Hymenoptera Sessiliventres, having the pronotum small, accurately adapted to the mesonotum; the anterior lobe of the latter is widely separated from the posterior; there are two spurs on the anterior tibiae. The larvae usually live on leaves after the manner of caterpillars, but a few inhabit galls.
The sawflies are an important family of Insects, their species being numerous, while some of them are, in the larval state, very destructive to vegetables and fruit. Being quiet creatures, rarely seen on the wing, they are, though common Insects in this country, but little known, and few persons recognise a sawfly as such. They are usually of small or moderate size, and the numerous species have a great family resemblance. This remark requires some qualification in the case of the Cimbicides, they being Insects of larger size—usually surpassing the honey-bee—of more robust structure, and with greater powers of flight.
Fig. 343.—Lophyrus pini. Britain. A, Larva; B, ventral aspect of pupa; C, imago, male. (After Vollenhoven.)
The antennae are remarkably variable in form and structure. Cameron considers that nine should be taken as the normal number of their joints; but there are only three in Hylotoma, while in Lyda there may be forty or more. The head is usually held closely applied to the thorax, but is really borne on a neck capable of much elongation (Fig. 332). The pronotum forms a part of the alitrunk, but is not soldered thereto. Usually the prosternum is more or less completely concealed by the side-pieces, but in Cimbicides it is larger and conspicuous, the side-pieces being in this group smaller than usual. The dorsal pieces of the mesothorax have their relative proportions different to what we find them in the other families of Sessiliventres, and even in most of the other Hymenoptera. There is first an antero-median lobe of triangular shape projecting, like a wedge, far backwards, into the great lateral lobes. These latter form the larger part of the area of the mesonotum; they meet together in the middle line, and behind are separated by a deep depression from the posterior lobe, or scutellum of the mesothorax, which is frequently divided into two parts, the anterior being the so-called scutum. The pieces of the metanotum are short and obscure, owing to the great unevenness of their parts; on each side of the middle there is a small membranous space of pallid colour. The cenchri, as these spaces are called, are, in Lyda, delicate, membranous, depressed spaces, in front of each of which there stands up a flap of membrane. The function of the cenchri is quite unknown. The median plate is fastened to the hind margin of the metanotum, and looks quite like one of the dorsal plates of the following abdominal segments, from which, however, it is separated by a more or less conspicuous membrane. In the majority of the Tenthredinidae the median plate is divided along the middle, but in the Cimbicides this is not the case. The mesosternum is very large, and the metasternum small, so that the middle and hinder pairs of coxae are placed close together. The abdomen consists of nine segments, there being eight dorsal plates in addition to the median plate, and seven ventral plates besides the terminal armature. There is a pair of short cerci, each of a single segment. The trochanters are divided; each tibia bears two spurs at the extremity, and the tarsi are 5-jointed.
Fig. 344.—Saws of Cimbex sylvarum. A, The pair spread out and placed in a horizontal position; a, the lower margin of the saw proper; b, the upper margin of the support: B, two teeth of the saw more highly magnified.
The most characteristic and interesting of the structures with which the Insects of this family are provided is the apparatus from which the name of sawfly is derived. As long as two centuries ago these instruments excited the admiration of Vallisnieri and of Réaumur, who described them at length; and it is truly astonishing that any part of a living being should be changed into tools so mechanically perfect as these saws are (Fig. 344). They serve the purpose of assisting the female in depositing the eggs in a suitable situation, the place selected being frequently the tender stems of shrubs or other plants, or the interior of leaves. These organs are therefore of course possessed only by the female. They are placed on the lower aspect of the hinder extremity of the body, where they are enclosed and protected by a pair of sheaths, from which they can be made to protrude by a little pressure exercised on the parts immediately in front of them. Each female possesses a pair of these saws; they consist of thin laminae of very hard consistence, and are not only toothed at their edge, but in many cases each tooth is itself serrate; at the same time the outer face of the saw is sculptured or plicate in a remarkable manner, so that the saw in this way acts as a file or rasp. The Insect having selected a suitable place, uses the saws by placing the extremity of the abdomen against a twig or leaf, protruding the blades, which, moving with an alternate motion, one being thrust forward while the other is retracted, act on the plant so as to make an incision. Each saw is directed in its movement by the support, the pair of supports being united at the base by membrane as shown in Fig. 344. In the case of some species,—Hylotoma rosae, the common sawfly of our rose-bushes, for instance—there is no difficulty in observing the operation; indeed old Réaumur, when speaking of the placid disposition of the sawflies, suggests that it was given them so that we may easily observe their charming operations. We cannot but regret that in these days we are unable to take so complacent a view of the arrangements of nature. There is much variety in the details of the structure of these saws; so much indeed that it is possible to identify most of the species by means of the saw alone. According to certain observers, the eggs are laid by some kinds on, not in, the leaves, so that we may conclude that in these cases the saws are not used by their possessors. An incision having been made, an egg is placed in it, and also a drop of some liquid matter. The egg is at first small, but soon increases till it becomes twice or three times its former size, and the development of the embryo commences.
The larvae of the Tenthredinidae exhibit great variety, and are indeed in this respect more interesting than the perfect Insects. The usual rule is that the larvae much resembles those of Lepidopterous Insects, and feed exposed on plants in the same way as Lepidopterous larvae do. But the exceptions are numerous; sometimes the larva is covered with slime, and thus protected from various enemies. In other cases it is very depressed, a broad creature, of irregular outline, living closely attached to the leaf, somewhat after the fashion of a huge scale-Insect. Some larvae mine between the layers of a leaf, others roll up leaves; a few live in the stems of plants, and one or two inside fruits. Even this does not complete the list of their habits, for a few species of Nematus live in galls caused by the deposition of the egg. A species of Lyda forms for itself a case out of bits of leaves, and carries this habitation about with it after the fashion of the Phryganeidae. The number of legs in these larvae is unusually great, varying from eighteen to twenty-two—that is, three pairs of thoracic legs and eight of abdominal or pro-legs. This character offers a ready means of distinguishing, in the majority of cases, these larvae from those of the Lepidoptera in which the number of legs varies, but is only from ten to sixteen; moreover, the pro-legs in sawflies are destitute of the circles of hooklets that exist in Lepidoptera. This mode of identifying the immature stages of the Tenthredinidae is not, however, always satisfactory, as there are some of these larvae that have no pro-legs at all, but only the three thoracic pairs. Another point of distinction exists, inasmuch as the larvae of the sawflies have only one ocellus on each side of the head, whereas in the Lepidopterous caterpillars the rule is that there are several of these little eyes on each side. In addition to this, we should mention that the Lepidopterous larva never has any pro-legs on the fifth body-segment, whereas in the sawflies when pro-legs are present there is always a pair on the segment in question.
These larvae are of various colours, but the patterns and markings they exhibit are not quite like those of the Lepidoptera, though it would be difficult to make any correct general statement as to the nature of the differences. The variety of their postures is very remarkable; and in respect of these also Tenthredinidae differ considerably from Lepidoptera. Some of them hold the posterior part of the body erect, clasping the leaf by their anterior legs; others keep the posterior part of the body curled up (Fig. 343, A), and some combine these methods by curving the posterior part of the body and holding it away from the food. These attitudes, like the general form, are characteristic for each species. The Nematus larvae that inhabit galls possess all the characteristics of those that feed externally. As a rule the skin of the larva is naked and free from hair, but it is often minutely tuberculate, and in a few species it is armed with remarkable forked spines. These spines may exist during part of the larval life, and completely disappear at one of the moults. The creatures are as a rule very sluggish, and move about much less than Lepidopterous larvae; many of them, when alarmed, have the power of exuding a disagreeable liquid, either from the mouth or from pores in the skin; in the latter case it may be sent as a sort of spray to some little distance from the body. This operation is said to be very efficacious as a means of protecting the larvae from the attacks of parasitic flies that are desirous of laying eggs in their bodies. One peculiarity as to their colour has attracted the attention of Réaumur and subsequent naturalists, namely, that in the case of many species a great change takes place in the colour during the life of the larva, and more especially at the period of the last moult. The change to the pupal state usually takes place in a cocoon, and some species have the peculiar habit of forming a double cocoon, the outer one being hard and coarse, while the inner is beautifully delicate. The cocoon is sometimes formed in the earth, and in that case it may be to a large extent composed of earthy matter. The Insect frequently remains a long time in its cocoon before emerging as a perfect Insect; however long this time may be, it is nearly all of it passed in the larval state; when the Insect does change to a pupa it speedily thereafter emerges as a perfect Insect. In the pupa the parts of the imago may be seen enveloped in a very delicate, transparent skin.