CHAPTER V 



THE PUPA OF CHIRONOMUS 



The pro- Larvae about to Undergo pupation can be easily 



pupation, distinguisliecl by tlie tbickened tborax. If a number of 

 such larvae are observed continuously for a few liours. 

 the process of pupation can be studied without serious 

 difficulty. The first distinct sign of change is the 

 retraction of the epidermis and soft parts from the old 

 cuticle of the prothoracic feet. Very shortly after this 

 (about a minute) the same process -takes place in the 

 blood-gills, and a little later in the anal feet. After 

 a further interval of a few seconds, or at most a minute 

 or two, the head and thorax of the pupa protrude from 

 the dorsal surface, between the larval head and prothorax. 

 The larval head, which has been emptied by the retrac- 

 tion of its contents, then slips round to the ventral surface. 

 The order of these events is not quite constant. Now 

 and then the anal feet and other posterior appendages 

 are seen to be unchanged in a larva which has already 

 slipped off the larval head, Imt this is unusual. It is 

 probable that the contraction of the thoracic and anal 

 regions sets up a blood-pressure, which is the immediate 

 agent in the protrusion of the puj^al head. An inde- 

 pendent indication of the existence of such blood-pressure 

 at the time of pupation is given by the occasional escape 

 of a large quantity of blood, which fills the space between 



