1958] 
Christiansen — Collembola 
77 
setae. Fifth and sixth abdominal segments fused, remainder 
distinct. Manubrium slightly less than half as long as 
dens, dorsally with numerous setae, ventrally with a single 
pair of large setae near the base of the dens. Dens ven- 
trally with numerous short acuminate setae, and dorsally 
with six slender short setae. Mucro short, with two large 
curved teeth and a minute apical tooth. 
Only a few specimens of this unusual species were seen 
and all of these were apparently immature. The head body 
ratios and general condition of the animals would indicate 
that they were nearly adult and thus while significant 
change in some organs with growth cannot be ruled out 
(ex., manubrial chaetotaxy) it seems likely that the major 
characteristics described above will hold for the adult 
animals. The discovery of this species poses some interesting 
questions. If we accept the commonly used distinction 
between Proisotominae and Isotominae — i.e. two or less 
manubrial setae on venter — then this species would be 
in the Proisotominae and close to the genus Isotomina or 
Proisotoma. However, in all other structures the animal 
shows a clear relationship to the Isotominae, and has 
nothing in common with the genera mentioned above save 
the manubrial chaetotaxy. In the Isotominae it would 
appear to fall into Salmon’s genus Isotomidia. This genus 
was created for species generally similar to Pseudisotoma 
except for the lack of clavate tenent hairs and the presence 
of papillae on the antennae. Stach, apparently feeling that 
this insufficient grounds for separation, placed this as 
a synonym of Pseudisotoma. The present species fits the 
criteria for Isotomidia except for the absence of the anten- 
nal papillae, and further differs from Pseudistotoma by 
the presence of an apical biloped bulb, and the single pair 
of ventral manubrial setae. In his description of Isotomidia 
triseta Salmon (1944) says “ A few isolated setae occur 
on the manubrium ...” and in his figures he shows what 
might be considered an apical bulb. In view of this and 
the differences between the two forms I have thought it 
best to consider Isotomidia a subgenus of Pseudistoma 
until a thorough study of all the forms can be made. 
The species has been taken from two localities in 
