8o COLLEMBOLA OF MINNESOTA 



careful comparison of many shows that, aside from an occa- 

 sional spine which has strayed clear out of position, as sometimes 

 occurs, the variation has well-defined limits. One may always 

 expect to find the two most distal spines large; then, there 

 are commonly four or five much smaller ones before the large 

 proximal spine is reached. Occasionally the number of these 

 small spines varies to three, or even six. Often there is a small 

 spine even more proximal than the large one just mentioned, 

 and situated slightly more mesad. In one individual, three 

 small spines instead of one were present. A point of interest 

 may here be mentioned. The specimens from the northern 

 part of the state differed somewhat from those taken in the 

 central and southern parts in that the latter showed a tendency 

 toward a larger number of the small spines on the dentes. 

 Six small spines were not very uncommon, besides those sit- 

 uated proximal of the large proximal spine. These latter 

 small spines, one, two, or even three in number, were also more 

 likely to occur in the southern specimens than in the northern. 

 The minute denticle of the inferior claw, sometimes observed 

 in the northern specimens, was seldom or never present in the 

 others. Notwithstanding these slight differences, I believe 

 they are no more than locality variations, and certainly not to 

 be taken as indicating distinct species. 



From T. niger this species may be very readily distin- 

 guished by the form of the mucrones, which in T. niger, bears 

 a large, spur-like tooth close to its base, while in T.arcticus the 

 tooth is noticeably separated from the base, and there is a smaller 

 tooth set almost opposite, on the other (outer) edge of the mu- 

 crones. The mandibles are respectively four and five-toothed. 

 Rather common throughout the summer under stones, bark, etc. 



Tomocerus niger Bourlet. 

 PI. VI, Figs. 10-12; VII, Fig. 5. 



1839. Macrotoma nigra. Bourlet, Mem. Soc. Roy. Lille, p. 390. 



1839. Macrotoma ferruginosa. Bourlet, Mem. Soc. Roy. Lille, p. 390. 



1842. Tomocerus celer. Nicolet, Mem. Soc. Helv. 



1844. Macrotoma celer. Gervais, Ins. Apt. Walckenaer. Vol. III. p. 407. 



1844. Macrotoma nigra. Gervais, Ins. Apt. Walckenaer. Vol. III. p. 408. 



1844. Macratoma lepida. Gervais, Ins. Apt. Walckenaer. Vol. III. p. 409. 



1847. Tomocerus celer. Nicolet, Mem. Soc. Ent. France. 



