The _spiracles_, or orifices of the air-tubes, are situated on each side of the larva just above the legs. They are usually present on segments 2 and 5 to 12, but vary {x}considerably in different groups and families. The larva is provided with a very complete digestive system, which consists of the following organs (see Plate I., fig. 9): A, the oesophagus; D, the ventriculus; F, the clavate intestine; E, the ilium; H, the colon; K, the biliary vessels; and O, the spinning vessels. These last open at a small orifice in the labium termed the spinneret (fig. 2, 5). They supply the silken threads which are employed by most larvæ in constructing their cocoons, and which also serve in cases of danger as a rapid means of retreat. Many larvæ, which live on shrubs and trees, suddenly lower themselves to the ground by means of one of these silken threads, and thus often escape being devoured by insectivorous animals. The entire growth of the insect is accomplished during the larval condition, the increase in size being frequently very rapid. Owing to this circumstance larvæ are often compelled to shed their skin, and in many species a very considerable alteration both in the shape and colour takes place at each moult, or ecdysis as it is sometimes termed. THE PUPA. The pupa of a Lepidopterous insect is completely encased in a chitinous envelope. With the exception of a slight twirling of the abdominal segments it is incapable of any motion. In the pupa of _Micropteryx_ the mandibles and labial palpi are said to be functionally active, but this is a very exceptional though extremely interesting case. In conjunction with other evidence it would appear to indicate that the _Lepidoptera_ originated from insects with active pupæ. The number of free or movable segments of pupæ varies considerably in different groups and genera, and by some modern authors it is regarded as a character of much importance in the framing of their classifications. The various organs of the perfect insect are distinctly marked out on the otherwise uniform integument of the pupa. In some groups, notably the _Micropterygina_, these organs are much more distinctly indicated than in others. II.--ANATOMY. THE PERFECT INSECT OR IMAGO. In common with all other members of the class, the body of a Lepidopterous insect consists of three main divisions: (1) the head, (2) the thorax, and (3) the abdomen. THE HEAD. The front of the head is termed the _face_, the top the _crown_, the sides are nearly entirely occupied by the compound eyes (Plate I., fig. 11, AA), and the lower surface by the organs of the mouth. The _Eyes_ consist of a very large number of simple lenses arranged in the form of two hemispheres, one on each side of the head. The _ocelli_, or simple eyes, are situated on the crown, and are usually almost entirely covered by scales.