Let us now turn to (_a_) Plate XII., which shows part of the wings on one side of a bee’s body. Along the top edge of the lower wing there is a row of tiny hooks, and the lower edge of the upper wing is curled over, thus forming a kind of ridge. When the bee takes to flight the front wing is stretched out from over the back, and during this action it passes over the upper surface of the back wing. When the ridge reaches the hooks it catches upon them and is held fast. In this manner the two wings are locked together. (_b_) Plate XII. shows the wings hooked together ready for flying. When the bee comes to rest she folds her wings, and in doing this they are automatically separated, for the ridge slips away from the hooks that hold it. [Illustration: PLATE XII (_a_) From a photo-micrograph by] [E. Hawks Wing unhooked, showing Hooklets and Ridge (_b_) From a photo-micrograph by] [E. Hawks Wing hooked, as in Flying] The number of hooks varies, and there are sometimes more on one side of the body than on the other. As a general rule it is found that a worker bee has from eighteen to twenty-three of them, the one shown in (_a_) Plate XII. having nineteen, as you will be able to count. The queen does very little flying, and so her wings are not large, in proportion to her size. Therefore she has not usually so many hooks, and sometimes they are found to number as few as thirteen. The drone has large and powerful wings, and his hooks vary between twenty-one and twenty-six in number. Bees are able to move their wings very quickly, and you will agree with me in this when I tell you that it has been shown that the vibrations number at least 190 per second! The flight of the bee is greatly assisted by a number of air-sacs called _tracheæ_, contained in the thorax. These fill with air and make the body more buoyant, just as a lifeboat is made more buoyant by its air-chambers. When a bee has been at rest for a little time it cannot begin to fly straight away, for the air-sacs are empty. It therefore runs along the ground to get a start, as an aeroplane does, and by vibrating its wings fills the tracheæ. CHAPTER XVI THE ABDOMEN The hinder part of the bee’s body is called the abdomen, and it is here that the stomach is situated. The abdomen is larger than either the head or the thorax, and is joined to this latter by a thin waist. Insects do not possess skeletons, at least not internal skeletons of bones, such as we have. Their skeletons are outside the body, and take the form of a hard outer layer which protects the soft inner organs. This layer, or outer skin, is made of a horny substance, called _chitine_ (pronounced “ki-tin”), which comes from a Greek word meaning a tunic or outer dress.