COLLEMBOLA OF THE FAMILY ISOTOMIDAE 85 



sensory setae are long and numerous on the genital and anal segments, 

 where the largest setae are often minutely serrate. Length, 1.5 mm; 

 maximum, 2 mm. 



Remarks. — The tliird antennal segment, usually shorter than the 

 second, may become longer than the latter in large individuals. The 

 inner tooth of the unguis varies in size and is sometimes absent. 



The tooth of the unguiculus is at times reduced to an angle. The 

 teeth of the mucro vary greatly in form and relative size. Thus, 

 the first tooth may be as large as the second; and the lateral tooth 

 may be long or short, pointed or blunt; sometimes all four teeth are 

 subequal. 



In some of my specimens from Oregon, 111., the genital and anal 

 segments are ankylosed; as they are also in some of the English 

 examples that I received from J. M. Brown. 



In plate 28, figure 310, the strong bending of the two sense rods 

 is abnormal, being due to pressure from the cover glass used. The 

 rods appear in their normal attitude in plate 28, figure 318. 



My European material of this species consists of 9 specimens of 

 denticulata from Germany, given to me by Schaffer, and 1 1 examples 

 of arborea from England, from J. M. Brown. 



In this species the convergence of the dentes is a striking character, 

 and one known to Agren (1903) in only one other species of Isotoma, 

 namely, cinerea Nicolet. This convergence, as Agren notes, is evident 

 in living or stupefied individuals, but it sometimes does not show so 

 plainly in alcohoUc material. 



The three cotypes of MacGillivray's terminata in the Museum of 

 Comparative Zoology, Cambridge, Mass., as well as four others given 

 to me by A. P. Morse, agree exactly with the specimens of denticulata 

 that I received from Schaffer. 



I. synonyrnica MacGillivray is also tliis species, as I have found 

 from an examination of five cotypes: three in the museum just named 

 and two given to me by MacGiJlivray. 



Distribution. — Isotoma arborea is knoT\Ti from Sweden, Denmark, 

 Germany, Switzerland, Hungary, Siberia, and England. In both 

 Europe and North America it is most numerous in winter and spring, 

 and in Europe is said to occur in large colonies, sometimes on the 

 snow. The species lives prim.arily under the loose bark of trees or 

 logs in colonies, but may be found scattered on the ground under 

 damp leaves, and occurs in moss. 



Maine: Orono, March 10, F. L. Harvey. 



Massachusetts: Arlington, March 20, April 9. Cambridge, May 14, just 



hatched. 

 New York: Ithaca, February, March 20, 25, A. D. MacGiUivray (Cornell 



University); March 8, in a crow's nest, I. D. Dobroscky. Macedon, April 6, 



20, May 11, J. D. Hood. Potsdam, May 26, D. B. Young (New York State 



Museum) . 



