70 
IOWA ACADEMY OF SCIENCES. 
From such a primitive stock let us suppose the Collem- 
bola to have branched off. 
The abdomen of the Collembola possesses six somites 
and it is upon the third and fourth of these that the 
appendages were found useful and were retained. The 
pair which remained upon the fourth somite has been 
specialized into a very efficient organ for leaping, variously 
known as the “furcula,” “saltatory appendage,” “spring,” 
“spring-gabel,” and “bail.” The appendages upon the third 
somite have become an organ known as the “tenaculum” 
or “catch,” accessory to the furcula. 
This specialization of organs for springing is analogous 
to the super-development of legs for leaping in the fleas, 
flea beetles, crickets, etc.; only that in the Collembola 
organs- having no other use are specially set apart for the 
purpose. 
The furcula seems to be a pair of three-jointed append- 
ages which have their basal segments joined together, side 
by side, to produce one median basal piece. This first seg- 
ment which is usually more or less flattened and often 
shows characters indicating its double origin is called the 
manubrium. Figs. 1 and 2. From the distal end of the 
manubrium proceed two parallel or divergent pieces called 
the dentes, and each of these bears at its distal end a short 
segment called the mucrones or mucro. The mucrones 
usually bear one or more teeth of various forms and in 
various positions. 
The history of the development of the furcula seems to 
have followed a law which might be stated thus: “A pair 
of similar organs which habitually work together and only 
in the same direction, tend to become united, beginning at 
their bases.” Doubtless the development of the normal 
labium in the class Insecta from a second pair of maxillae 
to a united organ will fall under this law. 
In many of the Collembolans the furcula is apparently 
an appendage of the fifth somite, and has been so regarded 
by Sir John Lubbock and several other writers upon Col- 
lembola. It was held that in the Family Entomobryidae, 
the appendages of the fifth somite were represented, and 
