Both the bee and the wasp are armed with an exceedingly venomous sting, as is well known. This structure takes the form of a well-adapted mechanical contrivance, and is a weapon of offence as well as of defence. The sting consists of two barbed needle-points, of a sufficient length to pierce the flesh to some depth. From the peculiar arrangement of their serrated edges their immediate withdrawal cannot take place, and it is this circumstance, with the drop of poison injected into the open wound, that renders their sting of the most painful and irritating kind. The gland containing the poison is contained in a minute sac situated at the root of the piercing apparatus. In [Fig. 392] is shown the sting of the wasp and the bee.
Very many insects are provided with instruments for boring into the bark or solid wood itself. The female Cynip bores into the oak-apple for the purpose of depositing her egg. The larva, when full grown, eats its way out of the nut, and drops to the ground, where it attains the form of the perfect fly ([Fig. 393]).
Fig. 393.—Female Gall-fly and Larva.
There are numbers of species living exclusively upon the leaves of plants, to which they do much damage by the excrescences or galls they form. Each tree seems to be infested by its own species of gall-mite, the so-called nail-gall of the lime being caused by a species named Phytoptus tibiæ. These galls take the form of a pointed column, standing erect on the upper side of the leaf. Galls of much the same structure occur in the sycamore, maple, elm, and various fruit trees.
The gnat (Culex pipiens) is furnished with a sting curiously constructed ([Fig. 394]), and enclosed in a perfectly clothed sheath covered throughout by scales or feathers. This is folded up when not in use. The mouth is provided with a complete set of lancets for piercing the flesh; after having inflicted a severe wound, it injects an acid poison through the proboscis. The scales of the gnat vary in structure accordingly as these are found on the wing, the body, or the proboscis. A magnified wing is shown at No. 2, [Fig. 394], and a magnified scale from the proboscis at No. 3. In [Fig. 405], Nos. 3 and 5, more highly magnified wing and body scales are given. The proboscis is protected on either side by antennæ and feelers.
Fig. 394.
1. Head of Culex pipiens, female Gnat, detached from body; 2. Wing, showing nervature and fringed edges; 3. Scale from Proboscis; 4. Proboscis and Lancets. The reticulated markings on each side of the head show the proportionate space occupied by the eyes.
The giant-tailed wasp, Sirax gigas, is furnished with an even more curious mechanical boring apparatus ([Fig. 395]) than its congeners. This is a boring ovipositor, skilfully contrived for piercing the bark of trees, in which the insect deposits her eggs, and where the larva, when hatched, will find an ample supply of food to carry it through this stage of existence. The boring tube, it will be seen, is a perfect muscular structure (c, c, a, and x); in short, it is an endless form of drill, well known to the mechanic, such as is employed in fine work for drilling holes. The females are of some size, and may be surprised and taken in the act of boring through the bark of the pine tree, for which they have a preference.