Chapter XXIV: Introduction: Habits–classification–structure–chilognatha–chilopoda (18)

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 {513}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 {514}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 {515}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.

In Brazil _Dielocerus ellisii_, a sawfly allied to _Hylotoma_, constructs a nest in which the cocoons of many specimens are crowded together, being packed side by side like the cells in the comb of the bee, while the whole mass is protected by a thick outer wall. It is not known in what manner this communal work is carried out, but it is interesting to note that the cocoons assume to a considerable extent the hexagonal form of the cells in the comb of the bee. Some doubt was expressed as to the interpretation put on this structure by Curtis, but his observations have been confirmed by Smith and Peckholt.

Several species of sawflies are known to be very injurious to crops. One of these—the sawfly of the turnip, _Athalia spinarum_ (_centifoliae_ Panz.)—sometimes commits excessive depredations on the turnip crops in this country as well as on the continent of Europe; its life-history and anatomy were described by Newport {516}in an essay published by the Entomological Society in 1838. The eggs, it appears, are laid singly at the edges of the leaves in the month of May, as many as 200 or 300 being deposited by one female; as the parent flies are usually gregarious, appearing in large numbers in fields of turnips, it is not difficult to form an idea of the serious nature of their depredations. The egg grows very considerably; the development of the embryo is rapid, occupying, even in unfavourable weather, only seven or eight days, while in quite congenial circumstances it is probable that the eggs may hatch about the fourth day after their deposition. The young grub immediately begins to feed, and in about five days changes its skin for the first time; it repeats this operation twice at similar or slightly longer intervals, the third moult thus occurring when the larva is three or four weeks old; it is then that the larva begins to be most destructive. Sunshine and warm weather are very favourable to it, and under their influence it grows so rapidly that in a few days a field may be almost completely stripped of its foliage. This larva is of a sooty black colour, and will live on other Cruciferous plants quite as well as on the turnip. When full grown it buries itself to a slight depth under the surface of the earth, and forms an oval cocoon of a firm texture, and with many particles of earth closely adherent to it. The perfect fly emerges towards the end of July, and a second brood will be produced in the same season if circumstances are favourable; in that case the resulting larvae enter the ground for the formation of their cocoons in September or October, and pass the winter in their cocoons, but still in the larval state; changing to pupae in the following spring, and appearing as perfect Insects in May. From this account it appears not improbable that the offspring of a single female existing in the April of one year may amount by the following May—three generations having been passed through in the interval—to as many as 27,000,000 larvae. Fortunately the creatures are, as Frauenfeld observed, destroyed in very large numbers by a parasitic fungus and by a Nematode (_Filaria_).

We have, earlier in the chapter, alluded to the fact that the phenomena of parthenogenesis prevail somewhat extensively among sawflies. It is the rule in the family that males are very much less numerous than females, and there are some species of which no males have been discovered. This would not be of {517}itself certain evidence of the occurrence of parthenogenesis, but this has been placed beyond doubt by taking females bred in confinement, obtaining unfertilised eggs from them, and rearing the larvae produced from the eggs. This has been done by numerous observers with curious results. In many cases the parthenogenetic progeny, or a portion of it, dies without attaining full maturity. This may or may not be due to constitutional weakness arising from the parthenogenetic state. Cameron, who has made extensive observations on this subject, thinks that the parthenogenesis does involve constitutional weakness, fewer of the parthenogenetic young reaching maturity. This he suggests may be compensated for—when the parthenogenetic progeny is all of the female sex—by the fact that all those that grow up are producers of eggs. In many cases the parthenogenetic young of Tenthredinidae are of the male sex, and sometimes the abnormal progeny is of both sexes. In the case of one species—the common currant sawfly, _Nematus ribesii_—the parthenogenetic progeny is nearly, but not quite, always, entirely of the male sex; this has been ascertained again and again, and it is impossible in these cases to suggest any advantage to the species to compensate for constitutional parthenogenetic weakness. On the whole, it appears most probable that the parthenogenesis, and the special sex produced by it, whether male or female, are due to physiological conditions of which we know little, and that the species continue in spite of the parthenogenesis, rather than profit by it. It is worthy of remark that one of the species in which parthenogenesis with production of males occurs—_Nematus ribesii_—is perhaps the most abundant of sawflies.

Although many kinds of Insects display the greatest solicitude and ingenuity in providing proper receptacles for their eggs, and in storing food for the young that will be produced, there are extremely few that display any further interest in their descendants; probably, indeed, the majority of Insects die before the eggs are hatched, one generation never seeing the individuals of another. It is therefore interesting to find that a fairly well authenticated case of maternal attachment, such as we have previously alluded to as occurring in earwigs, has been recorded in _Perga lewisii_, an Australian sawfly of the sub-family Cimbicides. The mother, having deposited about eighty eggs on the leaf of a Eucalyptus, remains with them until they hatch, {518}after which she sits over her brood with outstretched legs, and with admirable perseverance protects them, so far as she is able, from the attacks of parasites and other enemies; she quite refuses to be driven away from her charges. Mr. Lewis, to whom we are indebted for this account,[427] states that the sawfly does not recognise her own special brood, but will give equal attention to another brood if she be transferred thereto; and he adds that many of the batches of larvae were destitute of any maternal guardian.

There are about 2000 species of sawflies known. A large majority of them are found in the European and North American regions; still, a good many are known to live in South America, and _Perga_—one of the genera of the family containing many species of large size—is peculiar to the Australian region. Although the family includes so many species, very few anomalies of structure have been detected in it; one species, _Pompholyx dimorpha_ Freymuth, is described as being apterous in the female, and as having the thorax curiously modified in its form. There are no very small Insects in the family, and none over the middle size. Nearly 400 species have been detected in Britain; this number could certainly be increased by persevering researches. The palaeontological record has hitherto given only a very meagre evidence about sawflies. Several species have been preserved in amber, and three or four are known from Tertiary strata in Europe and North America.

{519}CHAPTER XXIII

HYMENOPTERA PETIOLATA–PARASITIC HYMENOPTERA–CYNIPIDAE OR GALL-FLIES– PROCTOTRYPIDAE–CHALCIDIDAE–ICHNEUMONIDAE–BRACONIDAE–STEPHANIDAE– MEGALYRIDAE–EVANIIDAE–PELECINIDAE–TRIGONALIDAE.

We now pass to the consideration of the Hymenoptera of the sub-Order Petiolata, or Apocrita, as they are styled by Brauer. We should make use of the term Petioliventres, for it contrasts naturally by its termination with Sessiliventres, were it not that the word is so uncouth that we think it better to adopt the shorter and more euphonious expression, Petiolata.

The members of this sub-Order, without exception, have the hind body connected with the thorax by means of a deep constriction, so that the base of the abdomen (Fig. 336, B, _b_) is very narrow; the articulation between the two parts is effected by means of a complex joint allowing great play, and facilitating the operations of boring and stinging, processes that are of extreme importance in the economy of the great majority of the species. The petiole is sometimes extremely short, but it may be so long that it appears like a stalk, at whose extremity is borne the remaining part of the abdomen (Fig. 369). When the petiole is very short the abdomen reposes close to the back of the thorax (Fig. 331, C), and in this case the abdomen is usually described as sessile; while, when it is evidently stalked, it is said to be petiolate. These terms are, however, unsuitable, as the words sessile and petiolate should be reserved for the conditions characteristic of the two sub-Orders. We shall therefore use the terms pseudo-sessile and pedicellate for the two conditions of the Petiolata.

The Hymenoptera Petiolata comprises an enormous majority {520}of the Order. Although it includes many of the most interesting and important of Insects, its classification is but little advanced, for a great many of the forms are still rare or unknown. Three series may be adopted for the purposes of nomenclature.

1. _Parasitica._—Trochanters of two pieces, female with an ovipositor.

2. _Tubulifera._—Trochanters undivided; abdomen consisting of only three, four, or five visible segments.

3. _Aculeata._—Trochanters undivided; abdomen consisting of six or seven visible segments; female furnished with a retractile sting.

In the absence of any clear distinction between sting and ovipositor, these groups are merely conventional. The character furnished by the trochanters is unfortunately subject to some exceptions, there being a few parasitic forms in which the trochanters are not divided, and a few aculeates in which the reverse is more or less distinctly the case; moreover, the division, when it exists, is in some cases obscure, and the two pieces are of unequal size. Ratzeburg calls the upper division, which is frequently much larger than the other, the trochanter, and the lower division the apophysis. There is much reason for believing that the apophysis is really merely a secondary division of the femur. The Tubulifera are a comparatively small group, and will probably be merged in one of the other two, when the anatomy and morphology of the abdomen have been more thoroughly elucidated.

HYMENOPTERA PARASITICA OR TEREBRANTIA.

This is one of the most extensive divisions of the class Insecta. There can be little doubt that it contains 200,000 species, and possibly the number may be very much greater than this. It is, however, one of the most neglected of the great groups of Insects, though it is perhaps of greater economic importance to mankind than any other.

{521}Insects derive their sustenance primarily from the vegetable kingdom. So great and rapid are the powers of assimilation of the Insect, so prodigious its capacity for multiplication, that the Mammal would not be able to compete with it were it not that the great horde of six-legged creatures has divided itself into two armies, one of which destroys the other. The parasitic Hymenoptera are chiefly occupied in destroying the tribes of vegetarian Insects; the parasites do this by the simple and efficient device of dwelling in the bodies of their hosts and appropriating the nutriment the latter take in. The parasites do not, as a rule, eat the structures of their host,—many of them, indeed, have no organs that would enable them to do this,—but they absorb the vegetable juices that, in a more or less altered state, form the lymph or so-called blood of the host. The host could perhaps starve out his enemies by a judicious system of abstention from food; instead, however, of doing this, he adopts the suicidal policy of persistent eating, and as the result of his exertions, furnishes sufficient food to his parasites, and then dies himself, indirectly starved. Ratzeburg considers that the traditional view that the larvae of parasitic Hymenoptera live by eating the fat-body of their host is erroneous. They imbibe, he considers, the liquid that fills the body of the parasitised Insect.[428]

The wide prevalence of Insect parasitism is appreciated only by entomologists. The destructive winter moth—_Cheimatobia brumata_—is known to be subject to the attacks of sixty-three species of Hymenopterous parasites. So abundant are these latter that late in the autumn it is not infrequently the case that the majority of caterpillars contain these destroyers. Although Lepidoptera are very favourite objects with parasitic Hymenoptera, yet other Insects are also pertinaciously attacked; there is quite a host of Insect creatures that obtain their sustenance by living inside the tiny Aphididae, or "green-flies," that so much annoy the gardener. A still larger number of parasites attack eggs of Insects, one or more individuals finding sufficient sustenance for growth and development inside another Insect's egg. As Insects have attacked Insects, so have parasites attacked parasites, and the phenomena called hyperparasitism have been developed. These cases of secondary parasitism, in which another {522}species attacks a primary parasite, are extremely numerous. It is also pretty certain that tertiary parasitism occurs, and Riley is of opinion that even quaternary destruction is not outside the range of probability.

The physiological problems connected with Insect parasitism are of great interest to the entomologist; the modes of nutrition and respiration of these encaged creatures could not fail to be most instructive were we fully acquainted with them. It is obvious that when an Insect-egg is laid inside another Insect's egg, and the parasite has to undergo the whole of its growth therein, it is in the strangest condition as regards nutrition. It is unnecessary for the intruded egg to have yolk of its own; moreover, the embryonic mode of nutrition may be continued during what would, with other Insects, be the larval period. And it seems to be the case that both these conditions are actually met with in the lives of egg-parasites. The embryology and post-embryonic development of parasitic Hymenoptera have already been ascertained to be of the most extraordinary nature. Great variety, however, will no doubt be found to exist, as will be readily understood if we tabulate the conditions of the early life of various parasitic Hymenoptera.

1. The egg may be laid outside a larva, and the embryonic and larval developments may both be passed on the exterior.

2. The egg may be laid and the embryonic development passed through, outside the host, but the parasite on hatching may enter the host, so that the post-embryonic development is passed in the lymph of the host.

3. The egg may be laid inside the host, both embryonic and post-embryonic developments being gone through in the fluids of the host.

4. The egg may be laid inside another egg, the embryonic and post-embryonic developments being passed therein.

We shall find that all these conditions exist in the Insects we are about to consider.

We shall treat the series as composed of ten families; but we must remind the student that this great subject is still in a very unadvanced state; the combined efforts of generations of naturalists will be required to perfect it. Of the ten families five are comparatively insignificant in number of species. Many of the Cynipidae are not parasitic in habits, but live in galls. {523}After what we have said as to the mode of nutrition of parasites it will be understood that the physiological conditions of life may not be so different in a gall-dweller and a parasite as would at first be supposed; and it is perhaps not a matter for much surprise that good characters cannot be found to separate the gallicolous from the parasitic forms.

FAM. I. CYNIPIDAE—GALL-FLIES.

_Wings with very few cells, with no dark patch (stigma) on the anterior
margin; pronotum fixed to the mesonotum, and at each side extending back
to the point of insertion of the front wing. Antennae not elbowed but
straight, composed of a moderate number (12-15) of joints. Early stages
passed either in galls or as parasites in the bodies of other Insects._

The Cynipidae are always small, frequently minute, Insects; usually black or pitchy in colour. The simple structure of the antennae and the number of their joints are of importance as an aid in identifying a Cynipid. The mesonotum is usually remarkably convex, and has, behind, a prominent scutellum, which more or less overhangs the small metanotum and the median segment; these are perpendicular in their direction; the sculpture of these posterior parts of the alitrunk is usually deep and remarkable. The abdomen has usually only a short petiole, so as to be pseudo-sessile; but there are some genera in which this part is rather long. The abdomen is generally so very much changed in outer form that its structure is not easily understood. The visible portion is frequently in larger part made up of the greatly enlarged dorsal plate of the second or third segment, or of both. These large plates are really chiefly composed of free flaps, and on lifting them up the large ventral plates are disclosed, although these appeared previously to be nearly or quite absent. In the female {524}there is a very slender ovipositor, of which only a small part protrudes, although the organ is really elongate; it is drawn into the abdomen by means of a peculiar series of structures, the modified terminal segments to which it is attached being folded over into the interior of the body in such a way that the posterior part becomes situated anteriorly. In conformity with this arrangement, the ovipositor is bent double on itself, the anterior and the middle portions of the borer being carried into the body, leaving only a small part projecting beyond the extremity. The Cynipid ovipositor is an instrument of much delicacy, and is capable of a great deal of movement; it is usually serrate just at the tip, and although it looks so very different from the cutting apparatus of the sawflies (Fig. 344), it seems that it is really composed of pieces similar in their origin to those of the Tenthredinidae.

The wings frequently bear fine hairs; the paucity of nervures and the absence of the "stigma" are of importance in the definition of the family. The most important of the cells is one called the radial cell, situate just beyond the middle of the front part of the wing.

We cannot enter into a consideration of the classification of the family, as authorities are not agreed on the subject.[429] As regards their habits Cynipidae are, however, of three different kinds: (1) the true gall-flies, or Psenides, which lay an egg or eggs in the tissues of a growing plant, in the interior of which the larva lives after it is hatched; this mode of life may or may not, according to the species, be accompanied by formation of a peculiar growth called a gall: (2) Inquilines,[430] or guest-flies; {525}these lay their eggs in the galls formed by the gall-makers subsequent to the growth of the galls, of which they obtain the benefit: (3) Parasites; these live, like most Ichneumon-flies, in the interior of the bodies of other living Insects; they prey on a considerable variety of Insects, but chiefly, it is believed, on Aphididae, or on Dipterous larvae. These parasitic flies belong to the sub-family Figitides.

A great deal of discussion has occurred relative to the nature and origin of galls, and many points still remain obscure. Considerable light has been thrown on the subject by the direct observations of modern naturalists. Previous to Malpighi, who wrote on the subject two hundred years ago, it was supposed that galls were entirely vegetable productions, and that the maggots found in them were due to spontaneous generation, it having been an article of belief in the Middle Ages that maggots in general arose from the various organic substances in which they were found, by means of the hypothetical process called, as we have said, spontaneous generation. Malpighi was aware of the unsatisfactory nature of such a belief, and having found by observation that galls arose from the punctures of Insects, he came to the further conclusion that the growth of the gall was due to the injection by the Insect into the plant of a fluid he termed Ichor, which had, he considered, the effect of producing a swelling in the plant, something in the same way as the sting of a bee or wasp produces a swelling in an animal. Réaumur also made observations on the gall-Insects, and came to the conclusion that the latter part of Malpighi's views was erroneous, and that the swelling was not due to any fluid, but simply to irritation caused by the prick; this irritation being kept up by the egg that was deposited and by the subsequent development of the larva. Observations since the time of Réaumur have shown that the matter is not quite so simple as he supposed, for though in the case of some galls the development of the gall commences immediately after the introduction of the egg, yet in other cases, as in the Cynipidae, it does not occur till some time thereafter, being delayed even until after the hatching of the egg and the commencement of the development of the larva. Galls are originated {526}by a great variety of Insects, as well as by mites, on many plants; and it must not be concluded that a gall has been formed by Hymenoptera even when these Insects are reared from one. Extremely curious galls are formed by scale-Insects of the sub-family Brachyscelides on Eucalyptus trees in Australia; they are much inhabited by parasitic Hymenoptera, and Froggatt has obtained 100 specimens of a small black Chalcid from a single dead Brachyscelid.[431] The exact manner in which many of these galls originate is not yet sufficiently ascertained; but the subject of the galls resulting from the actions of Cynipidae has received special attention, and we are now able to form a conception of their nature. They are produced by the meristematic or dividing tissue of plants, and frequently in the cambium zone, which is caused to develop to an unusual extent, and in a more or less abnormal manner, by the presence of the Insect. The exact way in which a Cynipid affects the plant is perhaps not conclusively settled, and may be found to differ in the cases of different Cynipidae, but the view advocated by Adler and others, and recently stated by Riley,[432] seems satisfactory; it is to the effect that the activity of the larva probably affects the meristem, by means of a secretion exuded by the larva. The mere presence of the egg does not suffice to give rise to the gall, for the egg may be deposited months before the gall begins to form. It is for the same reason improbable that a fluid injected by the parent fly determines the gall's growth. It is true that the parent fly does exude a liquid during the act of oviposition, but this is believed to be merely of a lubricant nature, and not to influence the development. It is said that the gall begins to form in some cases before the larva is actually hatched, but the eggs of some Hymenoptera exhibit remarkable phenomena of growth, so that the egg, even during development of the embryo in it, may in these cases, exert an influence on the meristem. It is to reactions between the physiological processes of the meristem and the growing Insect that the gall and its form are due.

The investigations of several recent naturalists lend support to the view that only the meristematic cells of the plant can give rise to a gall. Riley says that the rate of growth of the gall is dependent on the activity of the meristem, galls on {527}catkins developing the most quickly; those forming on young leaves also grow with rapidity, while galls formed on bark or roots may take months to attain their full size.

It is a curious fact that Cynipid galls are formed chiefly on oaks, this kind of tree supplying a surprising number and variety of galls. The plants that furnish Cynipid galls in Europe are not numerous. A list of them is given by Cameron.[433] Several species, of the genus _Rhodites_, attack rose-bushes. One of the best known of our British galls is the bedeguar, found in various parts of the country on both wild and cultivated rose-bushes (Fig. 348), and caused by _Rhodites rosae_ (Fig. 349). This gall has the appearance of arising from a twig or stem, but it is really a leaf gall. Pazlavsky[434] has described the mode of formation of the bedeguar. The female _Rhodites_ in the spring selects a rose-bud—not a flower-bud—that should produce a twig and leaves, and pricks this bud in a systematic manner in three places. The three spots of the bud pricked by the Insect are the three undeveloped leaves that correspond to a complete cycle in the phyllotaxis of the plant. The three rudiments do not develop into leaves, but by a changed mode of growth give rise to the bedeguar. Usually this gall, as shown in our figure, is of large size, and contains numerous cells; but abortive specimens are not infrequently met with; sometimes a small one is seated on a rose-leaf, and it is thought that these are due to a failure on the part of the Insect to complete the pricking operation. {528}Cynipidae will not go through their gall-making operations except under natural conditions. Giraud[435] attempted to obtain oviposition, on gathered twigs of oak, from flies in confinement; but, although he experimented with thousands of specimens, they on no occasion laid their eggs in the fresh shoots placed at their disposal, but discharged their eggs in little heaps, without attention to the twigs. The same observer has also called attention to the fact that after being deposited in a bud the eggs of certain species of _Cynips_ will remain dormant without producing, so far as can be seen, any effect on the tree for a period of fully ten months, but when the bud begins to develop and the egg hatches then the gall grows.

The exact mode in which the egg is brought to the requisite spot in the plant is still uncertain. The path traversed by the ovipositor in the plant is sometimes of considerable length, and far from straight; in some cases before it actually pierces the tissues, the organ is thrust between scales or through fissures, so that the terebra, or boring part of the ovipositor, when it reaches the minute seam of cambium, is variously curved and flexed. Now as the canal in its interior is of extreme tenuity, and frequently of great length, it must be a very difficult matter for the egg to reach the tissue where it should develop. The eggs of Cynipidae are very remarkable bodies; they are very ductile, and consist of a head, and of a stalk that in some cases is five or six times as long as the head, and is itself somewhat enlarged at the opposite end. Some other Hymenoptera have also stalked eggs of a similar kind (Fig. 357, A, egg of _Leucospis_). It has been thought that this remarkable shape permits of the contents of the egg being transferred for a time to the narrower parts, and thus allows the broader portion of the egg to be temporarily compressed, and the whole structure to be passed through a very narrow canal or orifice. It is, however, very doubtful whether {529}the egg really passes along the canal of the borer. Hartig thought that it did so, and Riley supports this view to a limited extent. Adler, however, is of a different opinion, and considers that the egg travels in larger part outside the terebra. It should be remembered that the ovipositor is really composed of several appendages that are developed from the outside of the body; thus the external orifice of the body is morphologically at the base of the borer, the several parts of which are in longitudinal apposition. Hence there is nothing that would render the view of the egg leaving the ovipositor at the base improbable, and Adler supposes that it actually does so, the thin end being retained between the divisions of the terebra. Riley is of opinion that the act of oviposition in these Insects follows no uniform system. He has observed that in the case of _Callirhytis clavula_, ovipositing in the buds of _Quercus alba_, the eggs are inserted by the egg-stalk into the substance of the leaf, and that the egg-fluids are at first gathered in the posterior end, which is not inserted. "The fluids are then gradually absorbed from this exposed portion into the inserted portion of the egg, and by the time the young leaves have formed the exposed [parts of the] shells are empty, the thread-like stalk has disappeared, and the egg-contents are all contained within the leaf tissue." He has also observed that in _Biorhiza nigra_ the pedicel, or stalk, only is inserted in the embryonic leaf-tissue, and that the enlarged portion or egg-body is at first external. The same naturalist also records that in the case of a small inquiline species, _Ceroptres politus_, the pedicel of the egg is very short, and in this case the egg is thrust down into the puncture made by the borer, so that the egg is entirely covered.

Some Cynipidae bore a large number of the channels for their eggs before depositing any of the latter, and it would appear that it is the rule that the boring of the channel is an act separate from that of actual oviposition. Adler distinguishes three stages: (1) boring of the canal; (2) the passage of the egg from the base of the ovipositor, where the egg-stalk is pinched between the two spiculae and the egg is pushed along the ovipositor; (3) after the point of the ovipositor is withdrawn, the egg-body enters the pierced canal, and is pushed forward by the ovipositor until it reaches the bottom.[436]

{530}About fifty years ago Hartig reared large numbers of certain species of gall-flies from their galls, obtaining from 28,000 galls of _Cynips disticha_ about 10,000 flies, and from galls of _C. folii_ 3000 or 4000 examples of this species; he found that all the individuals were females. His observations were subsequently abundantly confirmed by other naturalists, among whom we may mention Frederick Smith in our own country, who made in vain repeated attempts to obtain males of the species of the genus _Cynips_. On one occasion he collected in the South of England 4410 galls of _C. kollari_ (at that time called _C. lignicola_), and from these he obtained 1562 flies, all of which were females. A second effort was attended with similar results. Hartig, writing in 1843, after many years' experience, stated that though he was acquainted with twenty-eight species of the genus _Cynips_, he had not seen a male of any one of them. During the course of these futile attempts it was, however, seen that a possible source of fallacy existed in the fact that the Insects were reared from collected galls; and these being similar to one another, it was possible that the males might inhabit some different gall. Adler endeavoured to put the questions thus raised to the test by means of rearing females from galls, and then getting these females to produce, parthenogenetically, galls on small oaks planted in pots, and thus completely under control. He was quite successful in carrying out his project, and in doing so he made a most extraordinary discovery, viz. that the galls produced by these parthenogenetic females on his potted oaks, were quite different from the galls from which the flies themselves were reared, and were, in fact, galls that gave rise to a fly that had been previously considered a distinct species; and of this form both sexes were produced. Adler's observations have been confirmed by other naturalists, and thus the occurrence of alternation of generations, one of the two generations being parthenogenetic, has been thoroughly established in Cynipidae. We may mention one case as illustrative. A gall-fly called _Chilaspis lowii_ is produced from galls on oak-leaves at Vienna at the end of April, both sexes occurring. The female thereafter lays eggs on the ribs of the leaves of the same kind of oak, and thus produces a different gall from that which nourished herself. These galls fall off with the leaves in the autumn, and in July or August of the following year a gall-fly is produced from them. It is a different creature from the {531}mother, and was previously known to entomologists under the name of _Chilaspis nitida_. Only females of it occur, and these parthenogenetic individuals lay their eggs in the young buds of the oak that are already present in the autumn, and in the following spring, when the buds open and the leaves develop, those that have had an egg laid in them produce a gall from which _Chilaspis lowii_ emerges in April or May. In this case therefore the cycle of the two generations extends over two years, the generation that takes the greater part of the time for its production consisting only of females. Adler's observations showed that, though in some species this alternation of generations was accompanied by parthenogenesis in one part of the cycle, yet in other species this was not the case. He found, for instance, that some gall-flies of the genus _Aphilothrix_ produced a series of generations the individuals of which were similar to one another, and were all females and parthenogenetic. In some species of the old genus _Cynips_ no males are even yet known to occur. A very curious observation was made by the American, Walsh, viz. that of galls gathered by him quite similar to one another, some produced speedily a number of both sexes of _Cynips spongifica_, while much later on in the season the remainder of the galls gave rise to females only of an Insect called _Cynips aciculata_. It is believed that the galls gathered by Walsh[437] were really all one species; so that parts of the same generation emerge at different times and in two distinct forms, one of them parthenogenetic, the other consisting of two sexes. It has, however, been suggested that _Cynips spongifica_ and _C. aciculata_ may be two distinct species, producing quite similar galls.

Turning now to the questions connected with inquiline or guest-flies, we may commence with drawing attention to the great practical difficulties that surround the investigation of this subject. If we open a number of specimens of any kind of gall it is probable that several kinds of larvae will be found. In Fig. 350 we represent four kinds of larvae that were taken out of a few bedeguar galls gathered on one day in a lane near Cambridge. It is pretty certain that No. 1 in this figure represents the larva of _Rhodites rosae_, and that Nos. 2 and 3 are larvae of inquilines, possibly of _Synergus_, or of a parasite; while No. 4, which was engaged in feeding on No. 3 in the position {532}shown, is possibly a Chalcid of the genus _Monodontomerus_, or may be _Callimome bedeguaris_. It is clear that, as we cannot ascertain what is inside a gall without opening it, and thereby killing the tenants, it is a most difficult matter to identify the larvae; the only safe method is that of observation of the act of oviposition; this may be supplemented by rearing the flies from galls, so as to ascertain what variety of flies are associated with each kind of gall. This last point has been well attended to; but the number of cases in which oviposition of inquiline gall-flies in the galls formed by the Psenides has been ascertained by direct observation is still very small; they are, however, sufficient to show that the inquilines deposit their eggs only after the galls are formed.

Bassett recorded the first case of the kind in connexion with a North American species, _Cynips_ (_Ceroptres_) _quercus-arbos_ Fitch. He says: "On the first of June galls on _Quercus ilicifolia_ had reached their full size, but were still tender, quite like the young shoots of which they formed part. Examining them on that day, I discovered on them two gall-flies, which I succeeded in taking. They were females, and the ovipositor of each was inserted into the gall so deeply that they could not readily free themselves, and they were removed by force."

The great resemblance of the inquiline gall-fly to the fly that makes the gall both dwell in, has been several times noticed by Osten Sacken, who says "one of the most curious circumstances connected with the history of two North American blackberry galls is, that besides the _Diastrophus_, which apparently is the genuine originator of the gall, they produce another gall-fly, no doubt an inquiline, belonging to the genus _Aulax_, and showing the most striking resemblance in size, colouring, and sculpture to the _Diastrophus_, their companion. The one is the very counterpart {533}of the other, hardly showing any differences, except the strictly generic characters! This seems to be one of those curious instances, so frequent in entomology, of the resemblance between parasites and their hosts! By rearing a considerable number of galls of _D. nebulosus_ I obtained this species as well as its parasite almost in equal numbers. By cutting some of the galls open I ascertained that a single specimen of the gall frequently contained both species, thus setting aside a possible doubt whether these Insects are not produced by two different, although closely similar galls."[438]

The substance of which galls are composed, or rather, perhaps, a juice they afford, is apparently a most suitable pabulum for the support of Insect life, and is eagerly sought after by a variety of Insects; hence by collecting galls in large quantities many species of Insects may be reared from them; indeed by this means as many as thirty different kinds of Insects, and belonging to all, or nearly all, the Orders, have been obtained from a single species of gall. Some galls are sought by birds, which open them and extract their tenants, even in cases where it might be supposed that the nauseous flavour of the galls would forbid such proceedings.

Not more than 500 species of Psenides and Inquiline Cynipidae are known from all parts of the world; and of described Parasitic Cynipidae there are only about 150 species. The British forms have recently been treated by Cameron in the work we have already several times referred to.[439]

A few Cynipidae have been found in amber; and remains of members of the family, as well as some galls, are said by Scudder to have been found in the Tertiary strata at Florissant.

FAM. II. PROCTOTRYPIDAE, OR OXYURA.

_Small Hymenoptera, with few, or even no, nervures in the wings: the
pronotum closely adherent to the mesothorax, and at the sides reaching
backwards to the points of insertion of the wings. The abdomen is
pointed, and the pointed apex is frequently deflexed; the ovipositor is
not coiled, but is retractile, and when extruded is of tubular form, and
{534}apparently a continuation of the tip of the body. The earlier stages
are passed in the bodies, or in the eggs, of other Arthropods._

The Proctotrypidae is one of the most difficult groups of Hymenoptera to define; some of its members exhibit a great resemblance to Aculeate Hymenoptera. This is the case with the Insect we figure (Fig. 351). It, however, is an undoubted Proctotrypid, but there are other forms that approach very closely in appearance to the Aculeata, or stinging Hymenoptera; so that until a better comprehension is reached as to the distinction between a sting and an ovipositor the separation between Proctotrypidae and Aculeata must be considered somewhat arbitrary.

There is extreme variety in the family; the wings differ considerably in shape and neuration; they are not infrequently altogether absent in one or both sexes. The chief distinction of the family from other parasitic Hymenoptera is the tubular form of the ovipositor; which part appears to be a continuation of the tip of the body. This latter is more definitely acuminate than usual, and has given rise to the term Oxyura, by which name the Proctotrypidae are distinguished in many books. From the Chalcididae they are distinguished also by the angles of the pronotum attaining the tegulae. In this character they agree with the Cynipidae, but the ovipositor and abdomen are very different in form in these two groups, and the Proctotrypidae very frequently have a pigmented spot or stigma on the front wings which is absent in Cynipidae. As if to add to the difficulties the systematist meets with in dealing with this family, some of its members have the trochanters undivided, as in the case of the stinging Hymenoptera. The larvae of all that are known lead a completely parasitic life in the bodies or eggs of other Insects or of Spiders. Sometimes half a dozen specimens may find the means of subsistence during the whole of their development in a single Insect's egg. Usually Proctotrypids pupate in {535}the position in which they have fed up, enclosed each one in a more or less distinct cocoon. In Fig. 352 we represent a very remarkable case of Proctotrypid pupation; a larva of some beetle has nourished many specimens of a species of the genus _Proctotrypes_, and the pupae thereof project from the body of the host, a pair of the parasites issuing from each segmental division in a remarkably symmetrical manner.

Comparatively little is known as to the habits of the members of this family, but such information as has been obtained leads to the conclusion that great variety will be found to exist in this respect. We have already mentioned that numerous species have been ascertained to feed inside the eggs of Insects or of Spiders; others have been reared from larvae or from galls of the minute Dipterous midges of the family Cecidomyiidae; others have been obtained from Cynipid galls, a few from ants' nests and from green-fly; some species are known to attack Coleoptera. The distinguished Irish entomologist, Haliday, has written an account of the proceedings of a species of _Bethylus_,[440] from which it has been supposed that this Insect carries off living caterpillars, and stores them in a suitable receptacle as food for its progeny, thus anticipating, as it were, the habits of the fossorial division of the Aculeata, in which group this instinct has, as we shall subsequently relate, attained an astonishing degree of perfection. Haliday's observation was unfortunately incomplete and has not been subsequently confirmed. The Bethylides are remarkable for their great approach in structure to the Aculeates, so much so that entomologists are not agreed as to whether certain Insects are Proctotrypids or Aculeates. _Pristocera_, with a very wide distribution, may be mentioned as illustrative of these doubtful forms; but other genera of the Bethylides are in many respects very similar to the Aculeates, and it is not matter for surprise that Haliday should have considered the Bethylides to be a tribe of the stinging Hymenoptera. {536}The genus _Scleroderma_ consists of small Insects much resembling ants, and, as well as some of its allies, is of great interest from the remarkable phenomena of polymorphism presented by certain species. The males in this genus are winged, the females completely apterous; yet at times winged females are produced—as exceptional individuals in a brood of wingless specimens—the females in these cases being not only winged, but possessed of ocelli like the females of other winged Hymenoptera. Particulars of a case of this kind have been given by Sir Sidney Saunders,[441] and Ashmead also mentions[442] the exceptional occurrence of these winged females. Westwood[443] was of opinion that there are three forms of the female sex. This subject is of importance in connexion with the production of the various castes in ants. Although the presence of wings in these Insects is always accompanied by the existence of ocelli (which, it will be remembered, are normally absent from the wingless individuals), yet the converse is not always the case, for a form of the female of _Cephalonomia formiciformis_, without any wings, yet having ocelli, as well as eyes, well developed, is figured by Westwood.[444]

The development of some of the Proctotrypids has been partially described by Ganin and others, and is of an extraordinary character. Ganin's observations[445] were most complete in the case of a species of _Platygaster_, which he found in the larva of a very minute Dipteron of the genus _Cecidomyia_. The _Platygaster_ larva changes its form very much in the course of its life, resembling at first a minute Crustacean rather than an Insect-larva; it has a very large rounded anterior portion, while behind it terminates in two, or more, tail-like processes. By a {537}very peculiar kind of metamorphosis this _Cyclops_-like larva changes into an almost unsegmented, oviform larva, destitute of appendages; by a second change this creature assumes a third condition, in which it is similar to the ordinary form of parasitic Hymenopterous larvae. Sometimes several of the _Platygaster_ larvae are found in a single host, but only one of them reaches this third stage. Afterwards the third larval instar passes into the pupal stage, which lasts five or six days, and then the perfect Insect appears. It is worthy of remark that the internal organs undergo quite as remarkable a change as the outer form does. The metamorphoses of some other Proctotrypidae have been examined by Ganin, and appear to be of an equally interesting character.[446]

There is reason to suppose that these _Platygaster_ parasites are of great economic importance as well as of scientific interest, for _Platygaster herrickii_ is one of the enemies of the larva of the destructive Hessian fly, _Cecidomyia destructor_.