120 PROCEEDINGS ENTOMOLOGICAL SOCIETY 



According to a system of classification which will soon be pub- 

 lished, these beetles represent a super-family, with representatives 

 in every part of the world where woody plants grow. The four 

 families are represented in all of the great faunal areas, while the 

 subfamilies, genera, and species become more and more restricted 

 with the descending rank. There are, of course, some notable 

 exceptions to be found in genera and species which are more or less 

 cosmopolitan. There are, on the other hand, a great many genera 

 and species which are, so far as known, exceedingly restricted in 

 their distribution. 



While, as a rule, species and groups of closely allied species fol- 

 low the distribution of their hostplant species, there are examples 

 of species of insects which are more restricted in their distribution 

 than that of their host plant. 



There are also a great many examples of allied plant species in 

 different countries being the hosts of the same genera and very 

 similar species of insects. Among these, there are to be found some 

 of the more striking examples of discontinuous distribution, es- 

 pecially under the broader conception of a species. Under a 

 more restricted conception, the number of such examples is greatly 

 reduced, so that under this conception the subject would resolve it- 

 self into one of discontinuous distribution of closely allied species. 



The subject of so-called paired or parallel species is of special 

 interest in connection with a study of geographical distribution. 

 There are some striking examples of paired species in the genus 

 Dendroctonus which if they occupied the same local faunal area 

 would be difficult to separate on account for their close resemblance 

 in structural characters. These paired species are brevicornis and 

 barberi, frontalis and arizonicus, mexicanus and parallelocollis, 

 monticolce and ponderosce, piceaperda and engelmanni, punctatus 

 and micans and terebrans and valens. In each case the pairs are 

 more or less widely separated from each other in their geograph- 

 ical distribution, as for example; micans of northern Europe and 

 punctatus of the Appalachians of North American ; frontalis of the 

 southern states and arizonicus of Arizona; barberi of Arizona and 

 New Mexico and brevicornis of the Pacific slope states, Idaho, Mon- 

 tana and part of Wyoming; ponderosce of the central and southern 

 Rocky Mountains and monticolce, of the northern Rocky Moun- 

 tains and Pacific Slope. 



There are a large number of similar cases of so-called paired spe- 

 cies in other genera and the supposition that some of them are one 

 and the same species has led to considerable confusion relating to 

 the true range of a species. Between North America and Europe 

 we have several examples such as Xyleborus pyri of America and 

 Xyleborus dispar of Europe, Dryoccetes autographus of Europe and 



