THE CANADIAN ENTOMOLOGIST. 193 



deciduous trees, the Oak, Beech and Hickory suffer severely, and there is 

 scarcely a northern tree but serves as food-plant for one or more species 

 of this family. As a rule each species has a limited number of food-plants, 

 but some few, like Pterocyclon mail, feed in many trees, both coniferous 

 and deciduous. 



According to their habits, the North American Scolytids may be 

 separated into four fairly well-marked groups : the Bark-beetles, the 

 Timber- or Ambrosia-beetles, the Twig-beetles, and a fourth group 

 containing a it\i species of varying habit. 



The Bark-beetles. — The first of the above-named groups includes 

 those forms which burrow in the bark, or between the bark and the wood. 

 The adults enter through a hole in the bark, cut in many cases by the 

 male, and drive a primary-tunnel, usually partly in the bark and partly in 

 the wood, and generally either parallel with or at right angles to the wood- 

 fibres. A few species burrow entirely in the bark, and a few species, 

 included here in the Bark-beetles, cut their tunnels just below and parallel 

 to the wood surface. The length of the tunnels varies in the different 

 species from less than an inch to more than a foot. The female, at least 

 in many species, does the greater part of the work, while the male guards 

 the opening and removes the chips and refuse. These main-tunnels are 

 always kept strictly clean. In sweeping the tunnels the beetles move 

 backwards, scraping the refuse with the mandibles back to the fore legs, 

 which pass it on to the middle, and these to the hind pair. When the 

 opening of the tunnel is reached the tip of the abdomen is protruded and 

 the refuse passed up to the hind pair of legs in the manner just indicated, 

 and by the hind legs pushed away from the opening. During this opera- 

 tion the beetle turns in the burrow, thus distributing the refuse evenly 

 about the opening. \w cutting the tunnels also, the beetles constantly 

 revolve, thus obtaining such perfectly cylindrical burrows. 



When not at work one beetle is usually guarding the entrance. By 

 backing into the entrance-hole the declivity of the elytra plugs the opening, 

 and thus presents a complete protection from many enemies. The 

 truncate character of the elytra serves well for this purpose, forming a 

 continuation of the surface of the wood. 



In niches along the sides of the primary-tunnel the whitish^ almost 

 transparent, eggs are laid, usually one, though sometimes several, in each 

 niche. In some species, however, e. i^., Dendroctoinis terebrans, they 

 are laid in clusters along the sides of the primary-tunnel. 



