To perform this by mere force, two things are necessary, power of muscles, and strength of sting; neither of which they seem to possess in a sufficient degree. Mr. J. Hunter thinks that it cannot be by simple force, because the least pressure bends the sting in any direction. It is probable that the serrated edges may assist, by cutting their way like a saw. The apparatus for the poison consists of two small ducts, which are the glands that secrete the poison; these lie in the abdomen among the air cells, they soon however unite into one oblong bag; at the opposite end of which a duct passes out, which runs towards the angle where the two stings meet, and, entering between them, forms a canal by the union of the two stings at this point. From the serrated construction of the stings the bee can seldom disengage them, and hence, when they pass into materials of too strong a nature, the bee generally leaves them behind, and often a part of the bowels therewith.[60] [60] Phil. Trans. for 1792, page 189. DISTINGUISHING CRITERIA OF INSECTS. It has already been observed, that the bodies of insects are covered with a hard skin, answering the purpose of an internal skeleton, and forming one of the characters by which they are distinguished from other animals. This external covering is very strong in those insects which, from their manner of life, are particularly liable to great friction, or violent compression; but is more tender and delicate in such as are not so exposed. The skin of insects, like that of larger animals, is porous; the pores in some species are very large; many insects often change or cast off their skin; this exuvia forms an excellent object for the microscope. Another distinguishing criterion of insects is the colour of their circulating fluid or blood, which is never red; this, at first sight, seems liable to some objections, on account of the drop of red liquor which is often procured from small insects when squeezed or pressed to pieces. It does not appear, however, that this is the blood of the little animal; when it existed as a worm there was no such appearance, and when transformed to the perfect, or fly state, it is only found in the eye, and not in the body, which would be the case if it circulated in the veins of the insect. It is probable there is a circulation of some fluid analogous to the blood in most insects: with the assistance of the microscope this circulation may be perceived in many; but the circulating liquor is not red. To these discriminating characteristics we may also add the following particulars: 1. That the body of insects is divided by incisuræ, or transversal divisions, from whence they take their name. 2. That they are furnished with antennæ, which are placed upon the fore part of the head; these are jointed and moveable in various directions.