HYMENOPTERA, the term used in zoological classification for that order of insects comprising ants, bees, wasps, and their allies the saw-flies. gall-wasps and ichneumon flies. These insects all exhibit the following characters: (I) The presence of two pairs of stiff membranous wings often with the venation reduced the perfection of their structure but also with regard to the re markable development of their instincts. In the latter respect they stand at the head of all invertebrate animals and their behaviour has been the subject of studies by many of the most famous nat uralists. About 6o.000 species are known but many thousands more still await discovery and even in the British Isles it is only the ants. bees and wasps that have been adequately collected and studied. Although the vast number of species are solitary like other insects, individuals of some species have acquired the habit of living together in great societies as is the case in ants, and cer tain bees and wasps: their social life and behaviour is fully dis cussed in the article SOCIAL INSECTS. Hymenoptera are also re markable for the highly evolved condition parasitism has reached in the order: tens of thousands of species betray this habit, and although they confer immense benefit to man as agents destroy ing other forms of insect life, they have been relatively little collected, and in many parts of the world are quite unstudied.
Hymenoptera have acute vision, the compound eyes are consequently large and there are usually three ocelli or simple eyes. The antennae are often very different in the two sexes and in the bees and wasps they are generally composed or almost absent : the hind-wings are smaller than the fore-pair and are interlocked with them by tiny booklets. (2) The mouth parts have biting jaws. but the labium is usually modified into a kind of tongue for lapping or sucking. (3) The abdomen is generally constricted at the base to form a waist and its first seg ment is joined up with the thorax: an ovipositor is present and used for sawing, piercing or stinging. (4) Metamorphosis is com plete and the larvae are either caterpillars or more often vermi form: the pupae are usually in cocoons and their appendages are
Hymenoptera form one of the largest and most highly developed orders of insects and are of great interest. not only on account of of 13 joints in the males and 12 joints in the females. The mouth parts exhibit their simplest form in saw-flies (fig. t) where they depart but little from the generalized, biting type. In most other Hymenoptera the mandibles are used for what may be termed industrial purposes more than for feeding, and the ligula is modi fied into an organ for lapping or sucking nectar. In the higher types of bees this organ is elongated into a kind of tongue which, in some cases, exceeds in length that of the entire insect. In these instances the labial palpi and maxillae are also correspond ingly lengthened and form with the ligula, a definite proboscis (fig. I) . The thorax (fig. 2) is chiefly characterized by the fusion of its last segment with the propodeum or first segment of the abdomen. In the saw-flies this union is scarcely evident, but in all other Hymenoptera it is a pronounced feature and the second abdominal segment (or first apparent segment) is constricted to form a waist or petiole. The wings have departed very widely from the primitive type of venation and almost every transition can be found from the well developed condition seen in saw-flies to some of the parasitic forms where there is only a single vein to the fore-wings or even no veins at all. At the bases of the f ore-wings are small scale-like plates or tegulae, which afford important characters used in classification. Most members of the order fly with the wings of a side interlocked by a row of hooklets. In the female the abdomen bears an elaborate ovipositor, typically composed of three pairs of valves. The first pair arises from the eighth segment and forms the lancets, while the other pairs arise from the ninth segment and form the lancet sheath and the so-called sting paips respectively (fig. 3) . In ad dition to functioning as an egg-laying instrument the ovipositor is used in saw-flies for sawing niches in plants into which the eggs are lodged : in ichneumon flies and their allies it is often em ployed in stabbing their insect hosts preparatory to laying their eggs within the bodies of the latter : in bees, wasps and some ants it is used for stinging, a habit which is found in no other insects.
Hymenoptera are grouped into two main sub-orders and these, along with their chief sub-divisions, are enumerated below.
