316 



Hahifx of the Greenhouse White Flu 



large round spine-bosses to be seen, situated: seven pairs near the 

 margin, a pair towards the middle line anteriorly, a pair in the 

 thorax, and a pair in each of the third and fourth abdominal 

 segments. Later, the glands at these points give rise to long waxy 

 processes (Figs. 26, 27), the arrangement of which is made use of in 

 systematic work. Like the preceding stages, this instar is at first 

 transparent and a pale greenish-yellow in colour and has small red eyes. 

 The length varies from 0-7 mm. to 0-82 mm. It differs from the others 

 in that much growth in length, breadth and thickness takes place. 

 There are six abdominal and two thoracic segments differentiated. 

 The body margin is at first entire, except for the cerci and posterior 

 spines, which are shorter than those in the preceding stage. Later, 

 waxy processes are developed along the margin generally to the number 



tiw.p 



Fig. 27. Fourth instar. 



Fig. 28. Fourth instar. 



of 38 pairs (Fig. 28). The mouth-parts are similar in structure to those 

 in previous stages. 



Tracheal system (Fig. 29). This is much more complicated than in 

 any previous stage, as numerous fine tracheal ramifications are present. 

 There are dorsal and ventral systems, which are in connection. 



Sjnracles (Fig. 29). There are four pairs of spiracles situated as 

 follows : a pair in the metathorax ; a pair of more anterior thoracic ; 

 a pair in the first abdominal segment; and a pair at the sides of the 

 vasiform orifice. 



Legs (Fig. 30). They are not functional for walking, and consist 

 of three segments, the basal one being very broad. No spines are 

 present. 



Antennae (Fig. 31). Each consists of two segments, the distal one 

 terminating in a rather fine point. 



