200 THE SCOLYTID BEETLES. 



cells at or toward the inner end of the social gallery, the walls and par- 

 titions of these cells consisting of an ambrosia-like substance mixed 

 "svith fine borings. 



Flight Habits. 



Further observations should be made on the flight habits of these 

 beetles, but from what we know of a few species it would appear 

 that in the same species flight may be either individual or collec- 

 tive. Iji one example noted by the writer (Hopkins, 1899a, pp. 346- 

 348), a large number of species, together with some of their associates, 

 scavengers, and predaceous enemies, were found in one great swarm. 

 The periods of flight vary with the number of generations in a sea- 

 son. Thus species with a single generation have but one definite 

 period of flight, while those with more than one generation have two 

 or more periods, or, when the generations overlap, there may be a 

 continuous period of flight throughout the season. 



Social Habits. 



In the social habits we find some features of special interest both in 

 their relation to taxonomy and to parallel lines of modification. In 

 the relation of the sexes there is a wide range of variation from simple 

 or unorganized and intensive polygamy to specialized or organized 

 polygamy, and a gradual reduction in the proportion of the number of 

 females, from 1 male and many females to 1 male and 2 females, and 

 finally to specialized monogamy. 



In Hypothenemus, Stephanoderes, Xyleborus, and allied genera the 

 males are much smaller than the females and very rare. In certain 

 species of Xyleborus as many as 60 females to 1 male have been found 

 in one brood gallery, and the proportion appears to be even greater 

 in Hypothenemus. In these groups there is no system in the relation 

 of the sexes or in eggs of the brood galleries of the females, and all live 

 together in the same galleries. In the other groups of genera of the 

 subfamily Cryphalinse where there is jio difference in the size of the 

 sexes there is more evidence of separate egg galleries for the different 

 females of the social group, and the galleries begin to take on more 

 definite and characteristic forms or patterns. In the Ipidse the tend- 

 ency toward a specialized polygamous relation of the sexes reaches 

 its highest development in Pityogenes, while in Ips there is a tendency 

 toward fewer females, the numbers of which in some cases are limited 

 to 2 or 3 to the gallery. In Corthylinse the sexes are more equally 

 divided, while m Crypturginae, Phloeotribinse, and Hylesininse there 

 is a tendency toward 2 females and 1 male or to 1 of each. The last 

 seems to prevail to a greater extent in the subfamily Scolytinse, or at 

 least in the genus Scolytus, than in other subfamilies. 



