FAUNA OF THE ALEUTIAN ISLANDS AND ALASKA PENINSULA 323 



often broke through into a maze of underground burrows. When 

 setting traps, I could hear the traps snapping a short distance 

 behind, as the mice were already being caught. Many of those 

 caught were young mice, and there was a remarkable pre- 

 ponderance of females. 



Several mice were infested with ticks, Ixodes angustus, some- 

 times three or four on one mouse. Many others were covered 

 with mites, a gamasid, probably Haeogamasus sp. 



There were numerous beetles in the mouse runways, some of 

 which fed on the trapped mice. One of these was the common 

 carrion beetle, Necrophorus sp., and two others, Nebria sp. and 

 Scaphinotus marginatus, were obtained there, though these latter 

 were not seen feeding. Dung beetles, Aphodius sp., and rove 

 beetles gathered at the anus of dead mice, attracted by the 

 traces of dung. 



Such an aggregation of more or less parasitic invertebrates in a 

 dense mouse population could be an important element in the 

 cyclic behavior of these rodents. 



At the time of this heavy peak population on Amak Island, 

 both lemmings and meadow mice were scarce on Alaska Penin- 

 sula. And none of the beetles, mentioned above, were noticed 

 that summer, either on Alaska Peninsula or on Unimak Island. 



Microtus oeconomus unalascensis 



This form is more readily distinguished from M. o. operarius 

 than most of the other subspecies. The skull shows wider nasals, 

 the convexity of the occipital plane is greater, and apparently it 

 is a somewhat larger animal. 



This mouse occupies Unalaska Island, and a specimen from 

 nearby Unalga Island is referable to this form. There is no 

 knowledge concerning its presence on Akutan and Akun Islands, 

 nor on other smaller islands in that vicinity. Meadow mice ap- 

 parently are not found on Umnak Island; at least, we obtained 

 only lemmings when we trapped there. Therefore, Unalaska may 

 be the westernmost point reached by Microtus in the Aleutians. 



On Unalaska Island, in 1936, I found meadow mice in the 

 characteristic grassy meadow habitat, just as on Alaska Peninsula 

 and Unimak Island. And, in 1925, Stevenson stated that in times 

 of heavy mouse population on Unalaska Island, numerous beetles 

 had ruined mouse specimens in the traps, just as they did on 

 Amak Island. 



