THE GENUS MESENTOTOMA 
(COLLEMBOLA: ENTOMOBRYIDAE) 
By Kenneth A. Christiansen 
Grinnell College, Grinnell, Iowa 
In 1942 Salmon described a new genus and species of 
littoral Collembola under the name of Mesentotoma exalga. 
As he pointed out, these animals had a number of peculiar 
characteristics, not the least of which was the littoral 
habitat. Two species previously described as members of 
the genus Entomobrya also display many of these peculiar 
characteristics, including the littoral habitat. These are 
E. laguna Bacon and E. dollfusi Denis, and both are 
clearly separated from the remainder of the members of 
Entomobrya. Of the three species mentioned above, dollfusi 
is most similar to the members of the genus Entomobrya , 
and laguna is least so. In text-figure one below, it can 
be seen that the three species give the appearance of 
steps in a linear series of species, becoming more differen- 
tiated from the typical Entomobrya body structure. The 
differences between dollfusi and the typical Entomobrya 
species is so great that it is difficult to tell to what group 
of the genus it is allied. The most likely candidate ap- 
pears to be the E. marginata group, and if we accept 
this hypothesis, then the idea of a linear relationship 
among the species is considerably reinforced by the struc- 
ture of the male genital plate and the empodial appendage. 
While the basal position of dollfusi is fairly clear, some 
specializations of exalga would tend to indicate that it is 
not directly ancestral to laguna, although much less dif- 
ferentiated from the members of the genus Entomobrya. 
In the remainder of this paper the genus is redefined, and 
the three species belonging to it are described and figured. 
Although the unusual habitat makes this animal difficult 
to find, eventually more species will probably be turned 
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