Homes Without Hands: Being a Description of the Habitations of Animals, Classed According to Their Principle of Construction
THOSE splendid insects which are popularly called RUBY-TAILED FLIES, or FIRETAILS, and scientifically are termed Chrysididæ, are also to be numbered among the parasitic insects.
They make no nests for themselves, but intrude upon those of various mason and mining bees, and several other insects. The Firetail does not, however, lay its eggs in the body of the larva, but makes its way into the nest while the rightful owner is absent, and places an egg near that of the bee. The egg of the parasite is sometimes hatched at the same time with that of the bee, but generally later. In the first instance, the larva feeds on the provisions which were supplied for the bee, and so starves the poor creature to death; and in the latter case, it is not hatched until the young bee is large and fat, and capable of affording ample subsistence to the parasite, which fastens upon it and devours all the after portions.
Then there are the CUCKOO FLIES (Tachinæ), which bear some resemblance to the common house-fly, but which are parasitic, feeding on the larvæ of other insects, and selecting the same species which are persecuted by the firetails. When the Tachina larva has eaten that of the mason bee, it forms an oval cocoon, and there remains until the time for becoming a perfect insect. A single larva of the mason bee seems to be sufficient for the Tachina grub, as Mr. Rennie has recorded an instance where two larvæ of the mason bee were in a nest into which a single egg of the Tachina had been introduced. The parasitic larva devoured one of the rightful inhabitants, but did not touch the other, and the cocoons of the bee and the Tachina were formed side by side.
SOMETIMES, as has already been mentioned, the chrysalis itself of a lepidopterous insect becomes the home of the parasite. I have found the pupæ of various butterflies absolutely filled with tiny ichneumon flies of the most brilliant colors; and in the British Museum there is an excellent example of a chrysalis, which has been filled by a single ichneumon fly, of such a size that the little chrysalis from which it was taken seems scarcely capable of holding it and its cocoon.
WE now pass to a remarkable series of insects belonging to the same order as the ichneumons, but parasitic upon vegetables and not on animals. Their scientific name is Cynipidæ, and they are popularly known as GALL FLIES, because they cause those singular excrescences which are so familiar to us under the name of Galls. This group comprises a vast number of species, all of which have a strong family resemblance, though they greatly differ from each other in size, form, and color.
British Gall.
<!-- pandi-media-anchor:figure-p509-o82-d951fb621cc9aa02 -->Leaf Galls of Oak. Bedeguar of Rose. Cynips Kollari (slightly magnified). Oak Apple.
Galls of Cynips Kollari. Currant Galls of Oak.
In the accompanying illustration are given several examples of British Galls, most of which are tolerably common in this country, and some of which can be found in plenty.
In the left-hand upper corner of the illustration is a figure of an oak leaf, upon which are two globular projections. These are the well-known "cherry galls," which are made by a little insect called Cynips quercus-folii. They are beautifully colored, some being entirely scarlet, while others are white, orange, and red, in various gradations, something like the color of a nearly ripe peach, or those of a Newtown pippin. Perhaps they bear more resemblance to the apple than to the peach, because their surface is highly polished and shining, much like that of the American apple.
These galls may be found in profusion upon the oak leaves, and are most plentiful upon pollard oaks, upon the youngest trees, or upon the oak underwood that sprouts around a felled trunk. In such cases the leaves are much larger, and fuller of juice than those which spring from adult trees, and the development of the gall is proportionately increased. Wherever there is a thick growth of oaken underwood, the numberless galls which stud the leaves have a remarkably beautiful effect, provided that the observer lies on the ground, or stoops sufficiently low to perceive the under surface of the leaves, to which the galls are attached.
If one of the galls be cut open with a knife, it will be found to consist of a soft pulpy substance, fuller of juice than an apple, and somewhat resembling the consistence of a hothouse grape. In the very centre of the soft mass the knife will meet with resistance in the shape of a globular cell of hard woody texture, and in the middle of the cell will be found a tiny grub, perfectly white, very fat, somewhat resembling the grub of the humble-bee, and curved so as to fit the globular cell in which it lies. This is the little being for whose benefit the gall was formed, and the little white grub feeds on the juices of the gall, precisely as the larva of the ichneumon fly feeds on the soft portions of the insect in which it temporarily resides.
On seeing the little creature thus snugly ensconced in the receptacle which serves it at once for board and lodging, a question naturally arises as to the manner in which it was placed there. No aperture is perceptible in the gall, not a hole through which air can reach the inclosed larva, which must, therefore, be capable of existing without more air than can pass through the minute pores of the vegetable substance in which it lies, or must be able to respire by means of the oxygen which is given out by living plants.
The question, indeed, is very like the well-known query as to the manner in which a model of a wagon and four horses can find its way into a bottle, the neck of which is so small as to prevent even the head of the wagoner from passing. The answer is similar in both cases. The bottle was ingeniously blown over the wagon and horses, and the gall was formed around the grub.
When the leaf is in its full juiciness, and the sap is coursing freely through its textures, a little black insect comes and settles upon the leaf. She is scarcely as large as a garden ant, but has four powerful and handsome wings, which can be used with much agility. An entomologist, on seeing her, would at once pronounce her to belong to the order Hymenoptera and to be closely allied to the ichneumon flies which have just been described.
Running to and fro upon the leaf, she fixes upon one of the nervures, and there remains for a short time, evidently busy about some task, which is very important to her, but which her minute size renders impossible to be observed with the naked eye. If, however, a magnifying-glass be applied very carefully to the leaf, the following process will be seen.
From the abdomen there projects a tiny hair-like ovipositor, which is coiled in such a manner that it can be protruded to a considerable length. This ovipositor is thrust into the leaf, so as to produce a hole, which is widened by the action of the boring instrument. Presently the blades of the ovipositor separate, and a single egg is seen to pass between them, so that it is lodged at the bottom of the hole. Into the same aperture is then poured a slight quantity of an irritating fluid, and the insect flies away, having completed her task. The whole proceeding, indeed, is, with the exception of the deposition of the egg, precisely the same as that which takes place when a wasp uses its sting, the ovipositor and sting being but two slightly different forms of the same organ, and the irritating fluid of the cynips being analogous to the poison of the wasp.
The effect of the wound is very remarkable. The irritating fluid which has been projected into the leaf has a singular effect upon its tissues, altering their nature, and developing them into cells filled with fluid. As long as the leaf continues to grow, the gall continues to swell, until it reaches its full size, which is necessarily variable, being dependent on that of the leaf. I have, for example, many specimens of these galls, of different sizes, from which the insects have escaped, showing that they had attained their full size. On the juices of the gall the inclosed insect lives until it reaches its full term of imprisonment, when it eats its way through the gall and emerges into the world. In some cases it undergoes the whole of its change within the gall, but in others it makes its way out while still in the larval state, burrows into the earth, and there changes into the pupal and perfect forms.
To the unassisted eye, the insect which forms the leaf gall presents no especial attraction, as it is simply, to all appearance, a little black fly. When placed under the microscope, however, it soon proves to be a really beautiful creature, though not possessing the brilliant and gem-like hues which distinguish many of its relatives. The body still retains its blackness, but has a soft tint on account of the white and shining hairs with which it is thickly studded. The eyes are large, stand boldly from the head, and the many lenses of which these organs are composed are so boldly defined, that even in so small an insect they can be distinguished with a very low power of the microscope. Indeed, the inch and a half object-glass is quite powerful enough to define them, while the half-inch glass makes them look like the pits in a lady's thimble.
The chief beauty of the insect, however, lies in the wings, which are very large in proportion to the size of the owner, are traversed by a few, but strong nervures, and glow with a changeful radiant lustre, like mother-of-pearl illuminated with living light. In order to see these wings properly, the insect should be laid on some black substance, and the light concentrated upon them by the various means which a microscopist can always employ.
THE oak is a tree that seems to be especially loved by gall insects, which deposit their eggs in its leaves, its twigs, its flowers, and even in its roots. One of the most familiar examples of oak galls is that which is called the oak apple, and which is produced by a species of insect called Cynips terminalis. Although the insect is not of very great size, the gall which it produces is sometimes enormous, being as large as a common golden pippin or nonpareil apple, and therefore very conspicuous upon the tree. It is colored in the same manner as the cherry gall, but seldom so brilliantly, and the exterior is not so smooth and polished.
The resemblance to a veritable fruit is much closer at the beginning of the season than in the autumn, as a number of small leaf-like projections surround its base, just as if they were a half-withered calyx. These, however, fall off as the summer advances, and are no more seen.
If the oak apple be cut with a knife, the first touch of the steel betrays a marked difference between its substance and that of the cherry gall. Its texture is neither so firm nor so juicy, but is of a softer, drier, and more woolly character. Moreover, the knife passes through several resisting substances, which, when the gall is quite severed, prove to be separate cells, each containing a grub. From each of these cells, which are extremely variable in number, a kind of fibre runs toward the base of the gall, and it is the opinion of some naturalists that these fibres are in fact the nervures of leaves which would have sprung from the bud in which the gall-fly has deposited her eggs, and which, in consequence of the irritating fluid injected into the tree, are obliged to develop themselves in a new manner.