THE PERIODICAL CICADA
cessation of digging and the tarsi are turned forward at right angles to the tibiae to serve as rakes (Fig. 116 B). The mass of earth pellets is scraped in toward the body, and—here comes the important part, the cicada's special technique—the little pile of rakings is grasped by one front leg between the tibia and the femur (Fig. 116 A, Tb and F), the former closing up against the spiny margin of the latter, the leg strikes forcibly outward, and the mass of loosened earth is pushed back into the surrounding earth. The process is repeated, first with one leg, then with the other. The miner looks like a pugilist training on a punching bag. Now and then the worker stops and rubs his legs over the protruding front of the head to clean them on the rows of bristles which cover each side of the face. Then he proceeds again, clawing, raking, gathering up the loosened particles, thrusting them back into the wall of the growing chamber. His back is firmly pressed against the opposite side of the cavity, the middle legs are bent forward until their knees are almost against the bases of the front legs, their tibiae lying along the wing pads. The hind legs keep a normal position, though held close against the sides of the body.
From what we know of the cicada's spring habits underground, we can infer that the nymphs construct their chambers on their arrival near the surface during April, and that, when the chambers are completed, the insects wait within for the signal to emerge and transform into the adult. Then they break through the thin caps at the surface and come out. It would be difficult to explain how they know when they are so near the top of the ground, and why some construct ample chambers several inches deep while others make mere cells scarcely larger than their bodies. Do they burrow upward till the pressure tells them that the surface is only a quarter of an inch or so away, and then widen the débris-filled tunnel downward? Evidently not, because the chamber walls are made of clean, compacted clay in which there
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