Hymenoptera

Hymenoptera (Gr. ὑμήν, a membrane, and πτερόν, a wing), a term used in zoological classification for one of the most important orders of the class Hexapoda (q.v.). The order was founded by Linnaeus (Systema Naturae, 1735), and is still recognized by all naturalists in the sense proposed by him, to include the saw-flies, gall-flies, ichneumon-flies and their allies, ants, wasps and bees. The relationship of the Hymenoptera to other orders of insects is discussed in the article Hexapoda, but it may be mentioned here that in structure the highest members of the order are remarkably specialized, and that in the perfection of their instincts they stand at the head of all insects and indeed of all invertebrate animals. About 30,000 species of Hymenoptera are now known.  
  
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After C. L. Marlatt, Bur. Ent. Bull. 3, N.S., U.S. Dept. Agric.  
Fig. 1.—A, Front of head of Saw-fly (Pachynematus); a, labrum; b, clypeus; c, vertex; d, d, antennal cavities. C and D, Mandibles. E, First maxilla; a, cardo; b, stipes; c, galea; d, lacinia; e, palp. B, Second maxillae (Labium); a, mentum; b, ligula (between the two galeae); c, c, palps. Magnified.  
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  | After C. Janet, Mem. Soc. Zool. France (1898).  
Fig. 2.—Jaws of Hive-bee (Apis mellifica). Magnified about 6½ times. a, mandible; b, c, palp and lacinia of first maxilla; d, e, g, h, mentum, palp, fused laciniae (ligula or “tongue”) and galea of 2nd maxillae. | Fig. 3.—Median section through mid-body of female Red Ant (Myrmica rubra). H, Head; 1, 2, 3, the thoracic segments; i., ii., the first and second abdominal segments; i., being the propodeum.  
  
Characters.—In all Hymenoptera the mandibles (fig. 1, C, D) are well developed, being adapted, as in the more lowly winged insects, such as the Orthoptera, for biting. The more generalized Hymenoptera have the second maxillae but slightly modified, their inner lobes being fused to form a ligula (fig. 1, B, b). In the higher families this structure becomes elongated (fig. 2, g) so as to form an elaborate sucking-organ or “tongue.” These insects are able, therefore, to bite as well as to suck, whereas most insects which have acquired the power of suction have lost that of biting. Both fore- and hind-wings are usually present, both pairs being membranous, the hind-wings small and not folded when at rest, each provided along the costa with a row of curved hooks which catch on to a fold along the dorsum of the adjacent fore-wing during flight. A large number of Hymenoptera are, however, entirely wingless—at least as regards one sex or form of the species. One of the most remarkable features is the close union of the foremost abdominal segment (fig. 3, i.) with the metathorax, of which it often seems to form a part, the apparent first abdominal segment being, in such case, really the second (fig. 3, ii.). The true first segment, which undergoes a more or less complete fusion with the thorax is known as the “median segment” or propodeum. In female Hymenoptera the typical insectan ovipositor with its three pairs of processes is well developed, and in the higher families this organ becomes functional as a sting (fig. 5),—used for offence and defence. As regards their life history, all Hymenoptera undergo a “complete” metamorphosis. The larva is soft-skinned (eruciform), being either a caterpillar (fig. 6, b) or a legless grub (fig. 7, a), and the pupa is free (fig. 7, c), i.e. with the appendages not fixed to the body, as is the case in the pupa of most moths.  
  
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Fig. 4.—Fore-Wings of Hymenoptera.  
  
1\. Tenthredinidae (Hylotoma)— 1, marginal; 2, appendicular; 3, 4, 5, 6, radial or submarginal; 7, 8, 9, median or discoidal; 10, sub-costal; 11, 12, cubital or branchial; and 13, anal or lanceolate cellules; a, b, c, submarginal nervures; d, basal nervures; e, f, recurrent nervures; st, stigma; co, costa.

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2\. Cynipidae (Cynips).

3\. Chalcididae (Perilampus).

4\. Proctotrypidae (Codrus).

5\. Mymaridae (Mymar).

6\. Braconidae (Bracon).

7\. Ichneumonidae (Trogus).

8\. Chrysididae (Cleptes).

9\. Formicidae (Formica).

10\. Vespidae (Vespa).

11\. Apidae (Apathus).  
  
Structure.—The head of a hymenopterous insect bears three simple eyes (ocelli) on the front and vertex in addition to the large compound eyes. The feelers are generally simple in type, rarely showing serrations or prominent appendages; but one or two basal segments are frequently differentiated to form an elongate “scape,” the remaining segments—carried at an elbowed angle to the scape—making up the “flagellum”; the segments of the flagellum often bear complex sensory organs. The general characters of the jaws have been mentioned above, and in detail there is great variation in these organs among the different families. The sucking tongue of the Hymenoptera has often been compared with the hypopharynx of other insects. According to D. Sharp, however, the hypopharynx is present in all Hymenoptera as a distinct structure at the base of the “tongue,” which must be regarded as representing the fused laciniae of the second maxillae. In the thorax the pronotum and prosternum are closely associated with the mesothorax, but the pleura of the prothorax are usually shifted far forwards, so that the fore-legs are inserted just behind the head. A pair of small plates—the tegulae—are very generally present at the bases of the fore-wings. The union of the first abdominal segment with the metathorax has been already mentioned. The second (so-called “first”) abdominal segment is often very constricted, forming the “waist” so characteristic of wasps and ants for example. The constriction of this segment and its very perfect articulation with the propodeum give great mobility to the abdomen, so that the ovipositor or sting can be used with the greatest possible accuracy and effect.

Mention has already been made of the series of curved hooks along the costa of the hind-wing; by means of this arrangement the two wings of a side are firmly joined together during flight, which thus becomes particularly accurate. The wings in the Hymenoptera show a marked reduction in the number of nervures as compared with more primitive insects. The main median nervure, and usually also the sub-costal become united with the radial, while the branches of radial, median and cubital nervures pursuing a transverse or recurrent course across the wing, divide its area into a number of areolets or “cells,” that are of importance in classification. Among many of the smaller Hymenoptera we find that the wings are almost destitute of nervures. In the hind-wings—on account of their reduced size—the nervures are even more reduced than in the fore-wings.

The legs of Hymenoptera are of the typical insectan form, and the foot is usually composed of five segments. In many families the trochanter appears to be represented by two small segments, there being thus an extra joint in the leg. It is almost certain that the distal of these two segments really belongs to the thigh, but the ordinary nomenclature will be used in the present article, as this character is of great importance in discriminating families, and the two segments in question are referred to the trochanter by most systematic writers.  
  
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After C. Janet, Aiguillon de la Myrmica rubra (Paris, 1898).  
Fig. 5.—Ovipositor or Sting of Red Ant (Myrmica rubra) Queen. Magnified. The right sheath C (outer process of the ninth abdominal segment—9) is shown in connexion with the guide B formed by the inner processes of the 9th segment. The stylet A (process of the 8th abdominal segment—8) is turned over to show its groove a, which works along the tongue or rail b.  
  
The typical insectan ovipositor, so well developed among the Hymenoptera, consists of three pairs of processes (gonapophyses) two of which belong to the ninth abdominal segment and one to the eighth. The latter are the cutting or piercing stylets (fig. 5, A) of the ovipositor, while the two outer processes of the ninth segment are modified into sheaths or feelers (fig. 5, C) and the two inner processes form a guide (fig. 5, B) on which the stylets work, tongues or rails on the “guide” fitting accurately into longitudinal grooves on the stylet. In the different families of the Hymenoptera, there are various modifications of the ovipositor, in accord with the habits of the insects and the purposes to which the organ is put. The sting of wasps, ants and bees is a modified ovipositor and is used for egg-laying by the fertile females, as well as for defence. Most male Hymenoptera have processes which form claspers or genital armature. These processes are not altogether homologous with those of the ovipositor, being formed by inner and outer lobes of a pair of structures on the ninth abdominal segment.

Many points of interest are to be noted in the internal structure of the Hymenoptera. The gullet leads into a moderate-sized crop, and several pairs of salivary glands open into the mouth. The crop is followed by a proventriculus which, in the higher Hymenoptera, forms the so-called “honey stomach,” by the contraction of whose wails the solid and liquid food can be separated, passed on into the digestive stomach, or held in the crop ready for regurgitation into the mouth. Behind the digestive stomach are situated, as usual, intestine and rectum, and the number of kidney (Malpighian) tubes varies from only six to over a hundred, being usually great.

In the female, each ovary consists of a large number of ovarian tubes, in which swollen chambers containing the egg-cells alternate with smaller chambers enclosing nutrient material. In connexion with the ovipositor are two poison-glands, one acid and the other alkaline in its secretion. The acid gland consists of one, two or more tubes, with a cellular coat of several layers, opening into a reservoir whence the duct leads to the exterior. The alkaline gland is an irregular tube with a single cellular layer, its duct opening alongside that of the acid reservoir. These glands are most strongly developed when the ovipositor is modified into a sting.

Development.—Parthenogenesis is of normal occurrence in the life-cycle of many Hymenoptera. There are species of gall-fly in which males are unknown, the unfertilized eggs always developing into females. On the other hand, in certain saw-flies and among the higher families, the unfertilized eggs, capable of development, usually give rise to male insects (see Bee). The larvae of most saw-flies feeding on the leaves of plants are caterpillars (fig. 6, b) with numerous abdominal pro-legs, but in most families of Hymenoptera the egg is laid in such a situation that an abundant food-supply is assured without exertion on the part of the larva, which is consequently a legless grub, usually white in colour, and with soft flexible cuticle (fig. 7, a). The organs and instincts for egg-laying and food-providing are perhaps the most remarkable features in the economy of the Hymenoptera. Gall-fly grubs are provided with vegetable food through the eggs being laid by the mother insect within plant tissues. The ichneumon pierces the body of a caterpillar and lays her eggs where the grubs will find abundant animal food. A digging-wasp hunts for insect prey and buries it with the egg, while a true wasp feeds her brood with captured insects, as a bird her fledglings. Bees store honey and pollen to serve as food for their young. Thus we find throughout the order a degree of care for offspring unreached by other insects, and this family-life has, in the best known of the Hymenoptera—ants, wasps and bees—developed into an elaborate social organization.

Social Life.—The development of a true insect society among the Hymenoptera is dependent on a differentiation among the females between individuals with well-developed ovaries (“queens”) whose special function is reproduction; and individuals with reduced or aborted ovaries (“workers”) whose duty is to build the nest, to gather food and to tend and feed the larvae. Among the wasps the workers may only differ from the queens in size, and individuals intermediate between the two forms of female may be met with. Further, the queen wasp, and also the queen humble-bee, commences unaided the work of building and founding a new nest, being afterwards helped by her daughters (the workers) when these have been developed. In the hive-bee and among ants, on the other hand, there are constant structural distinctions between queen and worker, and the function of the queen bee in a hive is confined to egg-laying, the labour of the community being entirely done by the workers. Many ants possess several different forms of worker, adapted for special duties. Details of this fascinating subject are given in the special articles Ant, Bee and Wasp (q.v.).

Habits and Distribution.—Reference has been already made to the various methods of feeding practised by Hymenoptera in the larval stage, and the care taken of or for the young throughout the order leads in many cases to the gathering of such food by the mother or nurse. Thus, wasps catch flies; worker ants make raids and carry off weak insects of many kinds; bees gather nectar from flowers and transform it into honey within their stomachs—largely for the sake of feeding the larvae in the nest. The feeding habits of the adult may agree with that of the larva, or differ, as in the ease of wasps which feed their grubs on flies, but eat principally vegetable food themselves. The nest-building habit is similarly variable. Digging wasps make simple holes in the ground; many burrowing bees form branching tunnels; other bees excavate timber or make their brood-chambers in hollow plant-stems; wasps work up with their saliva vegetable fibres bitten off tree-bark to make paper; social bees produce from glands in their own bodies the wax whence their nest-chambers are built. The inquiline habit (“cuckoo-parasitism”), when one species makes use of the labour of another by invading the nest and laying her eggs there, is of frequent occurrence among Hymenoptera; and in some cases the larva of the intruder is not content with taking the store of food provided, but attacks and devours the larva of the host.

Most Hymenoptera are of moderate or small size, the giants of the order—certain saw-flies and tropical digging-wasps—never reach the bulk attained by the largest beetles, while the wing-spread is narrow compared with that of many dragon-flies and moths. On the other hand, there are thousands of very small species, and the tiny “fairy-flies” (Mymaridae), whose larvae live as parasites in the eggs of various insects, are excessively minute for creatures of such complex organization. Hymenoptera are probably less widely distributed than Aptera, Coleoptera or Diptera, but they are to be found in all except the most inhospitable regions of the globe. The order is, with few exceptions, terrestrial or aerial in habit. Comparatively only a few species are, for part of their lives, denizens of fresh water; these, as larvae, are parasitic on the eggs or larvae of other aquatic insects, the little hymenopteron, Polynema natans, one of the “fairy-flies”—swims through the water by strokes of her delicate wings in search of a dragon-fly’s egg in which to lay her own egg, while the rare Agriotypus dives after the case of a caddis-worm. It is of interest that the waters have been invaded by the parasitic group of the Hymenoptera, since in number of species this is by far the largest of the order. No group of terrestrial insects escapes their attacks—even larvae boring in wood are detected by ichneumon flies with excessively long ovipositors. Not a few cases are known in which a parasitic larva is itself pierced by the ovipositor of a “hyperparasite,” and even the offspring of the latter may itself fall a victim to the attack of a “tertiary parasite.”

Fossil History.—Very little is known of the history of the Hymenoptera previous to the Tertiary epoch, early in which, as we know from the evidence of many Oligocene and Miocene fossils, all the more important families had been differentiated. Fragments of wings from the Lias and Oolitic beds have been referred to ants and bees, but the true nature of these remains is doubtful.

Classification.—Linnaeus divided the Hymenoptera into two sections—the Terebrantia, whose females possess a cutting or piercing ovipositor, and the Aculeata, in which the female organ is modified into a sting. This nomenclature was adopted by P. A. Latreille and has been in general use until the present day. A closely similar division of the order results from T. Hartig’s character drawn from the trochanter—whether of two segments or undivided—the groups being termed respectively Ditrocha and Monotrocha. But the most natural division is obtained by the separation of the saw-flies as a primitive sub-order, characterized by the imperfect union of the first abdominal segment with the thorax, and by the broad base of the abdomen, so that there is no median constriction or “waist,” and by the presence of thoracic legs—usually also of abdominal pro-legs—in the larva. All the other families of Hymenoptera, including the gall-flies, ichneumons and aculeates, have the first abdominal segment closely united with the thorax, the second abdominal segment constricted so as to form a narrow stalk or “waist,” and legless larvae without a hinder outlet to the food-canal. These two sub-orders are usually known as the Sessiliventra and Petioliventra respectively, but the names Symphyta and Apocrita proposed in 1867 by C. Gerstaecker have priority, and should not be replaced.

Symphyta.

This sub-order, characterized by the “sessile,” broad-based abdomen, whose first segment is imperfectly united with the thorax, and by the usually caterpillar-like larvae with legs, includes the various groups of saw-flies. Three leading families may be mentioned. The Cephidae, or stem saw-flies, have an elongate pronotum, a compressed abdomen, and a single spine on the shin of the fore-leg. The soft, white larvae have the thoracic legs very small and feed in the stems of various plants. Cephus pygmaeus is a well-known enemy of corn crops. The Siricidae (“wood-wasps”) are large elongate insects also with one spine on each fore-shin, but with the pronotum closely joined to the mesothorax. The ovipositor is long and prominent, enabling the female insect to lay her eggs in the wood of trees, where the white larvae, whose legs are excessively short, tunnel and feed. These insects are adorned with bands of black and yellow, or with bright metallic colours, and on account of their large size and formidable ovipositors they often cause needless alarm to persons unfamiliar with their habits. The Tenthredinidae, or true saw-flies, are distinguished by two spines on each fore-shin, while the larvae are usually caterpillars, with three pairs of thoracic legs, and from six to eight pairs of abdominal pro-legs the latter not possessing the hooks found on the pro-legs of lepidopterous caterpillars. Most saw-fly larvae devour leaves, and the beautifully serrate processes of the ovipositor are well adapted for egg-laying in plant tissues. Some saw-fly larvae are protected by a slimy secretion (fig. 6, c) and a few live concealed in galls. In the form of the feelers, the wing-neuration and minor structural details there is much diversity among the saw-flies. They have been usually regarded as a single family, but W. H. Ashmead has lately differentiated eleven families of them.

Apocrita.

This sub-order includes the vast majority of the Hymenoptera, characterized by the narrowly constricted waist in the adult and by the legless condition of the larva. The trochanter is simple in some genera and divided in others. With regard to the minor divisions of this group, great difference of opinion has prevailed among students. In his recent classification Ashmead (1901) recognizes seventy-nine families arranged under eight “super-families.” The number of species included in this division is enormous, and the multiplication of families is, to some extent, a natural result of increasingly close study. But the distinctions between many of these rest on comparatively slight characters, and it is likely that the future discovery of new genera may abolish many among such distinctions as may now be drawn. It seems advisable, therefore, in the present article to retain the wider conception of the family that has hitherto contented most writers on the Hymenoptera. Ashmead’s “super-families” have, however, been adopted as—founded on definite structural characters—they probably indicate relationship more nearly than the older divisions founded mostly on habit. The Cynipoidea include the gall-flies and their parasitic relations. In the Chalcidoidea, Ichneumonoidea and Proctotrypoidea will be found nearly all the “parasitic Hymenoptera” of older classifications. The Formicoidea are the ants. The group of Fossores, or “digging-wasps,” is divided by Ashmead, one section forming the Sphecoidea, while the other, together with the Chrysidae and the true wasps, make up the Vespoidea. The Apoidea consists of the bees only.  
  
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After Marlatt, Ent. Circ. 26, U.S. Dept. Agric.  
Fig. 6.—a, Pear Saw-fly (Eriocampoides limacina); b, larva without, and c, with its slimy protective coat; e, cocoon; f, larva before pupation; g, pupa, magnified; d, leaves with larvae.  
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After Howard, Ent. Tech. Bull. 5 U.S. Dept. Agric.  
Fig. 7.—Chalcid (Dibrachys boucheanus), a hyper-parasite.  
  
a, Larva.

d, Its head more highly magnified.

b, Female fly.

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c, Pupa of male.

e, Feeler.  
  
Cynipoidea.—In this division the ovipositor issues from the ventral surface of the abdomen; the pronotum reaches back to the tegulae; the trochanter has two segments; the fore-wing (fig. 4, 2) has no stigma, but one or two areolets. The feelers with twelve to fifteen segments are thread-like and straight. All the insects included in this group are small and form two families—the Cynipidae and the Figitidae. They are the “gall-flies,” many of the species laying eggs in various plant-tissues where the presence of the larva causes the formation of a pathological growth or gall, always of a definite form and characteristic of the species; the “oak-apple” and the bedeguar of the rose are familiar examples. Other flies of this group have the inquiline habit, laying their eggs in the galls of other species, while others again pierce the cuticle of maggots or aphids, in whose bodies their larvae live as parasites.

Chalcidoidea.—This division resembles the Cynipoidea in the position of the ovipositor, and in the two segmented trochanters. The fore-wing also has no stigma, and the whole wing is almost destitute of nervures and areolets, while the pronotum does not reach back to the tegulae, and the feelers are elbowed (fig. 7). The vast majority of this group, including nearly 5000 known species, are usually reckoned as a single family, the Chalcididae, comprising small insects, often of bright metallic colours, whose larvae are parasitic in insects of various orders. The “fig-insects,” whose presence in ripening figs is believed essential to the proper development of the fruit, belong to Blastophaga and other genera of this family. They are remarkable in having wingless males and winged females. The “polyembryonic” development of an Encyrtus, as studied by P. Marchal, is highly remarkable. The female lays her egg in the egg of a small ermine moth (Hyponomeuta) and the egg gives rise not to a single embryo but to a hundred, which develop as the host-caterpillar develops, being found at a later stage within the latter enveloped in a flexible tube.

The Mymaridae or “fairy-flies” are distinguished from the Chalcididae by their narrow fringed wings (figs. 4, 5) and by the situation of the ovipositor just in front of the tip of the abdomen. They are among the most minute of all insects and their larvae are probably all parasitic in insects’ eggs.  
  
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After Riley and Howard, Insect Life, vol. i.  
Fig. 8.—Ichneumon Fly (Rhyssa per-suasoria) ovipositing.  
  
Ichneumonoidea.—The ten thousand known species included in this group agree with the Cynipoidea and Chalcidoidea in the position of the ovipositor and in the jointed trochanters, but are distinguished by the fore-wing possessing a distinct stigma and usually a typical series of nervures and areolets (figs. 4, 8). Many of the species are of fair size. They lay their eggs (fig. 8) in the bodies of insects and their larvae belonging to various orders. A few small families such as the Evaniidae and the Stephanidae are included here, but the vast majority of the group fall into two large families, the Ichneumonidae and the Braconidae, the former distinguished by the presence of two median (or discoidal) cells in the fore-wing (figs. 4, 7), while the latter has only one (figs. 4, 6). Not a few of these insects, however, are entirely wingless. On account of their work in destroying plant-eating insects, the ichneumon-flies are of great economic importance.

Proctotrypoidea.—This group may be distinguished from the preceding by the position of the ovipositor at the extreme apex of the abdomen, and from the groups that follow (with very few exceptions) by the jointed trochanters of the legs. The pronotum reaches back to the tegulae. The Pelecinidae—included here by Ashmead—are large insects with remarkably elongate abdomens and undivided trochanters. All the other members of the group may be regarded as forming a single family—the Proctotrypidae, including an immense number of small parasitic Hymenoptera, not a few of which are wingless. Of special interest are the transformations of Platygaster, belonging to this family, discovered by M. Ganin, and familiarized to English readers through the writings of Sir J. Lubbock (Lord Avebury). The first larva is broad in front and tapers behind to a “tail” provided with two divergent processes, so that it resembles a small crustacean. It lives in the grub of a gall-midge and it ultimately becomes changed into the usual white and fleshy hymenopterous larva. The four succeeding sections, in which the ovipositor is modified into a sting (always exserted from the tip of the abdomen) and the trochanters are with few exceptions simple, form the Aculeata of Linnaeus.

Formicoidea.—The ants which form this group are readily distinguished by the differentiation of the females into winged “queens” and wingless “workers.” The pronotum extends back to the wing-bases, and the “waist” is greatly constricted and marked by one or two “nodes.” The differentiation of the females leads to a complex social life, the nesting habits of ants and the various industries that they pursue being of surpassing interest (see Ant).

Vespoidea.—This section includes a number of families characterized by the backward extension of the prothorax to the tegulae and distinguished from the ants by the absence of “nodes” at the base of the abdomen. The true wasps have the fore-wings folded lengthwise when at rest and the fore-legs of normal build—not specialized for digging. The Vespidae or social wasps have “queens” and “workers” like the ants, but both these forms of female are winged; the claws on their fret are simple. In the Eumenidae or solitary wasps the female sex is undifferentiated, and the foot claws are toothed. (For the habits of these insects see Wasp.) The Chrysididae or ruby wasps are small insects with a very hard cuticle exhibiting brilliant metallic colours—blue, green and crimson. Only three or four abdominal segments are visible, the hinder segments being slender and retracted to form a telescope-like tube in which the ovipositor lies. When the ovipositor is brought into use this tube is thrust out. The eggs are laid in the nests of various bees and wasps, the chrysid larva living as a “cuckoo” parasite. The Trigonalidae, a small family whose larvae are parasitic in wasps’ nests, also probably belong here.

The other families of the Vespoidea belong to the series of “Fossores” or digging-wasps. In two of the families—the Mutillidae and Thynnidae—the females are wingless and the larvae live as parasites in the larvae of other insects; the female Mutilla enters bumble-bees’ nests and lays her eggs in the bee-grubs. In the other families both sexes are winged, and the instinct and industry of the females are among the most wonderful in the Hymenoptera. They make burrows wherein they place insects or spiders which they have caught and stung, laying their eggs beside the victim so that the young larvae find themselves in presence of an abundant and appropriate food-supply. Valuable observations on the habits of these insects are due to J. H. Fabre and G. W. and E. Peckham. The prey is sometimes stung in the neighbourhood of the nerve ganglia, so that it is paralysed but not killed, the grub of the fossorial wasp devouring its victim alive; but this instinct varies in perfection, and in many cases the larva flourishes equally whether its prey be killed or not. The females have a wonderful power of finding their burrows on returning from their hunting expeditions. Among the Vespoid families of fossorial wasps, the Pompilidae are the most important. They are recognizable by their slender and elongate hind-legs; many of them provision their burrows with spiders. The Sapygidae are parasitic on bees, while the Scoliidae are large, robust and hairy insects, many of which prey upon the grubs of chafers.

Sphecoidea.—In this division are included the rest of the “digging-wasps,” distinguished from the Vespoidea by the short pronotum not reaching backward to the tegulae. They have usually been reckoned as forming a single, very large family—the Sphegidae—but ten or twelve subdivisions of the group are regarded as distinct families by Ashmead and others. Great diversity is shown in the details of structure, habits and nature of the prey. Species of Sphex, studied by Fabre, provisioned their brood-chambers with crickets. Pelopoeus hunts spiders, while Ammophila catches caterpillars for the benefit of her young. Fabre states that the last-named insect uses a stone for the temporary closing of her burrow, and the Peckhams have seen a female Ammophila take a stone between her mandibles and use it as a hammer for pounding down the earth over her finished nest. The habits of Bembex are of especial interest. The female, instead of provisioning her burrow with a supply of food that will suffice the larva for its whole life, brings fresh flies with which she regularly feeds her young. In this instinct we have a correspondence with the habits of social wasps and bees. Yet it may be thought that the usual instinct of the “digging-wasps” to capture and store up food in an underground burrow for the benefit of offspring which they will never see is even more surprising. The habit of some genera is to catch the prey before making their tunnel, but more frequently the insect digs her nest, and then hunts for prey to put into it.

Apoidea.—The bees which make up this group agree with the Sphecoidea in the short pronotum, but may be distinguished from all other Hymenoptera by the widened first tarsal segment and the plumose hairs on head and body. They are usually regarded as forming a single family—the Apidae—but there is very great diversity in structural details, and Ashmead divides them into fourteen families. The “tongue,” for example, is short and obtuse or emarginate in Colletes and Prosopis, while in all other bees it is pointed at the tip. But in Andrena and its allies it is comparatively short, while in the higher genera, such as Apis and Bombus, it is elongate and flexible, forming a most elaborate and perfect organ for taking liquid food. Bees feed on honey and pollen. Most of the genera are “solitary” in habit, the female sex being undifferentiated; but among the humble-bees and hive-bees we find, as in social wasps and ants, the occurrence of workers, and the consequent elaboration of a wonderful insect-society. (See Bee.)

Bibliography.—The literature of several special families of the Hymenoptera will be found under the articles Ant, Bee, Ichneumon-Fly, Wasp, &c., referred to above. Among earlier students on structure may be mentioned P. A. Latreille, Familles naturelles du règne animal (Paris, 1825), who recognized the nature of the “median segment.” C. Gerstaecker (Arch. f. Naturg. xx., 1867) and F. Brauer (Sitzb. K. Akad. Wiss. Wien. lxxxv., 1883) should also be consulted on this subject. For internal anatomy, specially the digestive organs, see L. Dufour, Mém. savants étrangers, vii. (1841), and Ann. Sci. Nat. Zool. (4), i. 1854. For nervous system H. Viallanes, Ann. Sci. Nat. Zool. (7), ii. iv. 1886-1887, and F. C. Kenyon, Journ. Comp. Neurol. vi., 1896. For poison and other glands, see L. Bordas, Ann. Sci. Nat. Zool. (7) xix., 1895. For the sting and ovipositor H. Dewitz, Zeits. wiss. Zool. xxv., 1874, xxviii., 1877, and F. Zander, ib. lxvi., 1899. For male genital armature S. A. Peytoureau, Morphologie de l’armure génitale des insectes (Bordeaux, 1895), and E. Zander, Zeits. wiss. Zool. lxvii., 1900\. The systematic student of Hymenoptera is greatly helped by C. G. de Dalla Torre’s Catalogus Hymenopterorum (10 vols., Leipzig, 1893-1902). For general classifications see F. W. Konow, Entom. Nachtr. (1897), and W. H. Ashmead, Proc. U.S. Nat. Mus. xxiii., 1901; the latter paper deals also especially with the Ichneumonoidea of the globe. For habits and life histories of Hymenoptera see J. Lubbock (Lord Avebury), Ants, Bees and Wasps (9th ed., London, 1889); C. Janet, Études sur les fourmis, les guêpes et les abeilles (Paris, &c., 1893 and onwards); and G. W. and E. G. Peckham, Instincts and Habits of Solitary Wasps (Madison, Wis. U.S.A., 1898). Monographs of most of the families of British Hymenoptera have now been published. For saw-flies and gall-flies, see P. Cameron’s British Phytophagous Hymenoptera (4 vols., London, Roy. Soc., 1882-1893). For Ichneumonoidea, C. Morley’s Ichneumons of Great Britain (Plymouth, 1903, &c.), and T. A. Marshall’s “British Braconidae,” Trans. Entom. Soc., 1885-1899. The smaller parasitic Hymenoptera have been neglected in this country since A. H. Haliday’s classical papers Entom. Mag. i.-v., (1833-1838) but Ashmead’s “North American Proctotrypidae” (Bull. U.S. Nat. Mus. xlv., 1893) is valuable for the European student. For the Fossores, wasps, ants and bees see E. Saunders, Hymenoptera Aculeata of the British Islands (London, 1896). Exhaustive references to general systematic works will be found in de Dalla Torre’s Catalogue mentioned above. Of special value to English students are C. T. Bingham’s Fauna of British India, “Hymenoptera” (London, 1897 and onwards), and P. Cameron’s volumes on Hymenoptera in the Biologia Centrali-Americana. F. Smith’s Catalogues of Hymenoptera in the British Museum (London, 1853-1859) are well worthy of study.

(G. H. C.)

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