18 



(PI. 4, fig. 4) . The tunnels of Dryocoetes are in some species, affaber, variably 

 irregular, but are in others, such as confusus Sw., distinctly of this type. 



Certain species of Ips and Pityophthorus have a preference for following 

 the grain of the wood, and in some of these a few very long egg-tunnels are 

 developed, more or less parallel to each other throughout much of the length, as 

 with Ips calligraphus Germ., and Ips perturbatus Eichh. (PI. 4, fig. 10). Certain 

 species of Pityogenes and Pityophthorus cut elongate egg-tunnels in the bark of 

 small twigs, and show a more or less distinct spiral arrangement. 



Cave Tunnels. — Species of the genus Cryphalus excavate an irregular cavity 

 in the bark, engraving the wood, in which the eggs are deposited (PI. 23, figs. 6, 7). 

 Pityophthorus opaculus Lee, and others, have a very similar habit, sometimes 

 combining the cave type with short irregular egg-tunnels. 



Pith Tunnels. — Certain species of Pityophthorus ,Stephanoderes and Micrads 

 cut their egg-tunnels through the pith of twigs (PI. 4, fig. 5). 



The Larval Mines. 



The larval mines of the bark-beetles have been described briefly on page 9, 

 and are dealt with in detail under their respective species in the remaining parts 

 of this series. 



Factors Influencing the Development of Bark-beetles. 



It is very noticeable that at different altitudes and latitudes and in different 

 seasons the broods of the Ipidse develop at different rates. A species which is 

 single-brooded in northern Canada may have two broods in the middle or 

 southern States. Certain species which have normally two broods, may have 

 but one or only a partial second brood in cold, wet seasons, in the same locality. 

 In the same locality, and during the same season, over-wintered individuals may 

 appear from cold, swampy sections or northern slopes several weeks later than 

 others of the same species, which have wintered in a sunny situation. It is 

 evident that the factors which influence the development of the larvae and the 

 time of appearance of the adults are of great interest, and are of particular 

 importance in economic studies. The chief of these factors are the moisture 

 content of the air in the tunnels, the temperature of the air and of the bark ,and 

 the sunlight. The beetles are particularly sensitive to any change in humidity; 

 they will leave all other activities to fill any openings made in the tunnel roof. 



Valuable experimental studies upon the effect of different degrees of heat 

 and moisture upon the development of bark-beetles have been made by several 

 European writers, expecially by Hennings upon Ips typographus Linn. The 

 results of these studies agree on the whole with more general observations 

 made in our forests under natural conditions. It has been a matter of common 

 observation in Canadian forests that the greater number of our bark-beetles 

 breed most rapidly in hot weather with a moderate supply of moisture. On 

 the other hand, broods developing in the bark in the open sunlight of clearings 

 are not uncommonly destroyed by the high temperature and dryness of the 

 bark, which render the latter unfit for food, and also directly affect the life 

 processes of the larvae. It may be noticed in very hot, dry seasons that while 

 broods in the thin bark exposed to the open sunlight may be partly or largely 

 destroyed, those breeding in the thick bark of the trunk or moister stump, or 

 in thick bark about the edge of the clearing, where the moisture has been partly 

 conserved by the shade, may breed successfully and with great rapidity. It 

 appears also that sunlight, aside from temperature, has a stimulating effect 

 upon growth. Hennings refers to a " heat paralysis " of the larvae which was 

 noticed sometimes at 24°C. dry (55 per cent to 56 per cent air moisture). 

 The highest life processes were reached just before that point. The addition 



