OSTEN SACKEN ON WESTERN DIPTERA. 349 



GENERAL REMARKS ON THE DIPTERA OF THE WESTERN REGION, AND 

 OF CALIFORNIA IN PARTICULAR. 



In the introductory paragraphs to the families of Diptera, of which 

 I have treated in the preceding pages, I have attempted some general- 

 izations concerning the character of the Western, and especially of the 

 Californian, faunae, and their relationship to other faunas. In trying now 

 to sum up these generalities, I become more than ever aware of the in- 

 sufficiency of our present knowledge of the Diptera of that fauna, and 

 of the meagerness of the results obtained. If I persist, nevertheless, 

 in my attempt, it is because I find that the general results thus far 

 reached for the order of Diptera coincide with those obtained in the 

 other orders of insects, and that their publication, even in their present 

 imperfect form, may tend to confirm the accuracy of those results. 



The belief of many, and under which I confess to have labored until 

 better informed, that the Eocky Mountains form a natural boundary for 

 a distinct entomological fauna, is erroneous. It is a well-known fact that 

 somewhere between the Eocky Mountains and the Mississippi there is a 

 line, west of which agriculture becomes precarious without artificial 

 irrigation. This line, which some observers place about longitude 98°, 

 marks the eastern limit of a region which extends to the Pacific Ocean, 

 and is characterized by peculiar conditions of life and a peculiar fauna. 

 Among these conditions, the principal, the one which determines the 

 most striking features of the whole region, is summer dryness. The 

 natural limits of this region, both north and south, are countries where 

 summer rains prevail. In the north, this limit marks the beginning of 

 northern forms, some of which are circumpolar ; in the south, the advent 

 of a tropical fauna. All living beings, and the insects among the rest, 

 have to adapt themselves to that condition of dryness. This explains 

 the prevalence of Heteromera among the Beetles the remarkably stout 

 carapace of which enables them to withstand desiccation for a surpris- 

 ingly long time (in Lacordaire's collection, an Eleodes remained alive 

 on its pin for seven months, of course without any food). Such Heter- 

 omera escape the heat of the day by their nocturnal habits. Their 

 usually black color is the concomitant of such habits. Certain Carabidce^ 

 also nocturnal, have the same black color, and often a remarkable re- 

 semblance to the Heteromera in their outward appearance. Dry soil 

 and sunny exposures attract the burrowing Hymenoptera — Bees, Sand- 

 wasps, Mutxllm — which form another characteristic feature of the region. 

 The nests of these are infested by numerous parasites — the Meloidw 

 among Coleoptera, the Bombylidce among Diptera. Such is the explana- 

 tion of the presence in the Western Plains of numerous species of 

 Cantharis (Hpicauta) and of the Bombylidce, which prevail among the 

 Diptera of the region as much as the Heteromera among the Beetles. 

 j The same conditions of life, with the same results, exist in other con- 

 tinents. There is a vast region in the Old World which resembles, in 

 that respect, the Xorth American western region. It is the so-called 



