ENTOMOLOGY

(Gr. entomon, an insect; logos subject), the science which comprises all the branches of knowledge connected with Insects. The richne of the insect world, with its 250,000 species, renders e. one of the most important of the many branches into} which Zoology is divided; and importance is further increased on|bears account of the harmful activities of| many kinds of insects. The study of {insect pests is generally termed Eco-| nomic Entomology. The Insect a or Hexapod a, are a cla | of arthropods characterized by the following features: (2) The body is seg-|crop, mented, and in the adult is divisible into|or three regions—the head, bearing a pair | of sensory antennae and a number of] highly specialized mouthparts; the thor-| ax, to which are attached three pairs of| legs (hence Hezapoda), and, generally,|discharge. two pairs of wings; and the abdomen. | (6) the mode of respiration is tracheal,| the tracheas communicating with the | exterior by means of special apertures termed stigmata. (c) The young invariably differ from the adult in a greater or le degree, and during development undergo slight, incomplete, or complete metamorphosis. In common with the remaining Arthropod a the body secretes an exoskeleton of chitin and the appendages are jointed. Insects constitute by far the largest number of terrestrial animals known, both generically and individually, the rapidity with which many multiply being little short of marvellous. Typically they are either aerial or creepimg in the adult condition, although some (e.g. Dytiscus) are aqua~ tic, but many pa a part or all of their larval life in the water. External Characters.—The head, which contains the so-called ‘brain’, and also bears the more important sense-organs (eyes, Ocelli, etc.), is usually rounded in character and is divisible into two main portions—the clypeus, situated between the two antennae and bearing the labrum or upper lip, and the epicranium. The mouth-parts consist primarily of three pairs of paired appendages—the mandibles and first and second maxillae_respectively—the last pair being fused to form the labium, but in many forms they are highly modified in connection with the suctorial habit. The thorax consists of three segments— namely, the pro-, me so-, and metathorax—each of which bears the wings where these are present. In the Cole opter a the anterior pair are modified as wing-cases (elytra), whilst in the Diptera, where only one pair occur, they are mesothoracic, and in some groups (springtails, lice, and fleas) they may be entirely wanting. The wings are outgrowths of the skin and are traversed and supported by nervures, the disposition of which is of considerable systematic importance, The abdomen consists of a variable number of segments— twelve, according to Heymons, being the maximum. They are typically without its|}appendages, but the hinder end often other specialized structures. Internal /Anatomy.—The alimentary canal consists of three main portions: (a) the stomodoeum, formed by the mouth, into which the products of the salivary glands pa , the oesophagus, and gizzard; (b) the mesenteron true gut, lined by endoderm and po e ing a number of blindly ending caecal glands; (c) the intestine, into the anterior portion of which the excretory organs, termed the Malpighian tubules, The blood is normally colourle , being mainly nutritive in character. It fills the body cavity and is circulated by the action of a pulsatile heart, surrounded by a pericardial cavity containing fat cells, and continued anteriorly into an aorta, whence it pa es into the coelom. The tracheal respiratory system is e entially a system of throughout the body and oxygenating the various parts directly. They are lined internally by chitin, and are supported internally by a closely coiled spiral thread. Each system is connected with its neighbours, and communicates with the exterior by means of a (usually protected) stigmatic aperture. The nervous system is typically invertebrate, consisting of a dorsal, ganglionic ma , united by a circum-oesophageal loop to the ventral nerve chain, which bears a pair of ganglia in each segment. In many forms, however, notably in the Diptera, this arrangement is considerably modified by ganglionic fusions. The eyes may be eihter simple ocelli or elaborate compound structures similar to those of the Crustacea, In addition there occur auditory and other organs, the significance of which is very imperfectly understood. In insects the sexes are distinct, but aphides and certain ‘stick insects’ are parthenogenetic whilst drone bees are also produced from unfertilized ova. The male sexual organs consist of paired testes opening into vasa deferent i a, which are often coiled. These swell posteriorly to form seminal vesicles in which the ripe sperms aggregate and are eventually discharged by way of the ejaculatory duct during copulation. The ovaries are also paired, and consist of ovarian tubules from which the ova are discharged into paired oviducts, these usually uniting posteriorly to form a muscular vagina. In the majority the life-history is very complex but three main types are distinguishable as follows: (a) Those in which the differences between young and adult are comparatively slight and in which the young, after a series of moults, a ume the mature form (e.g., Aptera, Orthoptera, Hemipter a); (6) those, such as dragon-fiy, in which the larva differs considerably both in structure and habits from the adult and is often aquatic; this larva never pupates, but after a period of quiescence sheds its skin for the last time and emerges a perfect insect; (c) in the third type the larva also differs markedly from the adult (¢g., maggot, caterpillar), but undergoes a definite, resting, pupal or chrysalid stage during which the larval organs disintegrate and are replaced by those of the adult (e.¢., and many Neuroptera). Such are said to undergo complete metamorphosis, as distinguished from the incomplete metamorphosis of the second type. The principal features utilized in the cla ification of insects are—the absence or occurrence and character of the wings, the structure of the mouth-parts, and the nature of the change from larva to adult; and upon this basis Sharp separates the nine vepeilco orders: % Aptera—springtails and bristletails; (2 Orthoptera—cockroaches, gra hoppers, earwigs, locusts, and crickets; (3) Neuflies, and termites; (4) Hymenoptera— bees wasps, ants, and saw-flies; (5) butterflies and moths; (7) Thy san opter a —thrips; (8) Diptera—flies proper; (9) Hemipter a—bugs, aphides. Economic.—LHither the adult forms or larvae of many insects are injurious to cereal crops, fruit trees, and numerous other cultivated plants. Where large crops are to be dealt with the only remedy seems to lie in the thorough breaking up of the soil in autumn, the burning of refuse and the encouragement in reason of natural enemies (e.g., moles and birds). In the case of trees and garden crops the treatment will vary with the type of insect; thus in the case of biting insects the food supply is poisoned, whilst suctorial insects are dealt with by contact spraying. There are three chief periods when spraying is useful—namely, winter, late spring, and summer. The object of the winter spraying is to cleanse the plant and to clear out all places where the eggs or young might be hidden. As the plants are dormant, a strong solution may be used, the following being a standard one: sulphate of fron 16 lb., lime \% lb., caustic soda 2 Ib., paraffin 5 pints, water to 10 gal. April and summer sprayings must be much weaker, their object being to exterminate any larvae which have escaped the previous sprayings. For contact spraying against mealy bugs and the like a solution of soft soap and qua ia, or an emulsion of soft soap and paraffin is generally used. For biting insects spray the foliage with the following: arsenate of soda 8 oz., lead acetate 1 lb. ‘water 24'gal.