consumer of the juices of Insects in its young stage as it is in the adult condition. Hence Brauer[86] divides Insects, as regards this point, into three categories. The forms in which both the young and adult take food by suction he calls Menorhyncha; those in which both the imago and immature forms feed by mandibles he calls Menognatha; while his Metagnatha consists of those insects that take food by jaws when young, but by suction with tubular mouths when mature. Besides these main divisions there are some exceptional cases to which we need not here allude, our present object being to indicate that in the Metagnatha the digestive organs are of a very different nature in the young and in the adult states of existence. The internal organs for the continuance of the species are known to be present in a rudimentary stage in the embryo, and it is a rule that they do not attain their full development until growth has been completed; to this rule there may possibly be an exception in the case of the Aptera. But little information of a comparative character exists as to the dorsal vessel and the changes it undergoes during metamorphosis. There is considerable difficulty in connexion with the examination of this structure, but it appears probable that it is one of the organs that changes the least during the process of metamorphosis. The exact nature of the internal changes that occur during metamorphosis is almost a modern subject. It is of course a matter of great difficulty to observe and record changes that go on in the interior of such small creatures as Insects, and when the phenomena occur with great rapidity, as is frequently the case in Insect metamorphosis, the difficulty is much increased. Nevertheless the subject is of such great interest that it has been investigated with a skill and perseverance that call for the {162}highest admiration. The greater part of the information obtained refers to a single Insect, the blowfly; and amongst those who have made important contributions to it we may mention Weismann,[87] Viallanes,[88] Ganin,[89] and Van Rees,[90] and it is at present under investigation by Lowne. A good deal, too, is becoming known about the processes in the case of the silkworm. INTEGUMENT AND ECDYSIS.