In the caterpillar there are no traces of the external sexual organs, so that the two sexes cannot be distinguished on superficial inspection; it was however long ago demonstrated by Herold[187] that the ovaries and testes exist in the youngest caterpillars, and undergo a certain amount of growth and development in the larval instars; the most important feature of which is that the testes are originally separate but subsequently coalesce in the middle line of the body, and become enclosed in a common capsule. In a few forms—especially of Liparidae—(Lymantriidae of modern authors)—the caterpillars are said to be of different colours in the two sexes. Most of what is known on this point has been referred to by Hatchett Jackson.[188] The Silk-glands of Lepidoptera are of great interest from the physiological point of view, as well as from the fact that they have furnished for many ages one of the most beautiful of the adornments made use of by our own species. The sericteria, or vessels that secrete silk, are of simple structure, and differ greatly in their size in the various forms of the Order; they sometimes become of great length; in the Silk-worm each of the two vessels is nearly five times as long as the body, while in _Bombyx yamamai_ and others, even this is exceeded. They {326}grow with remarkable rapidity, being in the young silk-worm only 3 mm. long, in the adult 22 mm. The increase in weight is still more remarkable; when the silk-worm is thirty-one days old, the sericteria weigh only 3 mgr., but when the age is fifty days their weight has increased to 541 mgr., being then ⅖ of the whole weight of the body. In the pupa they undergo a gradual atrophy, and in the moth they are, according to Helm, no longer to be found, though earlier authors were of a contrary opinion.[189] According to Joseph,[190] the silk-vessels begin to develop at an extremely early age of the embryo, and are very different in their nature from the salivary glands, the former being derivatives of the external integument (ectoderm), while the salivary glands belong to the alimentary system. This view is to some extent confirmed by the observations of Gilson as to the different manner in which these two sets of glands discharge their functions.