THE simplest form of burrow is that of the Tarantulas, which represent the largest known spiders. These huge araneads appear to depend wholly upon their size to resist the assaults of enemies who invade their den. At least I have not found satisfactory evidence that they erect any artificial barrier over the entrance to their tunnels.

A more complicated burrow, and one better serving for defense, is that of Leptopelma cavicula of northern Africa. The drawing (Fig. 1) shows a section view of the upper part of the burrow, the entrance to which is without any door or other defense as in the Fig. 1.—Burrow of Leptopelma cavicula. Section view of upper part. case of the tarantulas. The burrow descends perpendicularly for a little way, but at the top a special branch diverges laterally, which curves and again descends perpendicularly for a considerable distance. At the summit of this second and parallel perpendicular tube another branch issues, inclining upward toward the surface. A glance at this structure, if we suppose it to be characteristic of the species, and not an accidental formation, will show that it makes an admirable protection against heavy rains, which sink away into the first burrow as a kind of reservoir, enabling the spider to escape by the diverging branch. Against enemies who pursue it into its den, this structure also presents an effectual defense, for, while an enemy naturally would rush downward into the first direct passage, the spider may escape by the lateral branch. Supposing that the enemy, observing the mistake, ascends and follows along the branches, the spider has the opportunity to push up into the second branch while the pursuer, again following its natural instinct, would rush down the second perpendicular tube. I am here in the region of conjecture, but perhaps no better explanation presents itself.