Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/589

573 HYMENOPTEBA 573 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 convey ance to the hive. They are richly spiued 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 tibire 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 i (Oxyura). The abdomen may be united to the metathorax in two ien - 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 Siricidce and Tenthredinidce which have the abdomen united by the whole width. Thus the order becomes divided into two well-marked divisions, one repre sented by the Tenthredinidce (having the abdomen sessile) and the other by the Aculeata, Ichneumonidce (having it appendiculated). The abdomen shows little variation in structure or form in the Tenthredinidce ; 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, thread like pedicle (Pelopoeus, many Chalcididce, and Cynijndoe). 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 re gards the number of the segments, it varies. It is 8 with the Tenthredinidce and with the Siricidce, but in other groups it is less. Through one or other of the basal segments being greatly developed, and the terminal ones correspond ingly reduced in size, some groups appear to have only 3 or 4 (Chrysididce). The Aculeata have always the same num ber, namely, 7 in the males and 6 in the females. Some bees (Ccvlioxys) and Chrysididce have the abdomen armed with spines or teeth at the apex. nro- The male and female organs are situated at the end of aus&amp;gt; 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- larvse escape from the eggs and are ready to feed on them. In the rest of the order with ichneumons, saw-flies, &c. it is not used as an instrument of defence (some ichneu mons, 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 crush ing 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 papilla? situated on the under side of the ninth abdominal segment, while the two lateral (or rather lower) bristles have their origin in similar papilla? on the eighth. It would appear also highly probable that the parts are true appendages of the abdominal segments, rather than modi fied 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 Chrysididce. 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 accord ing 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 Metanio: through a regular metamorphosis appearing in four distinct forms, phoses. The egg is generally longer than it is broad, and rounded at both The egg. 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, Cyni2)S, Mymar) 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 Tryphon, 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 hymenopterons 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. The 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 work? of labour and time.
 * tive th e abdomen. They are rarely conspicuous externally