284 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 112 



the two specimens of transitans? from Jasper County, only the two 

 long setae on either side of XI are present, in perpusillum the long 

 and the short setae on either side and in one specimen of thiene- 

 manni all three setae are present. Probably the number of setae may 

 vary to some degree, as was already shown by Tuxen (1955); so until 

 a larger series of the three "species" is available, we cannot decide 

 whether to give them specific rank. 



Microentomon minutum Ewing 



Microentomon minutum Ewing, 1921b, p. 200. 

 Microentomon perpusillum Ewing, 1940, p. 533. 



The history of this species is a curious one (Tuxen 1956a, p. 249). 

 Ewing described it in 1921 from three immature specimens, although 

 he said that he had often seen specimens (supposedly adult) alive. In 

 1936 he stated that the tarsi are without sensillae, that the second ab- 

 dominal legs are one segmented, and that the genus should be included 

 in the subfamily Protentomoninae (with 2-segmented second abdom- 

 inal leg). In 1940 he even synonymized it with Berlese's Acerentulus 

 perpusillus. 



All three slides were originally labeled Microentomon minutum, but 

 two were later changed to perpusillum. Two slides including the 

 holotype bear the data, Takoma Park, Maryland, in decaying leaves 

 and twigs, H. E. Ewing collector, April 10, 1921; the third one, 

 Takoma Park, Maryland, under bark of decaying twig, H. E. Ewing 

 collector, April 24, 1921. All three specimens are immature and have 

 only 10 abdominal segments; all very distinctly possess one-segmented 

 second abdominal legs and furthermore have a distinct comb on the 

 eighth abdominal tergum. They are evidently young specimens of an 

 acerentomid, and very probably of Ewing's Acerentomon conurus, with 

 which they were found. 



The genus Microentomon should therefore be entirely abandoned, 

 and the species minutum considered as identical with Acerentomon 

 conurus, a junior synonym of Acerentulus americanus (Ewing). 



Acerentomon americanum Ewing 



Figures 45-54 

 Acerentomon americanum Ewing, 1921b, p. 197. — Ewing, 1940, p. 536. 



Two slides are labeled as this species, but they probably belong to 

 two different species, one of them undescribed. We shall here confine 

 ourselves to describing and drawing the holotype, a female. Data 

 for the holotype are Takoma Park, Maryland, from decaying leaves, 

 March 27, 1921, H. E. Ewing collector. 



The foretarsus (figs. 45-46) is typically acerentomid (Tuxen 1955). 

 The t 1 is placed proximally on the tarsus; d:p = 70:35( = 2). The 



