than the egg, as may be seen by referring to the diagrams in Plate III, where the eggs and ovipositors are all taken from photographs, and are drawn of the same amplification. It follows therefore that one end of the egg cannot be sunk in the plant tissue while the other is still in the canal, and the explanation of Hartig consequently fails.

Besides, it is not possible that the whole egg could be received into the ovipositor and could glide through it, for the ovipositor cannot be compared to a hollow tube. It consists, as already stated, of three parts which are firmly connected with each other. The upper is the ridged seta, on the under surface of which the two spiculae are mortised by two tenons. The seta certainly encloses a central cavity, but it has no connexion with the canal, and merely serves to contain a nerve branch, an air vessel, and some sanguineous fluid. Therefore the egg cannot pass through the ovipositor in the way Hartig supposed. On the other hand, there is sufficient space between the two spiculae to admit the egg-stalk.

It is still difficult to understand how the egg is finally protruded into the bud. We can clearly recognize the movements of the ovipositor made by the fly, but we are unable to follow the movements of the egg, and only in one way do we get any knowledge of this part of the operation. Neuroterus laeviusculus requires from fifteen to twenty minutes for the act of ovipositing. If a pricking fly were fixed in position, by being suddenly dipped into chloroform or ether, and the bud were opened, we could then see how far the ovipositor had penetrated, and whereabout the egg was: and if, during the fifteen

or twenty minutes occupied by the laying of the egg, a pricking fly could be fixed in position every successive minute, the buds would yield us a long series of preparations exhibiting the different stages in the birth of an egg. Unfortunately this idea cannot be carried out, on account of practical difficulties. In the first place the time occupied in pricking the buds is not always the same, and in the next each separate act is itself of uncertain duration, since the fly has greater obstacles to overcome in some cases than in others. It is therefore only possible to learn to recognize some of its different stages, and then from these to construct the whole operation.

We shall begin with the moment when the fly places its ovipositor on the bud. She always chooses the edge of one of the outer scales as a point of attack, and pushes her ovipositor under it. Then the ovipositor glides under the scales to the base of the bud-axis. Even this first act requires great strength on the part of the fly. We sometimes see it attack the bud repeatedly with its ovipositor, before it succeeds in getting it under the scales. It does not succeed with buds in which the scales are closely imbricated, hence it always prefers buds with loose-lying scales. When the ovipositor has arrived at the base, it is driven towards the bud-axis so as to reach the rudimentary leaves; but the path made by the ovipositor is always more or less curved. By making a careful preparation of any pricked bud, the canal can be plainly seen, and the path taken by the ovipositor followed. After the fly has finished the first part of its work, and

driven the ovipositor into the centre of the bud, there comes a moment of complete rest, and the fly sits motionless upon the bud. If it is fixed in this position by dipping it into chloroform, nothing is seen of the egg, it still remains in the vagina. Then follows the second part of the work, the pushing of the egg into the bud.

The egg slips, with its enclosed egg-body, to the base of the ovipositor between the origin of the two spiculae. The egg-body glides over the point where the two spiculae embrace the tenon of the seta, since the space remaining open between the two spiculae is too small to admit it. But the egg-stalk, which follows, slips between the two spiculae, is seized by them, and driven forward; in this way the egg is pushed downwards into the ovipositor, with the egg-body hanging out.

When at last the egg is about to enter the canal which has been bored into the centre of the bud, it becomes evident that it is impossible for the canal to admit the ovipositor and the egg-body to pass in at the same time. The egg-body is always of much greater diameter than the ovipositor; on this account the ovipositor is next partially withdrawn by the fly, until the pierced canal becomes empty; the egg-body then enters the pierced canal, and the ovipositor follows, pushing it before it. In short, the whole forward motion is dependent on the egg-stalk being propelled by the to and fro movements of the two spiculae, and the egg reaches the end of the bored canal, while the egg-stalk remains lying within it.

The process of egg-laying, though it seems rather com

plicated, can be divided into three stages. (1) The canal is bored, the ovipositor gliding under the imbricated scales to the base of the bud, and then being driven into the centre of the bud-axis. (2) The egg passes out of the ovarium to the base of the ovipositor, where the eggstalk 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.

If we consider all this varied procedure, we cannot but be astonished at the accuracy with which the fly carries it out, and repeats it many times in succession. Only one egg can ever pass through the same canal; there is no room for a second, because the egg-stalk of the first egg remains lying in the canal.

Those flies which lay their eggs in leaf surfaces have naturally a much easier task, since they have only to pierce a thin membrane. The mechanism of ovipositing however is exactly the same.

We shall next consider a provision existing in the terebra, by which the fly is enabled to carry out all the operations required in ovipositing, with the greatest exactitude. For this purpose its rigid chitinous armour is furnished with tactile hairs at various points. The tactile organs peculiar to insects consist of fine hairs, connected at their bases with ganglionic swellings of sensory nerve-matter, and they are found at various parts of the ovipositing apparatus. They occur constantly, in all hymenoptera, on the arch of the anterior plate. Their number varies in different species from

twenty to fifty. We must not ascribe any mechanical function in the process of egg-laying to these delicate hairs; they are merely tactile organs, each hair being in connexion with a nerve fibre. These nerve fibres all arise from the large abdominal ganglion, which also gives off motor fibres to the piercing apparatus1. The tactile hairs scattered over the arch have the important function of informing the fly as to the true position of the egg. While the egg is gliding over the hard chitinous covering of the ovipositor, the fly only becomes conscious of its progress from one stage to another, when it touches a tactile hair; these serve to signal its advance. Such hairs are consequently placed more closely on the arch, where the egg is caught between the two spiculae; and by their means the fly is guided exactly to where the egg-body is to be found. When the egg has reached the proper point, the egg-stalk is apparently seized by a rapid to and fro movement of the two spiculae. Thus all the time the egg is being pushed down to the ovipositor, there is a provision for keeping the fly informed of its progress by sensation. There are also on the seta, particularly towards its point, organs of sensation, not in the form of hairs but of slight projections of the chitinous membrane. There are besides, in some hymenoptera (Platygaster), perfect hairs on the point of the ovipositor. The sensory branches of the nerve, which is contained in the central

1 In no other order of insects is the abdominal ganglion so power. fully developed as in the hymenoptera; this is due to the fact that it has to undertake the innervation of the complicated piercing apparatus.