LARVA AND PUPA. 651 



metabola it may be high, in Ephemeridae sometimes as many 

 as twenty. 



The pupa resembles the imago much more closely than does 

 the larva. It may be exposed or enclosed in a cocoon. The 

 limbs of the pupa may stand out freely from the body as in the 

 Coleoptera, when it is known as free (pupa libera) ; or the limbs 

 and wings, very soon after the emergence of the pupa from the last 

 larval skin, may be glued to the side by a hardened secretion 

 as they are in the Lepidoptera, when the pupa is called obtecta ; 

 or finally, as in many Diptera, the pupa may remain enclosed 

 in the last larval skin, a condition known as a pupa coarctata. 



The larval body of those insects with incomplete meta- 

 morphosis grows gradually into the body of the adult. Apart 

 from wear and tear, the tissues of the one become* in time the 

 tissues of the other and the new organs such as the wings and 

 the genital armature are formed by simple growth. In the 

 insects with complete metamorphosis this is not the case. 

 Traces of the imago are clearly discernible in the later phases 

 of the larval stage, but when this ceases to move and to feed 

 and turns into the inactive pupa a profound reorganization 

 takes place in the body of the insect. A few of the systems 

 of organs persist from the larva to the imago and these are 

 amongst the most important such as the reproductive, the 

 nervous and the circulatory. The other organs however are 

 disintegrated. At the beginning of metamorphosis certain 

 toxins * are said to arise in the blood which seem to poison 

 certain definite tissues and cells, which then readily fall victims 

 to the phagocytes. Each toxin seems specific to one set of organs, 

 and the toxins arise in a definite sequence. The alimentary 

 canal with its associated glands, the respiratory system, the fat- 

 bodies, the epidermis and most of the muscles are destroyed and 

 replaced by new growths. The active agents in this process of dis- 

 integration, known as histolysis, are the blood corpuscles. These 

 become vigorously phagocytic and attacking the component cells 

 of the fated organs gradually break them up and absorb them. 

 At the same time new organs arise to play the same parts in 

 the imagines as did the evanescent organs in the larva. And 

 they arise at special foci or centres which are known as imaginal 

 discs. These are collections of cells which seem to have retained 

 * S. Metalnikoff, Biol. Centrbl. xxvii. 1907, p. 396. 



