1. No extensive study of the tracheal system of Collembola has previously been made.

2. The general plan of the tracheae has been worked out on Sminthurus viridis; two independent systems exist.

3. The presence of only a single pair of spiracles constitutes a unique condition among adult insects. The position of these is in the anterior region of the prothorax, and not in the head or cervicum as previously believed. The structure of the spiracles is extremely primitive and they possess no closing apparatus.

4. The tracheae branch dichotomously, but no anastomosis exists between the systems of the two sides of the body. Taenidia are present, but no ‘transition cells’ have been observed and the fine tracheae terminate in unbranched tracheoles.

5. The initial entrance of air into the tracheae is through the spiracles; the displacement of fluid is very slow and the whole system is not completely filled with air until about fourteen days after emergence from the egg.

6. Tracheae are found to be a constant feature in all members of the Sminthurinae examined.

7. Various methods of technique adopted are given.

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