Page:EB1911 - Volume 21.djvu/573

 The structural variations presented by the phyllites are comparatively few. The most finely crystalline specimens have generally the most perfect parallel arrangement of their constituents. The foliation is generally flat or linear, but in some rocks is undulose or crumpled, From the imperfection of their cleavage phyllites are rarely suitable for rooting materials; their softness renders them valueless as road stones, but they are not uncommonly employed as inferior building materials. They are exceedingly common in all parts of the world where metamorphic rocks occur; as in the Scottish Highlands, Cornwall, Anglesey, north-west Ireland, the Ardennes, the Harz Mountains, Saxony, the Alps, Norway, the Appalachlans, the Great Lakes district in America, &c.

 PHYLLOXERA (Gr. , leaf, and  , dry), a genus of insects belonging to the family of Aphidae, or Plant-lice, in the Homopterous section of the order Hemiptera. It is chiefly known from the causal relation of one of its species to the most serious of vine-diseases. The name was first given in 1834 to a plant-louse which was observed to “dry up the leaves” of oaks in Provence. About twenty-seven species are now known, all characterized by length not exceeding ·06 of an inch, flat wings, three articulations in the antennae, one or two articulations in the tarses, with digitules, but without cornicles on the abdomen.

The following full description of the only species which attacks the vine, the Phylloxera vastatrix, or grape-louse, is reprinted from the article in the 9th edition of this encyclopedia.

“The symptoms of the disease, by means of which an infected spot may be readily recognized, are as follows: The vines are stunted and bear few leaves, and those small ones. When the disease reaches an advanced stage the leaves are discoloured, yellow or reddish, with their edges turned back, and withered. The grapes are arrested in their growth and their skin is wrinkled. If the roots are examined

numerous fusiform swellings are found upon the smaller rootlets. These are at first yellowish in colour and fleshy; but as they grow older they become rotten and assume a brown or black colour. If the roots on which these swellings occur be examined with a lens, a number of minute insects of a yellowish-brown colour are observed; these are the root-forms (radicola of Phylloxera (fig. 1); they are about ·8 mm. long, of an oval outline and with a swollen body. No distinction between head, thorax and abdomen can be observed. The head bears small red eyes and a pair of three-jointed antennae, the first two joints being short and thick, the third more elongated, with the end cut off obliquely and slightly hollowed out. Underneath, between the legs, lies the rostrum, which reaches back to the abdomen. The insect is fixed by this rostrum, which is inserted into the root of the vine for the purpose of sucking the sap. The abdomen consists of seven segments and these as well as the anterior segments bear four rows of small tubercles on their dorsal surface. These root-dwelling insects are females, which lay parthenogenetic eggs. The insect is fixed by its proboscis, but moves its abdomen about and lays thirty to forty yellow eggs in small clusters. After the lapse of six, eight or twelve days, according to the temperature, the larvae hatch out of the eggs. These are light yellow in colour and in appearance resemble their mother, but with relatively larger appendages. They move actively about for a few days and then, having selected a convenient place on the young roots, insert their proboscis and become stationary. They moult five times, becoming with each change of skin darker in colour; in about three weeks they become adult and capable of laying parthenogenetic eggs. In this way the insect increases with appalling rapidity: it has been calculated that a single mother which dies after laving her eggs in March would have over 25,000,000 descendants by October. If, however, the insect were content with this method of reproduction the disease could be isolated by surrounding the infected patches with a deep ditch full of some such substance as coal-tar, which would prevent the insects spreading on to the roots of healthy vines. The fertility of the parthenogenetically produced insects would also diminish after a certain number of generations had been produced.

As the summer wears on a second form of insect appears amongst the root-dwellers, though hatched from the same eggs as the form described above. These are the nymphs, destined to acquire wings; their body is more slender in outline, and at first they bear well-marked tubercles. After several moults the rudiments of two pairs of wings appear, and then the insect creeps up to the surface of the earth, and on to the vine. Here it undergoes its fifth and last moult, and appears as a winged female, capable of reproducing parthenogenetically The winged form has a slender body with distinct head (fig. 2). The eyes are well developed,

with numerous facets; the antennae have three joints, the terminal one shaped like that of the root-dwellers. The wings are transparent, with few nervures, and are well adapted for flight. The anterior pair reach far beyond the end of the abdomen; the posterior are narrower and not so long. These winged forms are about 1 mm. long. They fly about from July till October, living upon the sap of the vine, which is sucked up by the rostrum from the leaves or buds. They lay their parthenogenetically produced eggs in the angles of the veins of the leaves, in the buds, or, if the season is already far advanced, in the bark. In very damp or cold weather the insect remains in the ground near the surface, and deposits its eggs there. The eggs are very few in number and of two sizes, small and large (fig. 3, b and c). From the larger a female (fig. 4) is hatched in eight or ten days, and simultaneously, for the first time in the life-history of the Phylloxera, a male (fig. 3) appears from the smaller egg. Neither male nor female has wings; the rostrum is replaced by a functionless tubercle; and there is no alimentary canal.

The female is larger than the male and differs from it and the other forms in the last joint of the antennae. The life of these sexual forms lasts but a few days, and is entirely taken up with reproduction. The female is fertilized by the male and three or four days later lays a single egg—the winter egg—and then dies. This egg is laid in the crevices of the bark of the vine, and as it is protectively coloured it is almost impossible to find it.

Here the winter eggs remain undeveloped during the cold months; but in the following spring, as a rule in the month of April, they give birth to a female insect without wings, which resembles the root dwelling forms, but has pointed antennae. These forms are termed the stock-mothers; they creep into the buds of the vine, and, as these develop into the young leaves, insert their proboscis into the upper side. By this means a gall is produced on the under side of the leaf.

Scheme of the Various Forms of Phylloxera vastatrix. A. Root-infesting forms, ♀

Root-infesting forms, 2nd generation, ♀ Winged forms, ♀ Root-infesting forms, 3rd generation, ♀ &c.
 * Wingless female Male.


 * Winter egg


 * Stock-mother.


 * Gall-producing ♀ Root-infesting ♀


 * ditto A.

The gall is cup-shaped, and its outer surface is crumpled and covered with small warts and hairs. The opening upon the upper surface