ENTOMOLOGIC. 49 



burrow. As this burrow or tunnel becomes longer, eggs are deposited in 

 it at more or less regular intervals, though in some cases all are deposited 

 in one or more little heaps. The larvae or worms hatching from these eggs 

 gnaw a burrow away from the one made by their mother, and thus fre- 

 quently form a very regular system of tunnels, as may be seen in the illus- 

 tration. Each species has its own method of work, so that an expert can 

 readily judge from a tunnel the species that made it. These larval tunnels 

 become larger and larger with the growing larvse until the full-grown 

 larvae cease their labors at the end of the tunnels. They now change to 

 pupae, and later to beetles, which, after hardening their outer skins, 

 reach the light by a straight burrow leading to the surface of the trunk. 



We have a large number of species of bark-beetles which vary consid- 

 erably in the method of leaving the burrow in which they grew, in the 

 method of swarming, in the manner they search for new homes for their 

 offspring, in the formation of the new tunnel, in the way they deposit 

 eggs, in the time required to grow, in the number of annual generations, 

 and in many other ways not necessary to mention here. Some species of 

 bark-beetles find their food only in the bark, others between the bark 

 and the wood, and still others in the solid wood itself. Of course, this 

 difference causes differences in other habits, but chiefly in the arrange- 

 ments of the holes through which they reach the surface. As no bark- 

 beetle will fly during a cold and rainy day, nor in cloudy weather, but 

 only during the time that the sun shines brightly, the irregularity of their 

 appearance outside the tree is readily accounted for. In selecting the 

 proper tree for the reception of its eggs, the female beetle is very care- 

 ful. Each species will investigate with great care a tree before it is 

 selected. Beetles that breed in pine trees will not fly to oaks, and such 

 as live in maple will not settle upon linden. Some will select the roots, 

 others the trunk, and still others only the branches or smaller twigs. 

 This constancy in selecting the exact kind of tree and exact spot upon it 

 is explained by the difference in the jaws and other organs of the insects, 

 svhich would make it very difficult to gnaw or burrow in other trees. 

 But they select with such great care, not only the kind of trees, but also 

 their conditions. Trees that are dead are never selected, as they would 

 not be able to furnish the required fluid food. Perfectly healthy trees 

 are also discarded, because the larvas would be drowned by too much 

 sap. Such species of bark-beetles as are hairy, can live in trees like 

 pines, notwithstanding their sticky sap, which would smother beetles 

 with a smooth skin. Trees that are injured by heavy winds, by careless 

 felling of neighboring trees, by the scorching of fire, or have become dis- 

 eased by other accidents form the best homes for bark-beetles and are the 

 ones that are always selected. If, however, such diseased or injured trees 

 should not be found, healthy trees have to be utilized by these insects. 



Bark-beetles are strongly influenced by climatic conditions, by cold, rain, 

 or absence of sunshine. To escape such unfavorable conditions the beetles 

 make as soon as possible a burrow in the chosen tree, so that they have a 

 retreat in bad weather. In these retreats they can also escape their 

 numerous enemies among birds and carnivorous insects. In trees with a 



