at the base of each larval 

 maxilla, there is a very large, invaginated histoblast, the developing 

 maxilla of the adult; these histoblasts become evaginated at the 

 close of the larval period, but the maxillae do not assume their defini- 

 tive form till after the last ecdysis. 



The extreme modification of the more usual course of development 

 of the mouth-parts is found in the footless and headless larvae of the 

 more specialized Diptera. Here the mouth-parts do not appear 

 externally until during the pupal stadium and become functional only 

 when the adult condition is reached. See the figures illustrating the 

 development of the head in the Muscidae (Fig. 220). 



It should be noted that the oral hooks possessed by the larvae of the 

 more specialized Diptera are secondarily developed organs and not 

 mouth-parts in the sense in which this term is commonly used. ' These 

 oral hooks serve as organs of fixation in the larvae of the CEstridae and 

 as rasping organs in other larvae. 



e. THE DEVELOPMENT OF THE GENITAL APPENDAGES 



The development of the genital appendages of insects has been 

 studied comparatively little and the results obtained by the different 

 investigators are not entirely in accord ; it is too early therefore to do 

 more than to make a few general statements. 



In the nymphs of insects with a gradual metamorphosis rudimen- 

 tary genital appendages are more or less prominent and their develop- 



