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



contracted state are much less. Being flattened and almost as 

 slender as a nematode, they probably are the smallest insects 

 known, although some others are shorter, e. g., minute beetles 

 and some of the parasitic Hymenoptera. 



Microentomon minutum, n. sp. 



When living not seen by the unaided eye except when on a dark background 

 and then appearing only as a minute white speck. Head about three-fifths as 

 broad as long; pseudoculi inconspicuous, lateral, pit-like; rostrum in the form of 

 a tubercle. Exposed part of prothorax about twice as broad as long. Meso-, 

 and metathorax subequal. Abdomen with sides parallel tor most of its length; 

 tergal apodemes very slender, extending from pleuron to pleuron and unbranched 

 laterally; seventh abdominal segment about twice as broad as long, with its 

 single transverse row of dorsal setae situated at the posterior margin of the 

 tergal plate and composed of six setae. Anterior legs, although larger and 

 differently conformed from the others, yet not so much so as in most all other 

 Proturans. This pair extends beyond the anterior margin ot the head by about 

 four-fifths the length of the tarsi. Length, well extended, 0.61 mm.; width, 

 0.09 mm. 



Type. Cat. No. 24,587 U. S. N. M. 



Described from type specimen taken from decaying leaves 

 and twigs at Takoma Park, Maryland. This minute insect 

 normally inhabits moist decaying twigs, being found under the 

 bark of the same. The species is probably very common, but is 

 not usually observed because of its minute size and habits. 

 Three mounted specimens are at hand, all of which lack the com- 

 plete number of segments and the genital papilla hence are not 

 mature. The generic characters of these specimens agree 

 exactly with those of the single female found by Berlese. Be- 

 cause of its common occurrence it appears desirable to make this 

 species the type of the genus, notwithstanding the fact that the 

 mature form has not yet been taken. I have frequently 

 observed live specimens of the species and studied them alive 

 in the laboratory. 



A Comparison Between the Palearctic and Nearctic Protura. 



The number of Proturan species now known from the Nearctic 

 Region is twelve, which number happens to be exactly the same 

 as reported from the Palearctic Region. Of the mors general- 

 ized forms which breathe through tracheae and constitute the 

 family Eosentomidae, four species are known from each of these 

 two zoogeographical regions. Of the forms without tracheae, 

 the Palearctic is represented by four genera, one of which has 

 not been found yet in the Nearctic; of these same atracheate 

 forms the Nearctic is represented by five genera, two of which 

 are yet not known to occur in the Palearctic. Most of the Pal- 

 earctic species thus far described are over one millimeter long 

 and are well chitinized; on the other hand most of the twelve 



