The Insect World: Being a Popular Account of the Orders of Insects

The fly always comes out at the same end of the cocoon, that is, at the end where its head is placed, and also where the head of the larva previously was. This end is composed of two parts — of two half cups placed one against the other. These can be detached from each other and from the rest of the cocoon. It is sufficient for the fly that one can be detached, and in order to eflect this, it employs a most astonishing means. It expands and contracts its head alternately, as if by dilatation; and thus pushes the two half cups away from the end of the cocoon. This is not long able to resist the

tattering of the fly's head, and the insect at length comes out triumphant. This fly, which should be blue, is then grey; it, however, comes quickly to perfection, at the end of three hours attaining its ultimate colour; and in a very short space of time every part of the animal becomes of that firmness and consistency which characterise it. At the same time, the wings, which at the moment it came into the world were only stumps, extend and unfold themselves bydegrees. The meat-fly is represented in Fig. 55.

One of the features in the formation of this fly which most attracted the attention of Reaumur, and which is likely to excite the curiosity of all those who take an interest in insects, is the composition of its trunk. We will, therefore, with that illustrious observer, take a glimpse at the remarkable and complicated apparatus by the aid of which the fly can suck up liquids, and can even taste solid and crystalline substances, such Blue-bottle fly {Callipforl\omitoria\ magnified. as sugar. It is no difiicult matter to make a fly show its trunk extended to its full extent. One has only to press between the finger and thumb either the two sides of the upper and under part of the thorax. It is thus forced at once to put out its tongue.

The trunk appears to be composed of two parts joined together, and forming a more or less obtuse angle (Fig. 56). The first portion

Fig 56.— Trunk of the Meat-flr. F'g 57.— Conical part of the trunk.

of the trunk, that which joins the head, is perfectly membranous and in the form of a funnel. We will call it the conical part, and show it separately (Fig. 57). The second portion terminates in a thick mass, in part cartilaginous or scaly, and of a shiny brown colour. Alcove

the conical portion are two oblong antennae, without joints, of chestnut colour, and furnished with hairs.

On ceasing to press the thorax, the membranous conical portion may be seen to draw itself back within its sheath (Fig. 58). The second portion is at the same time drawn into the cavity, but it raises itself by forming a more and more acute angle, so that when it reaches the opening of the cell it is parallel with, and its length is equal to that of the cell, which is quite large enough to receive it. The base lengthens and flattens a little, and conceals the trunk.

Let us cause the trunk to extend itself a second time, in order to observe its tip minutely. Here the opening is placed, which may b^ looked upon as the mouth of the insect, and is provided with two large thick lips (Fig. 59). These lips form a disc, perpendicular to

Fig. 58. _ Fig. 59. Retractile prolioscis of Bhie-bottle fly. Extremity on the proboscis of a fly.

the axis of the trunk; the disc is oval, and is divided into two equal and similar parts by a slit. The hps have each a considerable number of parallel channels situated perpendicularly to the slit. These channels are formed by a succession of vessels placed near each; other. On pressing the trunk we see that these vessels are distended by a liquid. Reaumur, from whom w^e borrow these details, discovered a few of the uses to which this trunk is applied. He covered the interior of a transparent glass vase with a light coat oi thick syrup. He then put in some flies, when it was easy to see some of them proceed to fix themselves to the sides of the vase, and regale themselves on the sugary liquid, of which they are very fond. He observed them carefully, and in his admirable work he recom mends those who are curious to try the experiment, with which, likf himself, they will certainly be satisfied.

While the body of the trunk is stationary its end is mucl agitated. It may be seen to move in diflerent ways, and with ar astonishing quickness; the lips acting in a hundred difierent ways and always with great rapidity. The small diameter of the disi

which they form lengthens and shortens alternately; the angle formed by the two lips varies every instant; they become successively flat and convex, either entirely or partly. All these movements, Re'aumur remarks, give a high idea of the organisation of the part which performs them. The object of all these movements is to draw the syrup into the interior of the trunk. If we observe the lips (Fig. 60) attentively, it will easily be seen that they touch each other about the •centre of the disc, and leave two openings, one in front, •the other at the back. The one in front is, one may •say, the mouth of the fly, as it is to this opening that the liquid is brought, which is intended to be and is soon introduced into the trunk. Without occupying ourselves for the present with the channel through which pig. 60. it 1rises,.,we may•'first, ask,-whatever,,..,.that channel.- may•'be,' V^^boscis?^ of*r^^a fly.i^''°" what IS the power that forces the liquid into it?

It is nearly certain that suction is the principal cause of the iiquid flowing up the trunk. It would thus be a sort of pump, into -which the liquid is forced by the pressure of the external air. The fly exhausts the air from the tube of its trunk, and the drop of liquid ^hich is at the opening penetrates and goes up this channel through the influence of the atmospheric pressure. To this physical phenomenon must be added the numerous and multiplied movements -which take place in the trunk, and which are intended to cause suflicient pressure to drive the liquor which is introduced jnto^ the ■channel upwards.;., a^' Reaumur wished to know how it was that very thick. syr,ups, and ■even solid sugar, can be sucked up by the soft trunk of the fly. What he saw is wonderful. If a fly meets with too thick a syrup, it can render it sufficiently liquid; if the sugar is too hard, it can dis- •solve small portions of it. In fact, there exists in its body a supply •.of liquid, of which it discharges a drop from the end of its trunk at Tvill, and lets this fall on the sugar which it wishes to dissolve, or on the syrup it wishes to dilute. A fly, when held between the fingers, .X)ften shows at the end of its trunk a drop, very fluid and transparent, of this liquid. "The water poured on the syrup," says Re'aumur, " would not always insinuate itself sufficiently quick into -every part of it; the movement of the fly's lips hastens the operation; the lips turn over, work, and knead it, so that the water can quickly penetrate it, in the same way as one handles and kneads with one's hands a hard paste which it is wished to soften, by causing the water by which it is covered to mix with it. This, again, is the same

^^ mE INSECT WORLD.

means the fly employs with sugar. When the trunk is forced to act upon a grain of irregular and rugged form, on which it cannot easily fasten, its end distorts itself to seize and hold it. It is sometimes very amusing to see how the fly turns over the grain of sugar in different ways; it appears to play with it as a monkey would with an apple. It is, however, only that it may hold it well in order to moisten it more successfully, and afterwards to pump up the water which has partly dissolved it."

Reaumur often observed a drop of water at the end of the trunks of flies which were perfectly surfeited with food. This drop ascended the trunk, then re-descended to the end, and this many times in succession. It appeared to him that it was necessary for these insects, as for many quadrupeds, to chew the cud, as it were; that, in order the better to digest the licjuid they had passed into their stomachs, they were obliged to bring it back into the trunk that it might return again better prepared. In order to assure himself directly of the reality of his supposition, Reaumur tested the Water which a fly, that he says " had got drunk on sugar," had brought back to the end of its trunk; he found this to be sugar and water. Also, having given a fly currant-jelly, he observed, after it had sufficiently gorged itself, several drops of red liquid in its tnmk, and having tasted it, found it had the flavour which, from its appearance, he guessed it would have.

The illustrious observer, who had already made all these discoveries on the formation and functions of the trunks of insects, often reflected on the fact that the liquors of which flies are most fond are enclosed under the skin of certain fruits, such as pears, plums, grapes, &c., or even under the skin of some animals of which they suck the blood. In order that the trunk of a fly may act under such circumstances, itis necessary for it to pierce and open the skin. If this is the case, flies ought to be possessed of a lancet. He looked a long time for this lancet, and at last found it. It is situated on the upper side of the part of the trunk which is terminated by the lips; it is placed in a fleshy groove, and is enclosed in a case. It has a very fine point, and is of light colour (Fig. 6 1 ). The point is situated in the opening which is to be seen between the lips of the trunk, at its anterior end, through which liquids may pass. That is the only opening of the lips; and the sucker which takes up the liquid is the same part which we just now called the case of the lancet.

Reaumur is so interesting an author that it is difficult to cease quoting him; but we must continue our review of the principal kinds of Diptera.