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We have designated the abdomen as the urosome; the abdominal segments of insects and other Arthropods as uromeres, and the sternal sclerites as urosternites, farther condensed into urites. (See Third Report U. S. Entomological Commission, 1883, pp. 307, 324, 435, etc.)

Fig. 178.—Nymph of the pear tree Psylla, with its glandular hairs.—After Slingerland. Bull. Div. Ent. U. S. Dep. Agr.

The reduction takes place at the end of the abdomen, and is usually correlated with the presence or absence of the ovipositor. In the more generalized insects, as the cockroaches, the tenth segment is, in the female, completely aborted, the ventral plate being atrophied, while the dorsal plate is fused during embryonic life, as Cholodkowsky has shown, with the ninth tergite, thus forming the suranal plate.

In the advanced nymph of Psylla the hinder segments of the abdomen appear to be fused together, the traces of segmentation being obliterated, though the segments are free in the first stage and in the imago (Fig. 178). It thus recalls the abdomen of spiders, of Limulus, and the pygidium of trilobites.