The order _Lepidoptera_, which includes all those insects commonly known as Moths and Butterflies, is chiefly distinguished by its members possessing four wings clothed with numerous minute scales, the term _Lepidoptera_ being derived from the two Greek words, [Greek: lepis], a scale, and [Greek: pteron], a wing. The mouth of these insects is suctorial, the maxillæ forming a spiral proboscis which is coiled up between the large labial palpi when not in use (see Plate I., figs. 5 and 6). The other oral organs are rudimentary. To acquire this form these insects pass through three very distinct stages, viz., the Egg, the Larva, and the Pupa. I.--METAMORPHOSIS. THE EGG. The eggs of _Lepidoptera_ are generally somewhat globular, much flattened above and beneath. Some are very elaborately sculptured, whilst others are quite smooth. They are usually white or yellowish, but always change much in colour as the contained embryo develops. THE LARVA. The larvæ of moths and butterflies are popularly known as caterpillars. They always consist of thirteen segments, segment number one being the head. The head is furnished with several simple eyes (Plate I., fig. 2, AA), a pair of very short antennæ (BB), and a very powerful masticatory mouth. The mouth consists of the following organs: The labrum, or upper lip (1); a pair of mandibles, or upper jaws, working like scissor-blades (2,2); two maxillæ, or lower jaws (3,3), each carrying a jointed organ termed the maxillary palpus; and the labium, or lower lip (4); which bears another pair of minute jointed appendages--the labial palpi. Segments 2, 3, and 4, which answer to the thorax of the perfect insect, are each furnished with a pair of legs. They consist of the six following joints (fig. 2): (_a_) coxa, (_b_) trochanter, (_c_) femur, (_d_) tibia, (_e_) tarsus, and (_f_) claw. These legs correspond to those of the perfect insect. The remaining nine segments of the body constitute the abdomen. Usually segments 7 to 9 and 13, each have a pair of fleshy pads, which are termed prolegs and are furnished on their edges with a row of minute hooklets (see Plate I., fig. 14, proleg highly magnified). It is these hooklets which enable caterpillars to hold on by means of their prolegs with such great tenacity. The number of the prolegs varies considerably in different groups and families.