The wings have few nervures, and may be even entirely devoid of them; when present they proceed from the base of the wing, or from the costa (the part which bounds the wing in front) towards the apex, which they may or may not reach. Connected with the lateral nervures are others (recurrent nervures) which unite them together, and form in this way regular cellules. The first two lateral nervures (those nearest the top) are the most important, and are called the “marginal” and “submarginal” respectively. The cellules which they form by means of the cross or recurrent nervures are called the “marginal” and “submarginal cellules.” Not unfrequently there is, towards the apical third of the costa, a thickened spot (considered by some to act as a counterpoise when the insect is flying) termed the “stigma” or “pterostigma.” It is not always present, however, and when present may be very small, or, as in Pachylosticα, Nexγ large and projecting. In a similar way the hind wing may contain lateral and cross nervures, but they are fewer in number and in importance, and may bè entirely absent, although present in the larger pair. The arrangement of the nervures in the Hymenoptera undergoes, in the various families, great diversity; and, what is of great importance in classification, their form, in the different families, and even genera, is, within certain limits, remarkably constant. On this account great attention has been paid to them; and each nervure and cellule has received a distinct name. The use of the nervures and cellules does not lead to the formation of artificial groups; for we find that the existence of a particular arrangement of the nervures in a hymenopterous insect denotes the presence of other characters. Each family indeed has its own form of wing, as will be seen from the accompanying figures. The relative value, however, of the neuration in classification is not always the same. It is of much greater importance, for instance, with saw-flies than with bees. The wings are usually shorter than the body, and may be so short as to be useless for flight; they may be even entirely absent. Apterous species are found in almost all the families. In most cases it is the females only which are thus deprived of the power of flight. But the opposite of this may exist; as, for instance, with a curious species of Chalcididoe which lives in'the nest of bees as a parasite. With ants, again, the neuters are always wingless; and the females lose their wings when they commence the formation of a colony. It is worthy of remark, too, that some Chαlci∙ didce and Oxyura, which are normally winged, occasionally appear in an apterous or semi-apterous condition. On the anterior margin of the hind wings are placed a number of very minute hooks, which fit into a thickened rim on the posterior margin of the front wings, so that the two become united, and strike the air as one whole. The wings are usually transparent, and, in certain arrangements of the light, are seen to be highly iridescent. Occasionally they are coloured in patches, or are entirely black or blue; in the latter case, they are not unfrequently of a thickish texture and have a metallic lustre, as, e.g., in Hylotoma. The smaller forms (Chalciclidoe and Oxyurd) have few or no nervures. Some of these have the wings deeply fringed; and Mymar anàFlabrinus have the wings as it were cleft and stalked; that is to say, there is a long, thin stalk projecting from the thorax for a certain distance; then it dilates into a number of deeply fringed branches (see fig. 5).

The head is seldom broader than the thorax. It is usually of a more or less globular shape, but may be much flattened and long. The compound eyes are placed along the sides, while the simple eyes, or ocelli, are arranged in a triangle on the vertex. Certain ants and Chalcididoe are blind. Others have eyes, but want the ocelli. The mouth organs are, except with bees, adapted solely for mastication or for prehension; while, in addition to performing these functions, wasps use them for building up or digging out their nests in the ground or wood. Some worker ants and other aculeates (especially males) have the mandibles enormously developed. Dlastophaga, an insect found in figs, has a curious mandible, remarkable, not in itself, but by having attached to it an appendage whose use is supposed to be to clean away the juice of the fig from the mouth. The maxilla and labium are provided with jointed palpi.

The number of joints in the maxillary palpi is usually five, but may be more or less than that; the labial palpi are two-to four-jointed. It is with bees that the mouth parts (and especially the ligula) have their greatest development and specialization. With them the various parts (except the mandibles) are elongated to form a sucking tube, by means of which they lap up the nectar of flowers.

The form of the antennae is very variable. They may have three (Hylotoma) to sixty joints (ichneumons); may be very long and thin as in ichneumons, or scarcely projecting beyond the head as in Perga ; of uniform thickness, or distinctly clavate as in Cimbeχ∙, bare, or very hairy. Then they often differ very much in the males. Lophyrus (Tenthre- dinidoe)oxLd many Chalcididoe have them pectinated in various degrees; they are deeply forked in Schizoceτa ; and many others have them covered with long hair, although they are quite bare in the other sex. In some families the number of joints varies very much, but most Aculeata have the same number, namely, 13 in the male and 12 in the female. These appendages serve as sense organs, especially for the discovery of food, and, in the male, for finding the female. At any rate an ichneumon (for instance) when searching for a larva in which to lay its eggs (and this has especially been noticed with those which oviposit in concealed larvae) keeps them in a state of continuous trembling motion, and males have also been observed to do the same when searching for the females.

The three divisions of the thorax—pro-, meso-, and metathorax—have pretty much the same relative proportions in all the families. The prothorax is small. The upper part is strongly articulated to the mesothorax, while the lower is freer; and it is by means of this lower part that the head is united to the thorax. The metathorax is very large, as might be expected, from its having to take such an important part in flight. As for the scutellum it varies more than any other portion of the thorax. Mostly it is flat, or at least only slightly raised above the mesonotum; but in the smaller groups (Oxyura, Chalcididoe, Cynipidoe) its form is sometimes very curious. In Aegilips (Cynipidoe) and Agriotypes (Ichneumonidoe) it is produced into a sharp more or less curved spine; in Eucoela it is cup-shaped, that is to say, it is raised up, and has the centre hollow. It undergoes, however, its greatest development with some Chalcididoe. In Chirocerus, for example, it is lengthened so much that it reaches the middle of the abdomen. The metathorax is never very large, nor does it exhibit any marked peculiarities.

The legs show the same manifold diversity in form that we found to exist with the other appendages. The basal parts—thecoxae and trochanters—are not usually of any size, except in the lower tribes, e.g., Chalcididoe. In classification the trochanters are of value, for we find with the Aculeata they are joined to the femora by a single joint, whereas in all the other Hymenoptera there are two joints. Hence the bees, wasps, &c., form the division Moηotrocha; and the ichneumons, saw-flies, &c., are denominated Ditrocha. Some Chalcididoe have the femora greatly thickened, and toothed on the under side j but as a rule it does not show any striking peculiarity unless it be with some male insects. As might be expected from their more intimate connexion with the habits of the insects, the tibiae and tarsi vary according to the uses they are put to, apart from locomotion. With bees (at least with the non-parasitie species) they are employed to carry the pollen necessary for the nourishment of the young; for this purpose they are provided with hairs, or the apex of the tibiae and base of the tarsi are flattened out to form a plate on which the pollen is stored for its easier and more economical conveyance to the hive. They are richly spined in the sand-wasps, and are thus useful to the animals in digging out the nests which they form in the earth. At the base of the tibiae are placed two spurs (calcaria), which, however, may be absent. Finally the tarsi are provided with plate-like processes termed patellae, which may be very largely developed, especially with males; and the front pair may terminate in large jointed raptorial claws, as is the case with Chelogynus {f)xyura).

The abdomen may be united to the metathorax in two ways: it may be joined to it by its entire width, or by a narrow pedicle only. In the former case the thorax and abdomen form as it were one whole, so that the body has the appearance of being composed of only two parts; whereas in the other section the three divisions, head, thorax, and abdomen, are clearly separated, or specialized. It is only the Siricidae and Tenthredinidae which have the abdomen united by the whole width. Thus the order becomes divided into two well-marked divisions, one represented by the Tenthredinidae (having the abdomen sessile) and the other by the Aculeata, Ichneumonidoe (having it appendiculated). The abdomen shows little variation in structure or form in the Tenthredinidae ; but does so to a large extent with the others. Usually more or less globular and rounded, and not of any great length compared to the head and thorax, it is often very much compressed and sabre-like, as in Ophion, or very long and thread-like as in Pelecinus∙, then it may be joined to the thorax in such a way as to be semi-sessile, or by a very long, thin, threadlike pedicle {Peloρosus, many Chalcididae, and Cynipidae). Between these two extremes there are all gradations, the form of the abdomen depending on the habits of the insect, upon its manner of self-defence or of oviposition. As regards the number of the segments, it varies. It is 8 with the Tenthredinidae and with the Siricidae, but in other groups it is less. Through one or other of the basal segments being greatly developed, and the terminal ones correspondingly reduced in size, some groups appear to have only 3 or 4 {Chrysididae). The Aculeata have always the same number, namely, 7 in the males and 6 in the females. Some bees {Coelioxys) and Chrysididae have the abdomen armed with spines or teeth at the apex.

The male and female organs are situated at the end of the abdomen. They are rarely conspicuous externally with the males; but in some females they are of great length, and may indeed exceed considerably the length of the body. However different in the various groups of Hymenoptera the organs connected with the laying of the eggs may be, they are fundamentally constructed on the same plan in all of them, no matter how different they may appear or how various the ways in which they may be used. A typical ovipositor in the Hymenoptera may be described as being composed of three bristle-like organs,—one placed above, and the others below. This upper bristle is channelled throughout, and has (when in use) the others pressed to it in such a way that the three together form a narrow tube, through which the egg passes. The two lower bristles are toothed at the lower end. These three parts are enclosed between a couple of two-jointed valves, situated at their base, which serve them as supports. Thus the ovipositor consists of five different parts. It exists under two forms. In the bees and wasps {Aculeata) it takes the form of a sting, or weapon of defence; and it is connected with a gland secreting a poison (the principal constituent being formic acid) which the insect injects by means of the sting into any thing that attacks it. Besides this defensive (or offensive) use to which it is put, it is employed by the sand-wasps to benumb the larvae and other insects or spiders with which they store their nests for the use of their young, in such a way that they remain to all intents and purposes lifeless, yet still keeping fresh, until such time as the wasp-larvae escape from the eggs and are ready to feed on them. In the rest of the order—with ichneumons, saw-flies, <kc.— it is not used as an instrument of defence (some ichneumons, indeed, will attempt to pierce the hand with it when caught, but they are never able to do any harm); it is simply an instrument for laying the eggs, and is not connected with a poison gland, or at any rate with a gland secreting a poison similar to that of a wasp. A poison gland exists, for instance, with saw-flies, but its purpose and also the manner of its use are different from what they are in the case of the aculeates. Its function is to act on the plant in which the eggs are laid,—either to raise galls in which the larvae will find food and shelter, or to prevent the hole made by the ovipositor for the reception of the egg from closing in on the egg, and thereby crushing it; for we find that eggs laid, for example, on leaves are not closely pressed by the substance of the leaf, but have a more or less open space surrounding them.

From the observations of Kraepelin on the development of the ovipositor, it seems clear that the grooved central bristle and the two basal sheaths arise in the larva from papillae situated on the under side of the ninth abdominal segment, while the two lateral (or rather lower) bristles have their origin in similar papillae on the eighth. It would appear also highly probable that the parts are true appendages of the abdominal segments, rather than modified portions of the body walls.

The ovipositor is either hidden (as is mostly the case) or may be exserted to a greater or less extent. Its length varies with the habits of the species; that is to say, the longer it is, the deeper, in wood, or in any other substance, does the larva, on which the ichneumon is a parasite, live. Species with long ovipositors occur in all the parasitic families, except the Oxyura and Chrysididae. With the last-mentioned family it is tubular. It is strangely modified with the saw-flies. With them it forms a veritable sawing apparatus (hence the name of these insects), being broad, plate-like, and toothed in various ways and degrees according to the habits of the insects; according as the eggs are laid in leaves or in bark it is slender and thin or broad and thick. In the pupa state Hymenoptera with long ovipositors have them curled up on, and closely pressed to, the back. It is a curious circumstance that this embryonic condition of the ovipositor is retained in the perfect state by a few forms, as, e.g., Leucaspis.

As remarked at the beginning of this article, the Hymenoptera go throtigh a regular metamorphosis—appearing in four distinct forms. The egg is generally longer than it is broad, and rounded at both ends. The skin is always thin, never sculptured, and rarely coloured; the only instance of colour in any of their eggs known to the writer being in those of certain saw-flies which are more or less greenish, and this colour may have been imbibed from the leaf. Many parasitic species (Ophion, Cynips, Myrnar) have pedunculated eggs,—eggs provided with a long pedicle or stalk, by means of which they are attached to the plant or insect, as the case may be. A Τryphon, for example, attaches her stalked eggs to the larva’s skin; it hangs by the stalk; and when the young tryphon-larva’s development in the egg is matured it leaves it by the lower end, and then proceeds to bore its way into the inside of its victim. A curious phenomenon has been observed to take place in hymenopterous eggs shortly after being laid; it is that they swell up (perhaps by imbibing moisture, although this cannot be the sole cause of the swelling) to double or more than double the size they were when laid. Trie number of eggs laid by a female varies of course with the species. Colonial species lay the greatest number; but with them the eggs are not laid all at once, as is the case with solitary species. Some social forms lay comparatively few eggs, while, on the other hand, many saw-flies and ichneumons must lay some hundreds. Solitary bees and wasps do not lay many; but it must be remembered that the storing of food and the building and digging of the nest are workf of labour and time.

The larvae are of two sorts. Those of species with a petiolated. abdomen are white, footless grubs, incapable of any extended motion; nor is this necessary, for they have not to seek their food, Which is provided for them by their mothers,—either collected and stored up for them in nests made by the female, as with bees and wasps, or by the eggs being placed in the bodies of other insects on which they live as parasites, or in galls upheaved on plants. It is, however, of interest to note that, While the larva after leaving the egg shows no trace of legs, yet they were present when it was in the egg. This shows clearly that the legs were lost through disuse. And this view is confirmed by the fact that the larvae of the lowest division of the Hymenoptera, those with a sessile abdomen, have jointed thoracic legs, and often abdominal legs as well, while they, for the most part, lead a free existence. In the lowest group of this division, the Siriddae, as well as in those Tenthredinidae most nearly allied to them, the larvae have only the thoracic or true legs. They live either boring in wood, as Sirex, or in stems of plants, as Ceρhus, or in leaves rolled together by silken threads, as Lyda. Except Lyda, Cephus, and Xyela, all the saw-fly larvae have the usual thoracic and a variable number of false or abdominal legs, which are in fact merely prolongations of the ventral surface of the body, as with many phytophagous Coleoptera ; nor have they anything like the claspers of lepidopterous caterpillars. With their similar habits and the presence of these ventral legs, saw-fly larvae have a considerable resemblance to the caterpillars of Lepidoptera, but they may be known from them in two ways,—by always having more than five pairs of ventral legs (eight in Cimbex, seven in Nematus, and six in some Hylotomides), and never having more than two ocelli—one on each side of the head—instead of six on each side as in Leρidoptera. As with lepidopterous caterpillars, green is the prevailing colour of saw-fly larvae. They agree with them, too, in their general habits: they live on the leaves of various plants, devouring them in different ways, roll down leaves, raise galls, and mine leaves. It is interesting to note further that, as with lepidopterous caterpillars, larvae which are innoxious and eaten by birds are either entirely green, or green with black, pink, or white stripes along the sides and back, while noxious larvae—those with bad smells or secretions which render them unpalatable to...