[Illustration: From Carpenter, _Proc. R. Dub. Soc._, vol. xi.
1. _Isotoma hibernica_. Side view. 2. " Ocelli and post-antennal organ of right side. 3. " Tip of terminal antennal segment with antennal organ. 4. " Mandible. 5. " Tip of left dens with mucro. Outer view. 6. " Hind-foot with claws. × 240. 7. _Entomobrya anomala_. Catch.]
_Collembola._--In springtails, or _Collembola_, the jaws are sunk into the head, as in the entotrophous Thysanura; the head carries a pair of feelers with not more than six (usually four) segments, and there are eight (or fewer) distinct simple eyes on each side of the head (fig. 2, 1, 2). These are in some genera like the single elements (_ommatidia_) of a compound insect eye, in others like simple ocelli. The abdomen consists of six segments only. The first of these usually carries a ventral tube, furnished with paired eversible sacs which assist the insects in walking on smooth surfaces, and perhaps serve also as organs for breathing. From the researches of V. Willem it appears that the viscid fluid which causes the adherence of the ventral tube is secreted by a pair of glands in the head whose ducts open into a superficial 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.