Several species of these are of a gregarious nature, living together in extensive colonies under a thick net-work of silk, which serves them for a common protection while feeding on the foliage enclosed with themselves in a silken tent. Among these social net-weavers are the caterpillars of a fine insect, the black-veined white butterfly (_Pieris cratægi_), a rarity in some districts, but in certain localities, and at certain periods, abounding to such an extent as entirely to strip the hawthorn hedges of their foliage. Similar depredations are committed by the gaily-coloured progeny of the common lackey moth, and of the gold-tailed and brown-tailed moths; but the most formidable devastators, though the tiniest individually, are the little ermine moths (_Yponomenta_), small silvery-grey creatures, minutely spotted with black. The curious twig-like caterpillars of the brimstone moth (a pretty canary-coloured creature, with brown markings), and of several other geometers, are common upon hawthorn. Last summer (1864), the hawthorn trees and hedges about the parks and squares of London were entirely defoliated by caterpillars, which progressed from tree to tree in squads of numberless individuals, only seeking a new site of action when the former one had been despoiled of every vestige of leaf and bud. But it is not only the hawthorn which becomes attacked by insects: all other hedge-row trees and shrubs have their peculiar enemies, to describe which would take more space than we have to spare, and we therefore conclude the chapter with a few remarks upon the weeds of dirty hedge-rows. These harbour various insects, which migrate to our crops, and do an immense amount of injury. For instance, such plants as Jack by the Hedge (_Erysimum alliaria_), treacle mustard (_Sisymbrium officinale_), wild mustards, and other forms of _Cruciferæ_, in hedge-rows, afford a winter nidus for the turnip flea beetles (_Haltica concinna_ and _H. nemorum_),[25] from which they take their flight to the more delicate turnip and swede crops as soon as these come up. [25] See _How to Grow Good Roots_, pp. 43 and 44 of the present work. Birds need only here be mentioned incidentally, as there is still a conflict of opinions as to the use of the bird family to the farmer; and those species which mostly build in and frequent our hedges are perhaps those upon which evil suspicions are most universally held. Amongst these are the hedge-sparrow, finch, linnet, and others—and that these are mischievous at times, we are not prepared to deny; but we should be sorry if the curtailment of hedges, for which we are advocates, should result in the destruction of our small birds, as we conclude most of the species to be at times eminently useful. CHAPTER XXXVIII. ON THE MANAGEMENT OF HEDGE-ROW FENCES.