Social Insects

SOCIAL INSECTS.) Order XI. Psocoptera (Corrodentia) .—Very small sof t bodied insects, winged or wingless : wings membranous with few veins, anterior pair the larger with extensive stigma. Mouth parts for biting: tarsi two- or three-jointed: cerci wanting. In cludes the book-lice and their allies. The small recently discov ered group Zoraptera are sometimes included here.

Order XII. Anoplura.

Very small tough-bodied, flattened, wingless insects living on mammals or birds, with short three- to five-jointed antennae : eyes reduced or wanting. Mouth-parts highly modified either for biting or piercing: tarsi one- or two jointed : cerci wanting. Includes the true lice or Siphunculata and the bird-lice or Mallophaga. (See LousE.) Order XIII. Thysanoptera*.—Minute and very slender in sects with short six- to nine-jointed antennae and very narrow wings with long hair-fringes: mouth-parts stylet-like, for pierc ing: tarsi very short, one- or two-jointed, terminated by a small bladder-like organ : cerci wanting. Metamorphosis with a rudi mentary pupal stage. Includes the thrips.

Order XIV. Hemiptera*.

Mouth-parts for piercing and sucking with mandibles and first maxillae stylet-like and lying in a jointed, grooved rostrum formed by the labium. Wings very variably developed, with but few veins, and often wanting. Pro thorax large : tarsi with rarely more than three joints : cerci ab sent. An incipient pupal stage present in some cases. Includes the plant-bugs, cicadas (q.v.), leaf-hoppers, frog-hoppers, aphides (q.v.) and scale insects.

Division II.: ENDOPTERYGOTA Wings developed inside the body until the pupal stage, meta morphosis complete.

Order XV. Neuroptera*.

Small to rather large soft-bodied insects with two pairs of usually sub-equal, membranous, net veined wings which are closed roof-like over the body when at rest : hind-wings without a plicated posterior lobe. Mouth-parts for biting : tarsi five-jointed : cerci absent. Larvae campodeiform, terrestrial or aquatic : pupae free, usually in a silken cocoon. In cludes the alder-flies (q.v.), ant-lion flies (q.v.), mantis flies, lace wings and mealy wings.

Order XVI. Coleoptera*.

Fore-wings modified into horny elytra which meet in a straight line down the back, the hind wings (where present) membranous and folded beneath them. Prothorax large : mouth-parts for biting : cerci absent. Larvae of various types: pupae free, rarely in a cocoon. Includes the beetles.

Order XVII. Strepsiptera.

Minute insects : the males with branched antennae and degenerate biting mouth-parts. Fore wings minute club-like scales : hind-wings very large, membra nous and plicately folded. The females inert degenerate sac-like parasites living in the bodies of other insects includes the stylops (q.v.) and its allies.

Order XVIII. Mecoptera.

Soft-bodied insects with two pairs of similar membranous wings folded flat over the back in repose. Head generally prolonged into a beak: mouth-parts for biting: short cerci present. Larvae eruciform: pupae free, not in a cocoon. Includes the scorpion flies (q.v.) and their allies.

Order XIX. Trichoptera*.

Moth-like insects with two pairs of membranous hair-covered wings, with predominantly longi tudinal venation : hind pair usually the larger with a plicate posterior lobe. Mouth-parts impertect, mancii Dies wanting. Lar vae aquatic, more or less eruciform, living in cases : pupae free, with strong mandibles. Includes the caddis flies (q.v.).

Order XX. Lepidoptera*.

Small to very large insects usually densely clothed with flat scales. Mouth-parts auctorial: first maxillae usually modified into a spirally coiled proboscis: mandibles almost always absent. Larvae terrestrial, eruciform, with several pairs of abdominal feet : pupae obtected or partially free, usually in a cocoon. Includes the butterflies and moths.

Order XXI. Diptera*.

Insects with a single pair of mem branous wings: hind pair modified into minute knobbed organs or halteres: rarely wingless. Mouth-parts for sucking or for piercing also, and elongated to form a proboscis. Larvae vermi form : terrestrial, or parasitic or more rarely aquatic : pupae free or coarctate. Includes the true flies (q.v.).

Order XXII. Siphonaptera. (Aphaniptera).

Very small wingless insects, laterally flattened, with tough integument. Mouth-parts for piercing and sucking. Blood-sucking parasites living on warm-blooded animals. Larvae vermiform : pupae free, in a silken cocoon. Includes the fleas (q.v.).

Order XXIII. Hymenoptera*.

Wings tough and membra nous : hind pair the smaller and connected with fore pair by a series of minute hooks. Mouth-parts for biting and licking : lab ium usually modified into a tongue of variable length. Prothorax small and ring—like: abdomen usually basally constricted to form a waist, its first segment fused with the thorax: a sawing or pierc ing ovipositor present. Larvae eruciform or vermiform but some times embryonic : pupae usually in a cocoon. Includes the saw flies, ants, bees, wasps and ichneumon flies and their allies (qq.v.) : see also SOCIAL INSECTS.

Geographical Distribution.

The class Insecta as a whole, is cosmopolitan as are almost all its individual orders. The range of any great group of animals is governed by many factors, whose individual effects are difficult to evaluate separately. In the case of insects the possession of wings is a feature of supreme im portance, since it imparts to its possessors a capacity for dispersal lacking in other terrestrial animals not so endowed. The power to disperse freely allows a species to escape from intense competi tion, and the effects of rigid selection which results from over crowding within limited areas. Habits and food-supply are clearly also highly important : aquatic insects cannot exist where there is no water and bark beetles are not to be found on steppes or plains devoid of all trees. Yet, on the other hand, many insects have a distribution much more restricted than their available food, cli matic and other barriers precluding their spread. High mountain ranges may present insuperable obstacles by way of climate and changed vegetation, while deserts and oceans are even more pow erful barriers to the dispersal of almost all insects. Many facts of distribution, however, cannot be interpreted under present condi tions of the earth's topography and climates, and recourse has often to be made to a study of geological changes. The close simi larity of the insects of Europe and North America, for example, is explained on the basis of a former more favourable climate in the Arctic region, when it did not interpose as a barrier to dispersal as happens under the frigid conditions of to-day. The presence of allied spring-tails in New Zealand and Chile suggests a former ex tension of the Antarctic continent between them, since such deli cate insects can scarcely be credited to have attained their present distribution by other means. There are again insects which ex hibit a wide but discontinuous distribution, such groups often ap pearing to be scattered remnants of forms which once enjoyed an extensive, continuous range, but now widely separated by climatic or topographic changes. Moths of the family Uraniidae and but terflies of the genus Erebia exhibit a distribution of this character.

Some individual insects are almost cosmopolitan and possess great capacity for migration and of adaptation to changed condi tions. Examples of such species include the hawk-moth Celerio lineata, and the butterflies Pyrameis cardui and Anosia plexippus. The extensive distribution of the spring-tail Hypogastrura armata has been accounted for on the basis of former land-connections between certain of the great continents, as already alluded to. Other cosmopolitan insects such as the house-fly and many beetles affecting grain and other stored products have attained their wide dispersal through human agencies.

The spring-tails have the widest range of any group of insects and have adapted themselves to the greatest diversity of sur roundings, while ants approach them very closely in these two respects. Ants, as W. M. Wheeler remarks, are found "from the Arctic regions to the Tropics, from timber line on the loftiest mountains to the shifting sands of the dunes and sea-shores, and from the dampest forests to the driest deserts." More detailed facts relative to insect distribution are given in the separate arti cles devoted to the principal orders.

Geological Distribution.

Insects are relatively scarce as fossils and are only found in certain geological formations espe cially favourable for their preservation. Most of the specimens probably originally got drowned, and were quickly covered with silt or other deposits before they had time to decay ; consequently the richest remains occur in vegetable deposits such as coal, lig nite and peat, and in ancient fresh-water basins. A great many also occur in amber where they became entangled in the resin, which quickly enveloped them, thus securely sealing them up in a wonderful condition of preservation. Among the most famous localities for fossil insects are the Carboniferous rocks of Corn mentry, France, the Permian beds of Kansas and of New South Wales, the Triassic beds of Queensland, the lithographic lime stones of Jurassic age in Bavaria, the Oligocene shales of Floris sant in Colorado, the Miocene beds of Oeningen in Bavaria and Baltic amber of the same period.

The study of fossilized remains indicates that no undisputed in sect has been found earlier than Upper Carboniferous times and even these very ancient insects were already highly evolved, af fording little information as to the ancestry of the class as a whole. The oldest known insects are all Exopterygota and comprise totally extinct orders, some of which are allied to the existing Orthoptera and others (Palaeodictyoptera) exhibit the combined characters of several of the most generalized living orders; in cluded among these fossils are also ancestral dragon-flies of gigantic size. The only still existing insects represented in Car boniferous times are the cockroaches, which have persisted with relatively little modification through the intervening ages to the present day. In Lower Permian times the first true dragon-flies appear, with Ephemeroptera, Hemiptera and some anomalous insects (Protohymenoptera), which are regarded as the far-off an cestors of the Hymenoptera. In rocks of this age there are also found the earliest remains of the Endopterygota which are repre sented by some small species of Mecoptera. In the Upper Permian are found the first recorded Coleoptera and Neuroptera together with some remarkable insects (Paramecoptera) which are held to be the ancestors of Lepidoptera, Trichoptera and Diptera. It is not until Jurassic times that true Diptera and Trichoptera appear, while Hymenoptera are first found in the Cretaceous and the oldest Lepidoptera are of Oligocene age. In rocks of later date the insect remains are very like those of living forms and those found in Baltic amber even belong to existing genera.

A survey of fossil insects shows that the Exopterygota appeared first and though no remains of the most primitive insects (Thysa nura) have been found till Tertiary times, it is probable that their delicate organization has not favoured their preservation through the great periods of time when they presumably were existing. Although the Endopterygota are of great antiquity, those orders which are most specialized and exhibit the greatest differences between the larva and the imago were the last to appear. Further details respecting fossil insects will be found in separate articles dealing with the principal orders.

The Life-cycle.

In temperate and cold regions the life-cycle of most insects involves a period of dormancy or hibernation dur ing the winter months, whereas in the uniform hot and moist cli mate of the Tropics, hibernating does not exist and one generation succeeds the other without any such dormant period. Hiberna tion may take place in any stage in the life-cycle in different in sects. Among British butterflies, for example, the Hairstreaks hibernate as eggs, the White Admiral, Fritillaries, etc., overwinter as larvae, the garden whites and others as pupae and the Brim stone, Peacock and Tortoiseshells pass the winter as perfect in sects, sheltering in old buildings, hollow trees and other likely situations.

Some insects such as the Cotton Boll Weevil may pass through as many as eight generations in the year, while on the other hand, the click beetle (Agriotes obscurus) requires about five years to complete a single generation, most of that period being spent as a larva. The house-fly under favourable conditions of food, tem perature and moisture can complete its life-cycle in ten days, but the shortest cycle is found in some minute Chalcid wasps whose larvae live as parasites in the eggs of other insects, and require only seven days from when their eggs are laid till the perfect insects appear.

Plant-relations.

A vast number of insects only exist in as sociation with vegetation and every kind of phanerogamic plant, and every part of the plant may serve as food for one or other species of insect. Larvae of moths, saw-flies and many beetles feed openly and devour the foliage, and other larvae live concealed within the leaves, stems, roots or the solid wood. Thrips, aphides and other plant-bugs pierce the plant-tissues and extract the sap. There are also insects which confine themselves to devouring the flowers, seeds or even the pollen, while bees and many moths and flies imbibe nectar. There are again other insects which, instead of destroying plants, modify them in such a way as to produce ab normal growths, or galls, which serve as food and shelter for their larvae. Most galls are produced by gall-wasps (Cyni pidae) , gall midges (Cecidomyidae) and certain saw-flies while a few are pro duced by beetles, aphides and other insects. The actual cause of gall-formation has been much discussed and these growths are due to the activity of the meristem under the influence of a definite stimulus. In some cases gall-formation follows immediately after the insect has laid its eggs in the plant-tissues and in these in stances it is believed to be due to a secretion injected into the plant at the time of egg-laying, since the gall develops after the egg has been killed by puncturing with a needle. In other cases it is in duced by the presence of the larva, the gall not developing till after the insect has issued from the egg.

The relationship between insects and flowers is mutually bene ficial: insects obtain nectar and pollen and in return ensure cross pollination by transferring the pollen from the stamens of one flower to the stigma of another. Flower-haunting insects often ex hibit special structural modifications enabling them to suck nectar from deeply seated nectaries, and bees are provided with organs for pollen-collecting. Flowers in their turn are coloured or other wise ornamented to attract insect-visitors and their scents are be lieved to serve the same purpose. Most flowers are also so con structed that insects inevitably pollinate them, and many such as orchids, iris, yucca, etc., are formed so that pollination is effected in a particular manner.

Among the lower plants affected by insects, fungi of various kinds afford sustenance and shelter to many small beetles and their larvae, along with those of fungus-midges and other flies. Certain of the primitive fungi are pathogenic to insects, entering and spreading within their bodies and ultimately killing them. Mention needs also to be made of species of ants and termites which cultivate fungi within special recesses of their nests where they serve to feed the brood. Lichens are fed upon by various moth larvae, but liverworts, mosses and ferns support compara tively few insects.

Scavengers.

Not only living plants afford food and shelter for insects, but decaying vegetable matter is also resorted to by a vast number of beetles and flies, together with certain of the spring-tails. Dung affords sustenance for many insects as well as decomposing carcasses and animal remains of all kinds. In a large degree the activities of scavenger insects are beneficial in facilitating the rapid decomposition of all kinds of decaying material.

Aquatic Insects.

A considerable number of insects are aquatic and are found in fresh water, while a few occur in saline waters and in the sea. Among fresh-water insects some are aquatic throughout their life, notably beetles of the family Dytiscidae, which live habitually submerged, and water-bugs or pond-skaters (Hydrometridae) which are surface dwellers. The Whirligig bee tles (Gyrinidae) live submerged as larvae, but the adult beetles congregate in companies on the surface. A far greater number of insects are aquatic only in their immature stages, the adults being aerial : familiar examples are midges, mosquitoes, stone-flies, may flies, caddis-flies and dragon-flies.

All aquatic insects are believed to have been derived from terres trial ancestors and have become adapted in diverse ways to their special mode of life : these adap tations mainly concern locomo tion and respiration. Among beetles and water-bugs the swim ming legs are flattened and oar like, often fringed with closely set hairs, and form efficient paddles.

Respiratory modifications are numerous and in pond-skaters the body is clothed with a dense vel vety pile which renders them in capable of being wetted, and also serves to retain a coating of air around the insect when sub merged : in this manner a supply of oxygen is provided for breathing and the insect can remain im mersed till it is used up. Some beetles carry a supply of air be neath the elytra when submerged, while others descend with a bubble of air at the apex of the abdomen. Water-scorpions are provided with a kind of caudal respiratory tube and, when neces sary, protrude the apex of this siphon through the surface film: air passes down this channel to enter a pair of spiracles situated at its base. Mosquito larvae breathe by means of a caudal siphon of a different character : it bears a pair of spiracles at its apex and air enters the tracheal system directly, when the siphon breaks the sur face film. Many other insects breathe the oxygen dissolved in the water: such insects either possess gills or respire cutaneously. Gills are outgrowths of the integument containing tracheae or more rarely only blood. In may-fly nymphs (fig. 4o) they are commonly in the form of lamellae attached to the sides of the abdomen, while in many dragon-fly nymphs they lie within the terminal chamber of the gut, water being inhaled and exhaled through the anus. The so-called bloodworms or larvae of certain midges (Chironomidae) are remarkable in that the blood contains haemoglobin and respira tion takes place through the skin.

The open sea is almost devoid of insect life excepting for a few midge larvae that have been dredged from tolerably deep water and the Hemipterous genus Halobates which frequents the surface of warm oceans often far from land. Between tide levels the shore is inhabited by the spring-tail Anurida maritima, a few beetles and their larvae and certain flies. These insects, or at least their larvae, are immersed during each tide when they retreat under stones or in the sand, while some midge larvae remain continuously sub merged. Those of the European genus Clunio occur in rock pools along with the curious wingless females, while the males, which are winged, skim over the water surface. Loss of wings is a common feature of marine insects and it has been explained as an advan tage which prevents such insects being blown out to sea.

Predators and Parasites.

There are diverse groups of insects that have adopted close relations with respect to food and shelter hot only with other insects, but also with vertebrate animals. Many insects such as ground beetles and their larvae are predators which seek out and devour other insects as food : or the relations may be of a closer nature and the prey becomes the host, as hap pens in the case of parasites. Some parasitic insects live externally on their hosts and are termed ectoparasites, while others—endo parasites—live within the bodies of their hosts. Fleas and lice, for example, are ectoparasites of warm-blooded animals from whose bodies they extract blood as their food. The grubs of warble flies are endoparasites of mammals, while those of Ichneumon flies and Tachinid flies live within the bodies of other insects which they finally kill. Some very minute Hymenoptera are endoparasites of the eggs of various insects, finding therein sufficient nutriment to complete their transformations : other parasites attack only lar vae or pupae and a few confine themselves to adult insects. A parasite may, in its turn, be parasitized by smaller insects of simi lar behaviour which are termed secondary parasites or hyperpara sites; tertiary or even quaternary parasites are also known and provide the most complex examples of hyperparasitism.

Parasitic insects exhibit varying degrees of adaptation to their mode of life : some ectoparasites are very little modified except that their claws are specially developed for clinging to their hosts and wings are totally absent. Among endoparasites all traces of limbs and sense organs are wanting, these degenerative changes being adaptations to a life in which the necessity to seek out food is no longer present. The tracheal system is often totally absent but, on the other hand, the integument is unusually thin so as to allow of the passage of oxygen contained in the blood of the host, at whose expense such parasites feed and respire. During most of their life endoparasites avoid the vital organs of their hosts and feed largely upon the blood and fat-body, since their own death would speedily follow that of the insects they parasitize : for the same reason such parasites do not void the contents of the gut till the last moult. One of the most advanced types of parasitism is found in the stylops and its allies where the female, after issuing from the egg, spends its whole life as an endoparasite in the body of a bee or a leafhopper. The adult is a degenerate sac-like creature specially adapted for this mode of life. The male stylops is a para site only as a larva and finally emerges as an active winged insect.

Inquilines.

Certain insects instead of devouring others feed at their expense and are termed inquilines. The latter are numer ous in the nests of social insects and a well-known example is the wax-moth whose larvae live in bee-hives and destroy the comb.

Larvae of the hover flies (Volucella) live in the nests of bumble bees and wasps, where they feed on waste organic material. The curious bristle-tail Atelura occurs in ants' nests and is stated to live by imbibing the food which is being actually regurgitated from one ant to another. There are again other insects which benefit their hosts in return for their keep. Thus, certain beetles obtain food and shelter in ants' nests and provide the ants in return with cutaneous secretions that are much sought after by their hosts. In extreme cases the association is even more intimate, the guest beetles being actually fed by the ants in return for the secretions provided, thus affording an example of symbiosis or mutual benefit.

(See SOCIAL INSECTS.) Gregariousness and Social Life.—Although the vast ma jority of insects are solitary in habit, individuals of certain species associate together collectively and are said to be gregarious. Whirligig beetles, for example, associate in companies on the sur face of the water : locusts and some butterflies become gregarious when migrating and ladybirds at times congregate in masses. Some caterpillars are gregarious and members of a brood live in a com mon web spun over the food-plant : such companies usually dis perse, as the food becomes used up, and its members live inde pendently. There are again other insects such as the earwig and the mole cricket in which the female guards the eggs and the young brood. A solitary bee of the genus Halictus constructs its cells in a group around a common entrance tunnel in the ground : these cells are provisioned and guarded by the parent, who may even survive till the daughter bees emerge. Examples of this kind where the parent guards and tends the offspring, at least during their early life, foreshadow the beginnings of social life. The lat ter exhibits its own peculiar features in different groups of insects which have developed the social habit, but in all cases each society is a family consisting of two parent insects, or at least the fecun dated female, and the offspring, and the members of the two gen erations live together in a common abode or nest in mutual co-op eration. True social insects include several groups of wasps and bees, together with the ants and termites. In the highest grades of social life specialization of certain individuals to perform specific functions for the common good is attained and a system which exhibits striking parallels in behaviour with human society has been developed. (See SOCIAL INSECTS.) Ancestry of Insects.—It is conceded that the most primitive insects are the bristle-tails (Thysanura) and these lowly creatures betray certain characters which are to be regarded as inheritances from an ancestral stock. The embryonic development of insects also reveals primordial characters : the evanescent appendages just behind the antennae and those borne on the abdominal seg ments in generalized insect embryos, indicate that the ancestors of insects carried five pairs of head appendages and had a body of I s segments, all of which except the last bore limbs. Those car ried on the first three body segments (legs of insects) and on the i4th segment (cerci) were evidently more important than the in tervening pairs, of which traces only remain in some larvae and in the Apterygota. It seems clear, therefore, that the ancestors of insects were many-legged animals and the nearest approach to such progenitors is to be found in certain small, white creatures living in moist earth and represented by Scolopendrella and its allies. They form the class Symphyla which is related to the Chilopoda (centipedes) and Diplopoda (millipedes). Structurally the Symphyla have so many features in common with such bristle tails as Campodea and Anajapyx as to justify affinity between the two groups. Other Thysanura, such as Machilis and its allies, show features in common with the higher crustacea particularly with regard to the mouth-parts. It must also be remembered that the small evanescent appendages found behind the antennae in some insect embryos, are to be regarded as relics of the second pair of feelers only found in Crustacea. The foregoing remarks indicate very briefly that it is probable that the ancestors of insects had features in common with the Symphyla and through them with the Chilopoda and Diplopoda, but at the same time, these ancestors were derived from a stock from which the Crustacea also arose. The Arachnida, or spiders and scorpions, are only very remotely related to insects and evidently diverged very early in their evolu tion from the primordial arthropod stock. (See also ARTHROPODA.) Origin of Wings.—The Thysanura are eminently terrestrial insects, devoid of wings, and undergo no metamorphosis. Since they are the most primitive known insects and the majority of that class are winged creatures, the derivation of the latter from the Thysanura is largely involved in the origin of wings. Many authorities believe that wings first arose as small lateral expansions of the tergal region of the thoracic segments. During their evolu tion these expansions became sufficiently large to function after the fashion of parachutes in insects which possessed a tendency to leap. At a later stage they acquired direct articulation with the thorax, became supplied with tracheae and developed the power of independent motion. By others it is contended that wings were derived from plate-like thoracic tracheal gills which had lost their original function and became adapted for purposes of flight. Such gills, it is maintained, resemble those found in many may-fly nymphs, and the theory implies that the ancestors of winged in sects became temporarily aquatic and thus acquired gills, which developed into wings when their possessors resorted to the land for a second time.

Relationships of the Orders of Insects.

The most general ized orders of winged insects are all included in the division Ex opterygota, but among this assemblage of forms several divergent lines of evolution are evident. The Orthoptera are the most primi tive living winged insects and were probably more or less directly derived from Thysanuran ancestors. The Dermaptera, Isoptera and Embioptera are all more closely related to the Orthoptera than to any other order of insects. The Plecoptera exhibit many primi tive features allying them with the Orthoptera, but their imma ture stages are aquatic as are those of the Odonata and Ephemero ptera. The two latter orders exhibit certain mutual relationships and are to be regarded as rather isolated developments from origi nal Orthopteran ancestors. The Hemiptera and Thysanoptera have much in common : both orders are highly specialized with piercing mouth-parts and although originally descended from in sects with jaws of the normal biting type, they have left little trace of their original lines of evolution. The Anoplura are degenerate parasites which are perhaps related through the Psocoptera with Orthopteroid ancestors. The Endopterygota are more closely re lated to each other than to any of the Exopterygota and no con necting links between the two divisions have so far been discov ered. It is true that an incipient pupal stage is present among Termites and in some of the Hemiptera, and even among certain members of the latter order the wings develop internally, but these instances appear to be rather nature's experiments towards an endopterygote condition than evidence of lines of descent of any of the true endopterygote orders. The fossil order Paramecoptera appears to be the forerunner of almost all the higher orders of in sects and the direct ancestors of the existing Mecoptera. The lat ter, along with the Neuroptera, Lepidoptera, Trichoptera and Dip tera, form an interrelated complex, which attains its highest spe cialization in the last-mentioned order. The Hymenoptera lie outside this complex but there appears to be evidence that the fos sil order Protohymenoptera is the far-off progenitor of the group, and possibly serves to connect them with a Paramecopteran stock. The Coleoptera and the related Strepsiptera exhibit little or no indications of their affinities : the campodeif orm larvae of many beetles are primitive in type and not unlike those of Neuroptera and it is probable that the order arose as an early offshoot in the endopterygote series. The Siphonoptera or fleas likewise leave little trace of their descent, but in their metamorphoses they come nearer to the Diptera than to any other order and they are ac corded a position next to the Diptera in the more generally accepted recent systems of classification.

See the articles COLEOPTERA; DIPTERA; HEMIPTERA; HYMEN OPTERA ; LEPIDOPTERA ; NEUROPTERA ; ORTHOPTERA ; PLECOPTERA ; THYSANURA ; TRICHOPTERA.

BIBLIOGRAPHY.-The literature on insects has assumed enormous Bibliography.-The literature on insects has assumed enormous proportions and at least 2,000 scientific books, memoirs and separate articles appear each year in various languages, apart from writings of a popular character.

General Text-books.—Among the best elementary manuals to be found on the subject are: J. W. Folsom, Entomology with special reference to its Ecological Aspects (1923) ; G. H. Carpenter, Insects: their Structure and Life (1924) ; W. H. Wellhouse, How Insects Live (1926) . For a larger book, D. Sharp, "Insects" in the Cambridge Natural History (vols. v., vi., 189S, 1898) is important.

General text-books of a more advanced character include: L. F. Henneguy, Les Insectes (1904) ; the great work of A. Berlese, Gli Insetti (Milan, 1909-25) ; C. Schroder, Handbuch der Entomologie (Jena, 1912-28) ; J. H. Comstock, Introduction to Entomology (192o) ; A. D. Imms, General Text-book of Entomology (1925) ; all these works contain copious bibliographies.

There are also a number of books which, although containing general