IOWA ACADEMY OF SCIENCES. 
71 
that therefore the furcula in that family was not really 
homologous with that of the Poduridse which bear the or- 
gan upon the fourth somite. I can not agree to this view. 
The furcula seems to me to belong to the fourth somite, and 
to be merely shunted backward in some cases to a position 
beneath the following segment. Its muscle attachments, 
I think, indicate its true position. In several genera of 
the Entomobryidse the fourth somite has a tergum consid- 
erably longer and larger than that of any other somite of 
the body. Perhaps this great development is for muscle 
attachment as it is among these that the furcula reaches 
its maximum development. 
The furcula is provided with flexor and extensor 
muscles, the latter being the stronger. As the furcula is 
usually carried with its ends pointing forward, these 
strong muscles are normally tense and ready always for a 
spring. To counteract this tension and to hold the furcula 
in position, the “ catch’ 7 or “ tenaculum on the third 
somite is provided with two short, roughened blades which 
pass down between the bases of the dentes, close to the 
manubrium and then close up under them, turning out- 
ward to either side, thus holding the furcula up close to 
the body. The short leverage obtained by these blades 
enables their muscles to balance the more powerful furcula 
extensors. Pig. 6 and 7. The whole device is a simple 
one, yet so effective that in Achorntes boletivorus, a 
species common on decaying mushrooms, I have seen leaps 
of about fifty times the animal’s own length; and this 
species has by no means a well developed furcula when 
compared with many other Collembolans. 
It is interesting before going farther to compare with 
the Collembolan furcula, the condition and use of some of 
the abdominal appendages found in another form. One of 
the most active of Thysanura is the Machilis, a genus 
which I think has not yet been recorded from our State 
but which I have taken several times along the Mississippi 
river bluffs in Minnesota and Wisconsin. This insect has 
a 10-segmented abdomen, and eight of these somites 
beginning with the second, bear each a pair of small, 
