196 PROC. ENT. SOC. WASH., VOL. 23, NO. 9, DEC., 1921 



This is because it unites the two quite distinctive families of the 

 Protura that have been recognized in the past; the Eosentomi- 

 dae, which have tracheae and the Acerentomidae, which do not. 

 The abdominal appendages of P. transitans are very similar to 

 those of Eosentomidae, but on the other hand the absence of the 

 tracheae and the presence of but a single transverse row of dorsal 

 setae on each abdominal segment are characters of some of the 

 species of Acerentomidae. The writer expects to make a more 

 detailed study of this species later. 



Protentomon transitans, n. sp. 



Living specimen white; mounted specimen entirely hyaline. Head short; 

 pseudoculi circular in outline, dorso-lateral; mouth-parts inconspicuous and 

 hyaline except for the minute labial palpi which are yellowish. Prothorax 

 large, exposed part being fully two-thirds as long as the mesothorax. Meso-, 

 and metathorax subequal, without chitinous plates and with a few very minute 

 setae. Abdomen long; dorsal setae minute except for segments seven to twelve 

 where they are fairly conspicuous; segment nine about two-thirds as long as 

 eight and eight two-thirds as long as seven. First and second abdominal 

 appendages large, conspicuous and of the same size and constitution, each is 

 noted for having an intersegmental plate at the junction of proximal with 

 distal segment; last abdominal appendages one-segmented yet conspicuous, 

 shaped as those of Acerentomon. Legs short, stout and similar; front pair only 

 slightly larger than the others and with a similar claw. Length when moder- 

 ately extended, 0.74 mm.; thickness, 0.08 mm. 



Type Cat. No. 24,581 U. S. N. M. 



Described from a single male, taken from decaying leaves at 

 Takoma Park, Maryland. This species is in some respects 

 probably the most generalized of any of the Proturan species yet 

 described. The anterior legs are but little specialized, the pro- 

 thorax is much more comparable to the mesothorax than in any 

 other of our American species, and the seventh, eighth and ninth 

 abdominal segments are remarkably similar except for size. 



Acerentomon Silvestri. 



This is the genus erected by Silvestri for the first species of the 

 Protura to be described, the much studied A, doderoi. It is 

 usually differentiated from the other genera of the family 

 Acerentomidae as recognized by Berlese upon variations in the 

 number of segments found in the maxillary and labial palpi. 

 An investigation into the composition of these structures by the 

 writer has caused him to doubt the occurrences of such differ- 

 ences as are .supposed to exist, hence it appears best for the 

 present to use other characters. 



Acerentomon is distinguished from the other genera that should be recognized 

 in its family by two good characters: First, in this genus some ot the tergal 

 apodemes are broadly branched laterally and, second, the second and third 



