ficial
groove leading from the second maxillae backward to the tube on the
first abdominal segment. The third abdominal segment usually carries a
pair of short appendages whose basal segments are fused together; this
is the "catch" (fig. 2, 7), whose function is to hold in place the
"spring," which is formed by the fourth pair of abdominal
appendages--also with fused basal segments. In most Collembola the
spring appears to belong to the fifth abdominal somite, but Willem, by
study of the muscles, has shown that it really belongs to the fourth.
The fused basal segments of the appendages form the "manubrium" of the
spring, which carries the two "dentes" (usually elongate and flexible),
each with a "mucro" at its tip (fig. 2, 5). The fifth abdominal segment
is the genital, and the sixth the anal somite.

The spring serves the Collembola which possess it as an efficient
leaping-organ (see SPRINGTAIL). But in some genera it is greatly reduced
and in many quite vestigial.

Most springtails are without air-tubes, and breathe through the general
cuticle of the body. But in one family (_Sminthuridae_) a spiracle,
opening on either side between the head and the prothorax, leads to a
branching system of air-tubes. The _Sminthuridae_ are further
characterized by the globular abdomen, which shows but little external
trace of segmentation, and by the well-developed spring.

In the _Entomobryidae_ the body is elongate and clearly segmented, but
the dorsal region (tergum) of the prothorax is much reduced and the head
downwardly directed; the spring is well developed. In the _Achorutidae_
the head is forwardly directed, the tergum of the prothorax conspicuous,
and the spring small or vestigial.

In many genera of springtails a curious post-antennal organ, consisting
of sensory structures (often complex in form) surrounded by a firm ring,
is to be noticed on the cuticle of the head between the eyes and the
feelers. It may be of use as an organ of smell. Other sensory organs
occur on the third and fourth antennal segments in the _Achorutidae_ and
_Entomobryidae_ (fig. 2, 3).

_Distribution and Habits._--The Aptera are probably the most widely
distributed of all insects. Among the bristle-tails we find the genus
_Machilis_, represented in Europe (including the Faeroe Islands) and in
Chile; while _Campodea_ lives high on the mountains and in the deepest
caves. The springtails have even a wider distribution. The genus
_Isotoma_, for ex