11 



THE ENTRANCE-TUNNELS. 



The entrance-tunnels of the ambrosia-beetles pass directly through the 

 bark and more or less deeply into the wood. There they give off side tunnels, 

 along which the greater number of the egg-niches are cut, or in which the eggs 

 are deposited free, according to the habit of the species. 



The entrance-tunnels of the True Bark-beetles either pass directly through 

 the bark, or in most cases traverse it more or less obliquely, to open into the 

 nuptial-chamber or directly into the egg-tunnel within. The length of the 

 entrance-tunnel is never great, and varies with the thickness of the bark in 

 which the beetles are working. In the thick bark of large pine trunks, thinner 

 places, the bark fissures, are frequently chosen for the location of the entrance- 

 holes. Some species of ambrosia-beetles prefer to start their tunnels on freshly 

 cut or broken surfaces. The entrance-tunnels are always perfectly cylindric, a 

 result of the shape of the beetles and their constant revolution during the 

 excavation. Certain species usually cut their entrance-tunnels obliquely upward, 

 so that it is possible to tell whether the tunnels have been cut before or after 

 the trunk has fallen. 



THE EGG-TUNNELS PROPER. 



The egg-tunnels of the True Bark-beetles are usually cut between the wood 

 surface and the bark, engraving both. Certain species cut the egg-tunnel 

 entirely within the bark. Orthotomicus codatus Eichh. has this habit when 

 working in the thick bark of mature white pine, although on branches and 

 trunks of smaller trees its egg-tunnels engrave the wood surface more or less 

 distinctly. The egg-tunnels of many species are almost entirely within the 

 bark, only scoring the wood slightly; such are those of Phlceosinus canadensis 

 Sw. in cedar. On the other hand the tunnels of Chramesus icorice Lee., Leperi- 

 sinus aculeatus Say, Pityopthorus canadensis Sw., and many others, score the 

 wood very deeply, and those of a few species, such as Pityopthorus ramiperda 

 Sw. and Lymantor decipiens Lee., are almost entirely or quite below and parallel 

 with the wood surface. Certain species of Hypothenemus, Stephanoderes, 

 Micracis, and Pityophthorus cut their primary tunnels within the pith of twigs, 

 and have, on this account, been termed " twig beetles." Some species of 

 Pityophthorus cut their egg-tunnels usually upon the wood surface of twigs, 

 while their larvae frequently bore to the centre and pupate in the pith. 



The egg-tunnels of the ambrosia beetles branch from the entrance tunnels 

 in various ways, to be described in more detail under the several species in later 

 papers. The species of Gnathotrichus, Pterocyclon, Trypodendron, and Corthylus 

 cut their egg-tunnels at a greater or less depth below the wood surface, according 

 to the species and particular conditions of the wood, and vary somewhat in indi- 

 vidual habits. All the species in these genera cut egg-niches above and below 

 along the walls of the egg-tunnels, and later even along the entrance tunnels. 

 These niches are similar to those cut by most True Bark-beetles, and the eggs 

 are usually packed in with boring-dust and excrement. The niches are widened 

 and lengthened by the larvae to form short side tunnels or " larval cradles," 

 usually at right angles to the egg-tunnel, and only slightly longer than the larva 

 itself (PL 3, fig. 8); compound. The egg-tunnels of Anisandrus and Xyleborus 

 are usually merely side tunnels arising from the sides or the distal end of the 

 entrance tunnel. The eggs, in these two genera, are deposited free in the 

 tunnels and the larvae live therein without cutting cradles; simple. The 

 tunnels of Xyleborus saxesceni Ratz. are peculiar in that the larvae excavate 

 cavities in_congress (PL 2, fig. 13). 



It is interesting to note that certain species of the genus Platypus (formerly 

 included in the Family Ipidce) , which occur in the southern and western portions 

 of the continent, lay their eggs, according to Hubbard and others, free in the 



