2o6 DISEASE IN PLANTS. 



Poplars and Willows, the young shoots of which 

 often show characteristic swellings with lateral 

 holes indicating the points of exit. From the ex- 

 ternal apertures comminuted wood, like saw-dust, 

 is frequently ejected in quantity and betrays the 

 presence of the insects. Certain wood-wasps 

 {Sirex) and the larvae of moths {Cossiis) also make 

 large perforations in the wood of Willows and other 

 trees, often destroying it completely. In the case 

 of these larger borers, whose tunnels may be as 

 broad as the little finger, the foul smell as well as 

 abundant " saw-dust " betray the evil. 



Excavations in wood are by no means caused 

 only by insects : several of the larger Hymenomy- 

 cetes Stereum, Thelephora^Polyporus, etc. tunnel 

 the timber in characteristic ways and often after a 

 fashion very suggestive of insects. They usually 

 obtain access through fractures. 



Tunnels in leaves are invariably due to the 

 activity of miners belonging to the smaller moths 

 and beetles e.g. Tinea, Orckestes, etc. the larvae 

 of which eat out the mesophyll but leave the cover- 

 ing epidermis or cuticle untouched, and since the 

 insect bores forwards only, in an irregular track, 

 and leaves its excrement in the winding passage, 

 the effect is very characteristic. 



Whitish leaf tunnels in Peas are excavated by 

 Phytoniyza. 



Characteristic foxy-red tunnels are mined in the 

 leaves of Apples by Lyonettia, Coleophora, etc. 



Falling of fruit, of Apples, Plums, Apricots, etc., 

 before they are ripe, is frequently due to insects, of 



