There are some species, however, which are more remarkable than others in the regularity of their processional marchings, particularly two which are found in the south of Europe, but are not indigenous in Britain. The one named by Réaumur the Processionary (_Cnethocampa processionea_, Stephens) feeds upon the oak; a brood dividing, when newly hatched, into one or more parties of several hundred individuals, which afterwards unite in constructing a common nest nearly two feet long, and from four to six inches in diameter. As it is not divided like that of the brown-tails into chambers, but consists of one large hall, it is not necessary that there should be more openings than one; and accordingly, when an individual goes out and carpets a path, the whole colony instinctively follow in the same track, though from the immense population they are often compelled to march in parallel files from two to six deep. The procession is always headed by a single caterpillar; sometimes the leader is immediately followed by one or two in single file, and sometimes by two abreast, as represented in the cut. A similar procedure is followed by a species of social caterpillars which feed on the pine in Savoy and Languedoc; and though their nests are not half the size of the preceding, they are more worthy of notice, from the strong and excellent quality of their silk, which Réaumur was of opinion might be advantageously manufactured. Their nest consists of more chambers than one, but is furnished with a main entrance, through which the colonists conduct their foraging processions. [In the accompanying illustration is shown a nest of the Processionary caterpillar, part of which has been torn away to show the interior. Inside may be seen the larva of a certain beetle (_Calosoma sycophanta_), which feeds on these caterpillars, and one of the beetles is seen below in the act of ascending the tree. This beetle, although exceedingly scarce in England, is very common on the Continent, and trees have been cleared of Processionary caterpillars by the simple process of putting a few female beetles upon the branches.] [Illustration: Nest of Processionary Caterpillars (_Cnethocampa_ and _Calosoma_).] _CHAPTER XVIII._ STRUCTURES OF SPIDERS. Modern naturalists do not rank spiders among insects, because they have no antennæ, and no division between the head and the shoulders. They breathe by leaf-shaped gills, situated under the belly, instead of spiracles in the sides; have a heart connected with these; have eight legs instead of six; and eight fixed eyes. But as spiders are popularly considered insects, it will sufficiently suit our purpose to introduce them here as such.