ACCOUNT OF COLLEMBOLA AND THYSANURA. 117
On the second abdominal stenite is a peculiar sucker-like
organ known as the ventral tube. From this a pair of elongate
filaments can be extruded at will.
Although all three thoracic segments are usually distinct,
in some forms, notably the species of Lepidocyrtus, the mesonotum
is very much produced over the pronotum, entirely hiding it
and giving a characteristic appearance to the insects.
The eyes are simple ocelli arranged in a group on each side
of the head, usually on deeply pigmented patches. Some
species are quite without eyes, but in no species are more than
eight eyes on each side known. The antennae are four jointed
but in a few species joints one and two may be once divided,
while in the Sminthuridae it is common for the fourth joint to
be subdivided into many small divisions.
This Order of insects is remarkable for the many forms of
sensory organs present. Various forms of sensory hairs and
rods are to be seen on the third and fourth joints of the
antennas. Of great importance for classification is the organ
found in many species just behind the base of the antenna;. For
want of better understanding we term this the post-antennal
organ. It varies much in form, from a simple oval ring-like
shape to a rosette of few to many peripherally arranged lobes,
or it may consist of a number of tubercles lying in a narrow
elongate groove.
Certain families of Collembola are covered with scales, with or
without the addition of various forms of hairs. When scaled,
living specimens exhibit a very pretty iridescent appearance.
The feet are armed with, usually, two claws, an upper and
a lower, but the latter is occasionally wanting. The great
variation in the structure of the claws is of importance in deter-
mining the different species.
Behind the upper claw and situated at the apex of the
tibiotarsus may sometimes be found a number of long clubbed
hairs.
The furca is composed of a basal piece, the manubrium,
and two dentes, each of which at the tip carries a mucro. The
structure of this organ, and especially of the mucro, is of great
systematic value. 
Little is known of the life-history of these creatures. Most
species are vegetable feeders, a few being of economic importance.