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IOWA ACADEMY OF SCIENCES. 
jointed appendages. The largest pair is the pair borne by 
the eighth somite and I think that these are usually 
carried directed downward like the f urcula. This Machilis 
not only runs swiftly but leaps with great agility by means 
of this pair of appendages, possibly aided to some extent 
by the others. 
The Collembolan furcula varies considerably throughout 
the different genera, and the shape of its terminal segments, 
the mucrones, afford excellent specific characters in many 
genera. See figs. 3, 4, and 5. 
Some curious and beautiful adaptations have taken 
place to suit different modes of life. For instance, in the 
more active species living a rather free, roving life as the 
Smino-thuridse and many of the Ento-Mobryidse, we find 
the organ usually long, slender and very supple, reaching 
forward in some cases almost to the head. Fig. 8. 
In some of the heavy bodied species among the Poduridse, 
the organ is short and stout and far stiffer in proportion to 
its length. Fig 9. These are species which usually inhabit 
places more or less enclosed as spaces under bark, in worm 
holes, etc., where there is less room to use a longer spring. 
In Xenylla the spring is weak Und apparently not much 
used while in Friesia it has almost disappeared by atrophy, 
I suppose. Fig. 10. 
Some Collembola grouped together in the Family 
Aphoruridae are without the furcula entirely, but they are 
generally found in situations where the springing power 
could rarely be exercised. As the gradations of atrophy in 
different genera and species are so complete, I have little 
doubt that their habits are responsible for the loss of the 
organ from disuse. Thus I would hold that their springless 
condition must not be looked upon as primitive and I there- 
fore regard them as regressive rather than as ancestral 
members of the group. 
I might mention one or two of the most remarkably 
modified furculas. Some of our species of Sminthurus live 
on the surface of ponds and have a fan-like furcula. Fig. 
11. The manubrium is short, broad and flattened and the 
dentes diverge widely. On the outer and inner sides of 
