330 Habits of the (Treenlumse WJiite FUj 



spines. These long leg-spines are absent in the other stages, and those 

 beneath the vasiform orifice, if present, arc short. 



When larvae are nearing the moult, they become fat and lose 

 their close contact with the leaves, so that they are then easy to see. 

 The first three instars do not gi'ow in length during their existence, 

 this taking place entirely at the moult: but they grow in thickness 

 (dorso-ventrally). Thus, the length for each of these three instars is 

 constant, and it may, therefore, be used as a criterion of the stage. It 

 varies within 2/i only. The fourth instar (pupa), however, grows con- 

 siderably (12jLt) in length, and also very much in thickness (Fig. 27). 



The honeydeic on exposure becomes very viscous, so that any stage 

 which feeds for any considerable time with its dorsal surface upwards, 

 is quite likely to have its anus clogged, as is the case in some of my whole 

 mounts of adults. In these cases the plug of honey dew is situated 

 on the dorsal side of the lingula, thus confirming the dorsal position of 

 the anus with relation to the lingula, which I demonstrated both in a 

 whole mount and by means of sections. 



Dorsal pores. On the dorsal surface of the second, third and fourth 

 instars are small pores, those in the abdomen and thorax being seg- 

 mentally arranged. They are probably pores for the passage to the 

 exterior of wax formed by the underlying cells, the body being covered 

 with it. But it is not a white powder or meal as in the adult. 



It may be asked : Why are all the stages generally on the under- 

 side of the leaves ? The follomng points require consideration ; 



(1) The character of the cuticle of the leaf. 



(2) Presence of stomata. 



(3) Protection from wet. 



(4) Phototropism. 



(5) Position of the anus and character of excreta. 



(1) The cuticle is certainly thinner, and the vascular bundles are 

 nearer the surface, on the underside of the leaf. 



(2) In all the sections of larvae in situ that I have examined, in no 

 case do the stylets penetrate a stoma. 



(3) All the stages have a waxy covering, so that the position as 

 a protection from wet would appear to be a subsidiary matter. 



(4) If a leaf with flies on the under-surface is turned over, after a 

 time all the flies migrate to the other side and nuiy lay eggs there. 

 This fact excludes points (1) and (2). If a leaf hangs with its surface 

 in a vertical plane, the females do not show any preference of leaf-surface 

 for oviposition. To catch the Hies, I turn a leaf ovei-, and, holding a 



