Page:The New International Encyclopædia 1st ed. v. 10.djvu/750

INSECT. 662 INSECTICIDE. and vary groatlj' in llieir devclopiiicnt. Tlic silk spun by many larva" comes from long silkglamU similar in form and situation to the simpli' tuliis of the salivary glands. Malpighian tubules are present in most insects in the hinder end of the body, opening into the alimentary canal Usually at the junction of the stomach and intestine. They vary greatly in length and number. The nervous system consists of a welldevelopod brain, and !io more than thirteen pairs of ganglia xliich may be more or less confused in tl^' s|)o- cialized orders. The eeplialic ganglia are placed above the oesophagus, while the other ganglia are ventral. There is an accessory sympathetic sys- tem which is complex and difhcult of dissection. Itespiration is carried on by means of a system. of air-vessels called traclicip, which ramify through all parts of the body. These tracheaj connect with the outer air through a series of spiracles which are orilices at the side of the body, there being usually one pair for each .seg- ment. The tracheic are elastic and consist of an outer cellular coat and an inner chitinous hiyer, the latter strengthened by a spiral fibre. With some insects, as witii certain bees and locusts, the tracheie expand in places to air-sacs, and these sacs, as well as the finest capillary branches of the tracheae, lack the spiral fibre. Circulation is carried on through the dorsal vessel or heart, which is situated in the upper part of the body (just beneath the dorsal skin), extending from the head or thora.x to the pos- terior extremity. It is an elongate tube consist- ing of a number of united chambers, and is usual- ly closed behind but open in front, having also several orifices at the side. There is a muscular laj'er with internal and external membranous layers. The blood is forced forward into the body-cavity, receiving oxygen from the terminal branches of the trachea?. The muscular system is extensive, nearly four thousand muscles oc- curring in certain caterpillars. C'LA.ssiFiCATiON. According to the generally accepted classification, there are nineteen orders of insects, as follows: Thjjsanura, springtails and bristletails. Fph cm erida, May- II ics. Odonata, dragon-flies. Plecoptera, stone-flies. Jsoptera, white ants. Corrodenlia, psocids and book-lice. Mnllophaqa, bird-lice. Euphxoptcru, earwigs. Orlhojitera, grasshoppers, crickets, etc. I'Inisopoda, thrips. Hemiptera, true bugs, plant-lice, scale-insects. Xeuroptera, aphis-lions, ant-lions, etc. Hecoptera, scorpion-fiies, etc. Trichoplera, caddis-flies. Lrpidoptrrn, butterflies and moths. Diptcrii. true flies. Siphonaptcra. fleas. Coteoptera, beetles. Jh/mcnoptera, bees, wasps, ants, saw-flies, etc. Representatives of the principal of these groups are illustrated by typical forms on the accom- panying colored plate, which are described in de- tail elsewhere, under their respective names. See also colored plates of Heetles, Butterflies, Drarox-flies, and Aloriis. Brm.lonRAPiiY. Packard, Text-hook of Ento- molnqii (Xew York, 1898) ; Comstock. ilattunl for the Study of Insects (Ithaca, 1895) ; Pack- ard, (juide to the litudy of Insects (Xew York, 1S80) ; Howard, The Insect Hook (New York, I'JOl) ; .Sharp, L'dmbridije yatural llistury, vols. V. and vi. (London, 1890-09). INSECTA (Lat. noni. pi., insects). A class of six jigged ar;hropod.s, equivalent to the Hexa- poda. ^See I.NSKir. INSECTICIDE (from Lat. iii«cc(i/i»i, insect -f eidn, nuirdemus, from cadere, to kill). . substance used by man to kill insects. lSeside» those materials properly include<l by this delini- j tion, however, there are many other agents of great importance as insect exterminators, of which the following may be classed as natural controls: Changes of temperature at critical times in the life histories of insects; rain, flood; drought; forest and prairie (ires; bacteria, fungi, mites, spiders, pre<lacoous and pamsitic insec'ls. and other invertc-brates; fish, reptiles, birds, ant- eaters, and other vertebrates. Though each of these exercises an important function in checking in.sect depredation, their ollices are beyond the scope of the present article, which deals with artificial or man-applied controls. For convenience of discussion the classification of insecticides may be made according to the two ways by which insects obtain food. Plant - feeding insects either chew plant tissue by means of movable jaws or else suck the juices through punctures in the epidermis made by their tube like beaks. The chewing species may , be subdivided into those that feed beneath the surface and those that feed in exposed positions. To the concealed feeders belong the leaf-miners, which live beneath the epidermis of leaves, etc., and the borers in living and dead plants, timber, I and grain. The only way these may be controlled are by poisonous gases (bean-weevil), by drown- ing (peach-borers, sometimes), burning infested parts (raspberry cane-borer) , or by digging them out (applc-bor^r) , the feasible method dejiending upon the habit of the species. To the exposed feeders belong caterpillars, grasshoppers, beetles, etc., which are usually controlled by intestinal poisons, .such as arsenical compoimds. especially Paris green used at the rate of a i)ound in from 100 to .300 gallons of water, Hordeaux mixture. (See FrxGiciDE.) .-VnotluT jiopular poison is arsenate of lead, which ailhercs to the foliage better than Paris green, and may be applied in larger quantities with less danger of injun'. Of the newer insecticides, green arsenoid is con- sidered valu.able. Hellebore is most frequently u.sed upon small plants, especially for the cur- rant-worm. It is also sprayed upon fruits ap- proaching maturity. Some soft-bodied insects may l)e controlled in the same way as sucking insects. Sucking insects, among which are some of the most destructive pests of crops, cannot be con- trolled by stomach poisons, because they obtain their food from beneath the epidermis of the plant. Because of their small size, insidious habits, enormous prolificacy, or capability of withstanding treatment, they are among the most dreaded crop pests. Some (e.g. the squash-bug) have never been efTectually and economically controlled by any method yet devised. In general the most feasible controls are such as either obstruct the respiratorv' passages of the insects (tobacco and insect powder), or have a caustic action upon their skins (kerosene and whale-oil