]\ II. AwATi 257 



to pressure. The leaves of the buds are pressed against each other and 

 the larvae like to be among those closely packed leaves. This habit 

 combined with (ii) is very advantageous to them, since both habits 

 enable the larvae to get at the most tender leaves of the buds. The 

 nymphs, on the contrary, are negatively thigmotropic. They avoid 

 being pressed together and come to live free, uncrowded on the open 

 surface of the leaves. 



(iv) Gregariousness. All the instars, except the fifth, are gre- 

 garious in habit. They are found crowded together, and try to come 

 together however much they may be separated from one another. I 

 have tried many cases when the larvae were put separately on different 

 buds of the same twig. On the second or at most the third day they 

 were found to be together. This gregarious habit of the larvae is 

 extremely useful since one can kill all of them in one lot. The nymphs, 

 however, are never gregarious. They are always separated from each 

 other and try to avoid the neighbourhood of other larvae and nymphs 

 as well, 



(v) Powers of endurance. The larvae can live without food for 

 two or three days, after which they collapse and die. The fact that 

 they can thus exist temporarily is very beneficial to the first instar, 

 as it may not immediately find a fully-opened bud on its twig. 



(vi) Situation. The larvae are always found in the recesses of the 

 buds. They are rarely found outside, except in the nymphal stage, 

 when they are found on the under surfaces of the leaves. The buds 

 infected by them can easily be recognised. They are all covered with 

 the waxy threads hanging down like icicles. The nature of the threads 

 has already been described above. The position of the larvae in the 

 bud seems to be fairly constant. The head is turned away from the 

 growing point of the apex of the bud and the abdomen is turned towards 

 the apex. In the case of nymphs, the position is very indefinite. The 

 larvae, when sucking, press themselves down against the surface of a 

 leaf and the whole body seems to be heaving up and down as the stylets 

 are withdrawn or thrust into the leaf. The length of the stylet is 

 enormous. In all stages they are twice or thrice as long as the whole 

 body. 



V. Effects produced by the Larva. 



Psijlla mali is most dangerous to apple trees in its larval stages. 

 While in the adult, it does practically no damage. As described above, 

 the larvae are gregarious in habit and plenty of them are thus found in 



