SURROUNDINGS OF GROWING INSECTS 207 



bark, pith, fruit, or whatever it may be, particles of which are 

 broken up and swallowed. But the apple-tree is also inhabited 

 by hosts of sucking insects of the order Hemiptera, which, 

 piercing the tissues with their slender needle-like mandibles and 

 maxillae, draw supplies of sap into their stomachs. The 

 apple-aphids (Aphis pomi and other species) that feed on the 

 leaves have already been mentioned (p. 72 ) affording examples 

 of insects which, in the immature stages live and feed in the 

 same way as the adults do. In connexion with our present 

 subject of environment, it is worthy of note that while the 

 sucking action of one kind of aphid (A. pomi) may lead to a 

 reaction of the plant tissues resulting in a curling of the leaves 

 that tends to furnish shelter for the insects, another species 

 (A. avenae) does not cause the leaves to curl in this way, and 

 may suck from the petals as well as from the foliage. This 

 difference goes along with a more important divergence in 

 habit, for while A. pomi remains on apple throughout the 

 summer its winged females leaving their native shoots simply 

 to fly to other apple trees A. avenae is a migratory aphid, 

 its winged virgin females leaving the apple altogether in spring 

 and going off to spend the summer in the ears of oats. Within 

 the blossom-buds in spring may be feeding the curious tiny 

 larvae and nymphs of the sucker (Psylla mali, Fig. 106, c d), 

 differing strongly in appearance as well as in behaviour from 

 the adult Psyllae (Fig. 106 a) which fly and jump actively about 

 the shoots later in the year. The eggs (Fig. 106 b) of these 

 insects, with short stalks and bright yellow in colour, are 

 attached to the bark of the twigs ; and the newly-hatched 

 larvae which come out of them crawl along to the unopened 

 buds, on which they wait till the young leaves begin to expand 

 and the shoots to lengthen, affording the insects the oppor- 

 tunity of entering the developing blossoms. On the bark of 

 the branches may be found crowded together the passive 

 females of the Mussel-scale (Mytilaspis pomorum, Fig. 49) each 

 fixed to the tree by her very long and flexible piercers, and 

 covered by the protective scale, which serves also a shelter 

 for the winter eggs ; the larval scale-insects (Fig 48, p. 89) 

 move about over the bark in spring time, till they assume the 

 nymphal condition and settle down for the resting period of 

 the life-history. In crevices of the bark may also be found 



