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 and progressive decay of the whole neurone, beginning usually at the terminal twigs and proceeding back towards the cell body with its contained nucleus. These primary degenerations involve systems of neurones, correlated by function rather than by anatomical situation. Examples are afforded by locomotor ataxy and progressive muscular atrophy, the former being a degeneration of the afferent sensory system of neurones, the latter of the motor efferent system. The cause of primary degenerations is probably a defect inherited or acquired in the “vita propria” of the neurones affected. They slowly atrophy and disappear, and their place is filled up by an overgrowth of the supporting neuroglia tissue (figs. 10 and 18). This overgrowth of dense tissue is termed sclerosis, and was erroneously considered to be the cause, instead of the effect, of the atrophy of the nervous tissue.

 NEUROPTERA (Gr. , a nerve, and  , a wing), a term used in zoological classification for an order of the class  (q.v.). No ordinal name used in the class has had so many varying meanings given to it by different authors. As first used by Linnaeus (1735) it included all insects with mandibulate jaws and two pairs of net-veined wings—dragon-flies, May-flies, stone-flies, lacewing-flies and caddis-flies—and it has been employed in the same wide sense by D. Sharp (Cambridge Nat. Hist. vol. v., 1895). But detailed study of these various groups of insects shows that beneath their common superficial resemblances lie important distinctions in structure, and essential differences in the course of the life-history. Some of the families—the stone-flies, for example—have the young insect much like the adult, growing its wings visibly outside the thoracic segments, and active at all stages of its life. The dragon-flies and May-flies are also active throughout their lives and possess external wing-rudiments, though the young insects differ rather strikingly from their parents. All such families—falling into the group Exopterygota as defined in the classification of the Hexapoda—were separated from the Neuroptera by W. E. Erichson (1839) and united with the Orthoptera, with which order some entomologists still associate them under the name of “Pseudo-neuroptera.” The other groups of the old Linnean order (such as lacewing-flies and caddis-flies)—which are hatched as larvae markedly unlike the parent, develop wing-rudiments hidden under the larval cuticle, and only show the wings externally in a resting pupal stage, passing thus through a “complete” metamorphosis and falling into the sub-class Endopterygota—were retained in the order Neuroptera, which thus became much restricted in its extent. More recently the subdivision of the Linnean Neuroptera has been carried still further by the separation of the caddis-flies and scorpion-flies as distinct orders (Trichoptera and Mecaptera respectively), and by the withdrawal of the “Pseudo-neuroptera” from the Orthoptera—with whose typical families they have little in common—and their division into a number of small orders. Altogether, eight orders are recognized in the classification adopted here, the first five of these belonging to the sub-class Exopterygota and the last three to the Endopterygota (see ).

The multiplication of orders is attended with practical difficulties, and the distinctions between the various groups of the Linnean Neuroptera are without doubt less obvious than those between the Coleoptera (beetles) and the Diptera (two-winged flies) for example. But if classification is to express relationship, it is impossible to associate in the same order families whose kinship to insects of other orders is nearer than their kinship to each other. And no student can doubt that the stone-flies are akin to Orthoptera and the caddis-flies to the Lepidoptera, while dragon-flies and May-flies stand in an isolated position with regard to all other insects. In the present article, for the sake of convenience, all the insects which have been regarded

by Linnaeus and others as “Neuroptera” are included, but they are distributed into the orders agreed upon by the majority of modern observers, and short characters of these orders and their principal families are given. For further details the reader should consult the special articles on these groups, to which cross-references will be found.

Sub-class

Order Plecoptera.

This order was founded (1869) by F. Brauer—the name having been long previously suggested by H. Burmeister (1832)—to include the single family of the Perlidae or stone-flies. They resemble the Orthoptera more nearly than do any other group of the Linnean Neuroptera, having the anal area of the hind-wings folding fanwise beneath the costal area and the whole hind-wing covered by the fore-wing when the insect is at rest, though the forewing is not firmer in texture than the hind-wing, as is the case in the Orthoptera. In the opinion of J. H. Comstock and J. G. Needham the wing-neuration in this order is the most primitive to be found in the Hexapoda. The tenth abdominal segment carries a pair of jointed cerci which are often elongate, and the feelers are always long, while the jaws are usually feeble and membranous, though the typical parts of a mandibulate mouth are present—mandibles, maxillae with inner and outer lobes and palps, and second maxillae (labium) whose lacinae are not fused to form a ligula. Both head and trunk are somewhat flattened dorso-ventrally, giving the insects a very distinct and characteristic aspect. The stone-flies further resemble the Orthoptera in their numerous Malpighian excretory tubes, which vary in number from twenty to sixty. The reproductive organs, both ovaries and testes, become fused together in the middle of the body. A remarkable point in the Plecoptera is the presence in some forms (Pteronarcys) of small branching gills on the three thoracic and the front abdominal segments. These organs appear, however, from the observations of H. A. Hagen not to be functional in the adult insect—they are merely survivals from the aquatic nymphal stage.

Life-history and Habits.—The nymphs of the Perlidae are closely like their parents and breathe dissolved air by means of tracheal gills on the thoracic segments, for they all live in the water of streams. They feed upon weaker aquatic creatures, such as the larvae of May-flies.

The perfect insects, whose flight is feeble, are never found far from the water. A curious feature among them is the frequent reduction of the wings in the males of certain species, contrary to the usual condition among the Hexapoda, where if the sexes differ in the development of their wings it is the female which has them reduced. The Plecoptera are world-wide in their range and fossils referable to them have been described from rocks of Eocene, Miocene and Jurassic age, while C. Brongniart states that allied forms lived in the Carboniferous Period.

The two families included in this order agree with the Plecoptera in the young insect resembling the parent, but they are all terrestrial throughout life. The hind-wings have no folding anal area and the wings of both pairs, when present, are closely alike (see fig. 1) whence the name Isoptera (＝equal winged) lately applied to the group by