256 Apple ami Fear Sucker 



The larvae are distinguished from stage to stage; the distinction 

 being in the increase of the number of segments of the antennae. The 

 first and last segments seem to remain unchanged, and the rest are inter- 

 calated from instar to instar. The external appearance of the last 

 segment with its two bristles is very characteristic through the whole 

 life- history of Psylla mali. 



The following table will give the characteristic differences of the 

 antennae of each instar: — 



Instars No. of segments 



1st instar (Fig. 4) 2 



2nd „ (Fig. 5) 3 



.3rd „ (Fig. 6) 4 



4th „ (Fig. 7) 5 



5th Nymph (Fig. 8) 7 



6th Adult imago (Fig. 9) 9 



IV. Habits of the Young Insects. 



A description of the habits of the larvae or the insects in general can 

 never be too important nor too detailed, since the basis of insecticides 

 is founded on that. The more we know of the habits of an insect, 

 the easier does it become to kill it. 



(i) Chemotrojnsm. The larvae are positively chemotropic, their 

 antennae are very well developed, especially the last segment with its 

 two bristles. The first larva, as soon as it is hatched, crawls towards 

 the apical bud of the twig. It is moving its antennae in the air. The 

 larva seems to be attracted by a smell emitted by the bud, to which 

 smell we seem to be insensible. The same phenomenon is repeated in 

 other instars, which are all attracted to the buds by means of some 

 chemical substances. 



(ii) Heliotropism. All the instars except the nymphs are negatively 

 heliotropic. They avoid light and are always found in the dark recesses 

 of the bud. As soon as they are brought to daylight, or an artificial 

 light (electric) is brought to bear upon them, they try to avoid it. 

 It is this habit which enables them to find the tender leaves of the buds 

 inside. The nymphs, on the other hand, are positively heliotropic. 

 As soon as the fourth instar moults for the nymph, the latter leaves 

 the darkness of the bud and comes to live on the open under surface 

 of the leaf. This change of habit may be beneficial to an imago emerging 

 from it, as the latter must have an open surface to fly. 



(iii) Thigmotroj)ism. This is peculiar to these larvae, except the 

 nymphs. They are positively thigmotropic, i.e. they react positively 



