Mouth suctorial, consisting of a spiral trunk or "antlia," composed of the greatly-elongated maxillae, and protected, when not in use, by the cushion-shaped hairy labial palpi. Maxilla forming two sub-cylindrical tubes, united together by inosculating hooks, and constituting an intermediate tube by their junction. Maxillary palpi minute; labrum and mandibles rudimentary. Head, thorax, and abdomen more or less covered with hair. Wings, four in number, covered with modified hairs or scales; wanting in the females of a few species. Nervures not very numerous, mostly longitudinal. Antenna composed of numerous minute joints.

This well-known and most beautiful of all the orders of Insects comprises the Butterflies (fig. 189) and the Moths (fig. 190); the former being diurnal in their habits, the latter mostly crepuscular or nocturnal.

The larvae of Lepidoptera (fig. 189), commonly called "caterpillars," are vermiform in shape, normally composed of thirteen segments, the first of which forms a distinct horny head, with antennae, jaws, and usually simple eyes. The mouth of the caterpillar, unlike that of the perfect insect, is formed for mastication. The labium, also, is provided with a tubular organ - the "spinneret" - which communicates with two internal glands, the functions of which are to furnish the silk, whereby the animal constructs its ordinary abode or spins its cocoon. The viscera are embedded in a largely developed fatty tissue (epiploon), which is absorbed by the pupa during its period of quiescence. The three segments behind the head correspond with the prothorax, mesothorax, and metathorax of the perfect insect, and each carries a pair of jointed walking-legs. Besides these thoracic legs, there is a variable number (generally five pairs) of soft fleshy legs, which are borne by the segments of the abdomen, and are known as "pro-legs." Each is usually furnished with a crown of several rows of small horny hooks, and they are never attached to the 4th, 5th, 10th, and nth segments behind the head (i.e, to the 1st 2d, 7th, or 8th abdominal segments).

In the Diurnal Lepidoptera, or Butterflies proper (fig. 189), the antennae are knobbed (hence the name of Rho-palocera often given to the group); the wings are usually held erect when the insect is in a state of repose; the larvae have six thoracic legs, and ten pro-legs; and the pupae are always naked, attached by the posterior extremity, or head downwards, and usually angular.

In the Crepuscular Lepidoptera, including those forms which are active during the twilight, the antennae are fusiform, or grow gradually thicker from the base to the apex; the wings are horizontal or a little inclined when the insect is at rest; the posterior wings generally have their front margins furnished with a rigid spine ("retinaculum") which is received into a hook on the under surface of the anterior wings; and the pupae are never angular.

The Nocturnal Lepidoptera have the antennae setaceous, or diminishing gradually from the base to the apex, often serrated or pectinated (fig. 190); the wings in repose are horizontal or deflexed, and the hind-wings are often furnished with a "retinaculum," as in the preceding section; the pupae are mostly smooth, sometimes spiny, and often enclosed in a cocoon.

The two groups of the Crepuscular and Nocturnal Lepidoptera are often included in a single division, under the name of Heterocera. It is to be remembered that many members of the Nocturnal division of the order, though they would ordinarily be called Moths, are active during the day, and in this respect resemble the true Butterflies.