Hymenoptera Sessiliventres.—This group has been variously called Hymenoptera phytophaga, H. securifera, H. sessiliventres, H. serrifera, H. symphyta. We prefer an old term, taken from a character that enables us to recognise at a glance which group a species belongs to. The division or sub-Order may be formally defined as follows:—
Abdomen nearly continuous in outline with the thorax, the two parts having a broad connexion instead of a small highly mobile articulation. Anal lobe of hind wings usually of considerable size. Trochanters ditrochous (transversely divided into two, Fig. 345). Extremity of body of female furnished with saws or boring instruments, usually concealed, in some cases visible in part. Larvae with complex mouth-parts; three pairs of thoracic legs (imperfect in Cephidae and Siricidae), and frequently with numerous abdominal legs, which are destitute of hooks. Food vegetable.
The Insects of this sub-Order never exhibit the highly specialised habits and activity of the better known petiolate Hymenoptera. Though the food in the larval stages is always vegetable, there is considerable variety in the larvae and their habits; some feed in galls, some in the twigs of plants, some in the hard wood of trees and shrubs. The majority, however, live on the leaves of plants. Those that live in wood (Fig. 342, C) resemble in appearance Coleopterous larvae that have similar habits, and those that live on leaves (Fig. 343, A) resemble Lepidopterous larvae that do likewise. There are four families included in the sub-Order, viz. Cephidae, Oryssidae, Siricidae, Tenthredinidae.
The British Sessiliventres—under the name Phytophagous Hymenoptera—have recently been monographed by Mr. Peter Cameron in a series of vols. published by the Ray Society.[[424]] These contain many figures and many details relating to natural history, in addition to the descriptions of genera and species.
Fam. I. Cephidae—Stem Sawflies.
Slender Insects, with weak integument; free, more or less elongate pronotum; one spine on the front tibia. Larvae living in the stems of plants or in the tender shoots of trees and shrubs.
The obscure little Insects composing this family have slender antennae of peculiar form, composed of eighteen to thirty joints, two of which are short and stout; then come several long joints, with more or less power of movement, the terminal portion consisting of an elongate club of many joints with little power of movement. The pronotum is longer than is usual in the Hymenoptera, and instead of being very closely connected with the mesonotum, it is free and mobile, although its base overwraps the front of the mesonotum. The median plate (i.e. the dorsal plate connecting the thorax and abdomen) is divided to the base along the middle, the divisions being separated by a membranous piece broader behind; the anal lobe of the posterior wings is small but distinct. The female bears a saw at the extremity of the body, but it is covered by two flaps; these form a short, terminal projection. Although too much neglected, the Cephidae are really of great interest as being of more imperfect or primitive structure than any of the other families of Hymenoptera. The larval history has been traced in several species. C. pygmaeus is sometimes very injurious to corn crops on the continent of Europe, and even in our own country its effects in this respect are considered to be occasionally serious. The egg is laid in the stem of the corn plant; the larva soon hatches and eats its way upwards in the stem. It is a soft grub, apparently footless, but really possessing six small projections in place of thoracic legs. It occupies all the summer in feeding, and when full fed and about to prepare for its metamorphosis, it weakens the stem by a sort of girdling process below the ear; it then descends in the stem to near the root, where it constructs a transparent cocoon, in which it passes the winter as a larva, changing to a chrysalis in the month of May, and completing its development by appearing as a perfect Insect shortly thereafter. The girdling operation is very injurious, and causes the corn stem, when ripe or nearly so, to break in two under the influence of a strong wind, so that the ears fall to the ground.
Fig. 340.—Cephus pygmaeus. Upper figure, larva; lower, female imago. Britain. (After Curtis.)
The history of C. integer has been given by Riley. This Insect attacks the young shoots of willows in North America. Riley states[[425]] that by a wonderful instinct the female, after she has consigned her egg to the twig, girdles the latter, preventing it from growing any further, and from crushing the egg by so doing. The larva after hatching eats downwards, sometimes destroying a length of two feet of the twig; when full grown it fills the bottom of the burrow with frass, and then previous to making its cocoon eats a passage through the side of the shoot about a quarter of an inch above the spot where the cocoon will be placed, thus making it easy for the perfect Insect to effect its escape; it leaves the bark, however, untouched, and is thus protected in its retreat. A delicate transparent cocoon is then spun in which the larva passes the winter, changing to a pupa in the following March, and emerging as a perfect Insect about six weeks thereafter.