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Rh 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.

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 ).

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 .) 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 .)

—The literature of several special families of the Hymenoptera will be found under the articles, , , , &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