INSECTS AND OTHER ORGANISMS 393 



in these insects may be responses to the environmental 

 stimulation from food or temperature. 



Reference has already been made to the destructive 

 effects on plants due to the feeding of insects. Such effects 

 are obvious to all observers in the ravages of leaf-eating 

 caterpillars or wood-boring beetles. Sucking insects such 

 as the aphids, plant-bugs, and their allies, pierce the plant 

 tissues to obtain sap, and the result of their feeding habits 

 may often be seen in withered, rolled, or blistered leaves. 

 Recently some attention has been given to the details of the 

 action of sucking insects on plants, and reference may be 

 made to the researches of J. Davidson (1923) and K. M. 

 Smith (1920, 1926). It appears that between the piercing 

 maxillae of the Hemiptera there are two fine channels, one 

 dorsal in position through which the sap is sucked into the 

 insect's pharynx, and the other ventral, through which the 

 saliva of the insect is injected into the plant-cells where it 

 acts on the dissolved sugar (Fig. 8, B). The proboscis 

 is thrust usually between the cells of the cortex and 

 reaches the vascular bundles, the bast- tissue (phloem) of 

 which seems a favourite source of food-supply (Fig. 82) ; 

 in order to obtain sap the insects usually pierce the 

 individual cells where the saliva causes " plasmolysis and 

 disorganisation of the cell-contents." It is of interest 

 to notice that the same plant-tissues may be differently 

 affected by the saliva of different species of sucking 

 insect. Thus there are several kinds of plant-bugs of 

 the capsid family which when immature feed on the 

 juices of young growing apples in summer time. One of 

 these {Plesiocoris rugicollis) has in recent years become an 

 orchard pest, because the injury which it does to the fruit 

 becomes apparent in autumn when the apples ripen, through 

 cracking and deformation of the surface. Smith has shown 

 that the saliva of Plesiocoris rugicollis alone among the 

 capsid bugs has this definitely poisonous effect on the fruit 

 tissues. Some sucking insects, by piercing the epidermis 

 of plants, open a way for the invasion of bacteria and other 

 minute organisms into the plant- tissues. Thus it is 



