inally consists of three pairs of tubercles,
each pair arising apparently from the seventh, eighth, and ninth (the
latter the penultimate) segments of the abdomen in the Hymenoptera. The
spring of the Podura seems to be the homologue of the third pair of
these tubercles, and is inserted on the penultimate segment. This
comparison I have been able to make from a study of the embryology of
Isotoma.

[Illustration: 163. Catch holding spring of Achorutes.]

Another organ, and one which, so far as I am aware, has been overlooked
by previous observers, I am disposed to consider as possibly an
ovipositor. In the genus Achorutes, it may be found in the segment just
before the spring-bearing segment, and situated on the median line of
the body. It consists (Fig. 163) of two squarish valves, from between
which projects a pair of minute tubercles, or blades, with four rounded
teeth on the under side. This pair of infinitesimal saws reminds one of
the blades of the saw-fly, and I am at a loss what their use can be
unless to cut and pierce so as to scoop out a shallow place in which to
deposit an egg. It is homologous in situation with the middle pair of
blades which composes the ovipositor of higher insects, and if it should
prove to be used by the creature in laying its eggs, we should then
have, with the spring, an additional point of resemblance to the
Neuroptera and higher insects, and instead of this spring being an
important differential character, separating the Thysanura from other
insects, it binds them still closer, though still differing greatly in
representing only a part of the ovipositor of the higher insects. (This
is a catch for holding the spring in place.)

But all the Poduras differ from other insects in possessing a remarkable
organ situated on the basal segment of the abdomen. It is a small
tubercle, with chitinous walls, forming two valves from between which is
forced out a fleshy sucker, or, as in Smynthurus, a pair of long tubes,
which are capable of being darted out on each side of the body, enabling
the insect to attach itself to smooth surfaces, and rest in an inverted
position.

The eggs are laid few in number, either singly or several together, on
the under side of stones, chips or, as in the case of Isotoma Walkerii,
under the bark of trees. They are round, transparent. The development of
the embryo of Isotoma in general accords with that of the Phryganeidae
and suggests on embryological grounds th