36 INSECTS AND MAN 



ably, by covering the leaves densely and blocking out air 

 and light. That insects do a vast amount of direct damage 

 to plant life is a commonplace fact, but their indirect effect 

 is no less potent and is often more difficult to remedy. 



A considerable amount of damage is done to forest and 

 fruit trees by various boring beetles, and those belonging to 

 the family Scolytidce are perhaps of the greatest interest. 

 One of these beetles, Xyleborus dispar (fig. 1, B), deserves 

 special mention on account of its habits. Trees of various 

 kinds may be attacked, though, in justice to this industrious 

 little insect, it should be mentioned that the trees most 

 usually attacked are those which have previously suffered 

 some injury. In the spring the fertilised females, having 

 discovered a suitable tree, begin operations by boring a 

 horizontal tunnel for a short distance into the wood, and 

 from this a vertical tunnel is bored (fig. 1, A). That tunnel- 

 ling operations are going on may be seen by the white 

 frass, or sawdust, trickling down the tree from the hole 

 made by the beetle. Having completed the horizontal 

 tunnel and the first vertical tunnel, the mother beetle de- 

 posits her eggs in the latter, and closes the entrance with a 

 wad of damp frass ; she then proceeds to excavate a regular 

 tunnel system, ovipositing, the while, in each vertical branch 

 and closing each entrance with a damp wad. The eggs are 

 laid in clusters of six ; the total number, depending on the 

 nature of the wood, varies from six to forty-five, and as 

 they hatch in a few days, many of the tunnels contain 

 larvae long before the mother has completed her tunnelling. 

 The larvse have relatively weak mouth parts and no gizzards, 

 so that they are unable to obtain any nourishment from 

 the tree in which they begin life. 



Unlike the majority of insects, this little mother beetle 

 lavishes a considerable amount of forethought on her off- 

 spring. Through the walls of the tunnels a considerable 

 amount of sap oozes and forms a moist lining, the moisture 

 being retained by the wad of frass, carefully placed at the 



