PLAXT-HOUSE ALEYRODES. • 9I 



branch and a smaller branch laterally. From the lateral branch 

 there arises successively a number of twigs going to the sides of 

 the abdomen. The ventral branch goes to the ventral longitudi- 

 nal trunk and the dorsal branch to the dorsal longitudinal trunk. 



In the abdominal region of the larva the rudiments of the 

 reproductive organs appear as a pair of prominent yellowish 

 organs made up of large cells. The imaginal discs of the exter- 

 nal genital organs appear during the larval period as two pairs 

 of invaginations of the hypodermis at the posterior part of the 

 abdomen. The imaginal discs of the hypodermis appear in the 

 abdomen at about the same time as those of the genital organs. 



There are three moults during the larval stage. The first one 

 occurs at from five to six days after hatching. The second 

 occurs four or five days later, and the third five to six days after 

 the second. At each of the moults there is a very appreciable 

 increase in the size of the larva, but there is no apparent change 

 other than the growth of the organs and the formation of the 

 imaginal discs already mentioned, although it is probable that 

 '.he imaginal discs of the digestive and other organs are formed 

 during larval life. With the occurrence of each moult there is 

 an increase in the number of the wax rods secreted around the 

 margin of the body. 



In the pupal stage (Fig. 43) which lasts from twelve to six- 

 teen days, there are many important changes taking place on the 

 interior. The whole external form of the insect is changed, 

 from that of a wingless, motionless larva, to an active, flying 

 form. The greater part of the organs of the larva, with the 

 exception of the nervous and reproductive systems, are entirely 

 made over to meet the requirements of the new mode of life. 

 All of the mu.scles are reduced to an almost structureless condi- 

 tion. The greater part of the muscles of the body undergo his- 

 tolysis at the same time. The imaginal discs of the thoracic 

 region become everted so that they lie outside of the body. The 

 developing legs take on their normal appearance, the wings are 

 folded up in the form of pads on the thoracic tergites. The 

 external genital organs are developed from the two pairs of 

 imaginal discs in the posterior parts of the abdomen. The cen- 

 tral part of the nervous system passes directly over from the 

 larva to the imago, but it undergoes farther concentration during 

 the pupal period. The reproductive organs pass over from the 



