The _Coccidæ_, or Scale Insects, are sometimes very injurious to cultivated plants, but they are also useful, producing cochineal, shellac, manna, and other substances of considerable importance. The Cochineal Insect (_Coccus cacti_) is a native of Mexico, and furnishes us with the most valuable and durable red dye that we possess, and the Lac Insect (_Coccus lacca_), an East Indian insect, produces the well-known lac-dye. The common Scale Insect (_Coccus adonidum_) is well known in hothouses and conservatories in our own country, though it is not indigenous, having been imported from abroad. [Illustration: Fig. 118.--Coccus Cacti (Mag.)] SUB-ORDER III.--ANOPLURA. This sub-order contains only one family, which is considered to be the last and lowest group of the _Hemiptera_, of which it may be regarded as a very degenerate form. The insects have no wings, and live upon the blood of other animals. _Family 1, Pediculidæ._--The "Lice" are a very extensive family, but at present not very well known. Almost every mammal has a louse peculiar to itself. [Illustration: Fig. 119.--Pediculus Capitis (Mag.)] Three species infest man. The Head Louse (_Pediculus capitis_), found on the head, especially in children. The Body Louse (_Pediculus vestimenti_), found in the clothes; and the Crab Louse (_Phthirius inquinalis_), a broader and shorter insect, found in the hair on the face and body of uncleanly persons. The lice infesting different races of men differ in colour--thus, those found on niggers are black; and those on Europeans are whitish. Other species of lice infest the dog, cat, pig, mouse, elephant, monkey, etc. The _Pediculidæ_ undergo no metamorphoses. ORDER DIPTERA. INCLUDING THE GNATS, MIDGES, CRANE FLIES, HAWK FLIES, BEE FLIES, BREEZE FLIES, BOT FLIES, AND THEIR ALLIES. The _Diptera_, or two-winged flies, are among the most numerous orders of insects, but they have been so little studied hitherto that we have but a very imperfect knowledge of them, especially as regards the tropical species. [Illustration: Fig. 120.--Midas Giganteus.] The number recorded in the British Isles is about 3,000, that of the whole world probably exceeds 30,000, and it is certain that this number will be increased immensely as time goes on. The _Diptera_ may be defined as insects with a sucking mouth (_Haustellate_), and with only two wings, which are membranous, while the hind wings are represented only by a pair of small knotted organs, called _halteres_. Their metamorphoses are complete. The _larvæ_ are footless "grubs," generally with a soft body, but sometimes leathery or even nearly horny. Many of them possess a distinctly marked head, which may contain _ocelli_, but in the majority the head appears as if it were simply one of the neighbouring segments. [Illustration: Fig. 121.--Craspedia Coriaria.]