Insect external structure
Insect internal structure
Development of insects
Relationships of insects
Insect identification > Lepidoptera
The Lepidoptera are the moths and butterflies, which form one of the largest and most noticeable groups of insects. Its members are found in all countries and their large size in many cases, their brilliant colors and the habits of their larvae as well as the injuries they cause, have attracted much attention.
The adults have four large, membranous wings in most cases (a few have lost their wings), more or less completely covered by overlapping scales, making the wings opaque where these are present. Colors of the wings are due either to the presence of pigments in the scales; to optical colors caused by the surfaces of the scales breaking up the light striking them; or to both factors together.
The mouth-parts of the adult are greatly modified from those of chewing insects, though enough remains to show that the ancestors of the group must have fed by chewing. The development of the parts varies in different species, some of the lower forms having, as a whole, a much closer resemblance to the condition in chewing insects than is the case with most of them. In one group, the mouth-parts are sufficiently of the mandibulate type to enable the insects
In general a labrum or front lip is evident, but the mandibles are practically lost. The maxillae are extremely modified, a portion of each contributing its half to the formation of a proboscis or tongue. This is a flexible organ varying greatly in length, its two halves so interlocking as to form a tube between them, through which, when completely developed, fluids may be drawn into the mouth. The degree of development of the proboscis differs greatly in different Lepidoptera, and while it is functional in perhaps the majority of the group it is only partly developed or even rudimentary and useless in others. Such Lepidoptera evidently do not feed while adult.
In some cases the maxillary palpus is developed; in others it is nearly or wholly lacking. The labium or hinder lip is also practically absent except for the labial palpi which are usually large, thickly covered by hairs or scales, and project forward at the sides of the head, often turning upward somewhat, and partially or wholly concealing the proboscis when this is coiled up under the head, the place where it is carried when not in use.
The mouth-parts of the larva (or caterpillar as it is usually called) are entirely different. In this stage they are chewing structures, similar to those of a grasshopper in a general way, and no special description is needed. In the center of the end of the labium, however, is a slender projection called the spinneret, which at its tip has the external opening of the duct leading to the silk glands.
The antennae of adult Lepidoptera are usually quite long but vary greatly in their form in different species.
In the butterflies they are slender but enlarged near the tip forming a club, or with this enlarged part bent into a sort of hook. These forms of antennae are almost never found in the moths, where they may be simple and thread-like; with small hair-like projections at the side; bristles in place of the hairs; clusters of the bristles; with tooth-like or saw-like side projections; with long projections on one or both sides, in the latter case giving the antennae a feather-like appearance; and other forms also occur.
The eyes are large, though in some cases partly concealed by hairs or scales, which as a rule thickly clothe the entire body. Ocelli are also sometimes present.
On the top of the prothorax a pair of projections or lobes often occurs, called patagia, sometimes very large and capable of some movement, at other times smaller or even reduced to mere traces. On the large mesothorax is a somewhat similar pair of structures, called the tegulae, which extend backward over the point where each forewing articulates with the body. The abdomen may be long or short, stout or slender, connected with the thorax by either a broad or a rather constricted attachment. The legs are quite long and slender.
Characters by which the members of this group may be distinguished are
Insects which as adults have (with a few exceptions) four membranous wings more or less completely covered by overlapping scales; mouth-parts for sucking. The larvae have chewing mouth-parts. Metamorphosis complete.
The Lepidoptera is such a large order that great differences in its members are very common. The smallest ones are almost microscopic, while the largest one known may measure about a foot between the tips of its expanded wings. The wings of each side, to obtain their greatest efficiency, are more or less completely coordinated for flight by one of three methods.
In the butterflies and some of the moths, the basal portion of the costal region of the hind wing is enlarged, forming a sort of shoulder over which the hind margin of the forewing lies, thus enabling the two to a large extent to function as a single wing.
In most moths, however, instead of a shoulder, a rather long, curved bristle or cluster of bristles, called a frenulum, arises near the base of the hind wing and runs forward and outward, passing under a small flap or through a tuft of scales on the underside of the forewing, so that, as the two wings move in flight, this frenulum slides backward and forward in its track under the forewing and holds the two together. A third type of connection, found in only a few moths, is a small lobe near the base of the hind margin of the forewing, which extends backward toward the hind wing. This lobe is called a jugum and is also probably more or less effective in producing coordination in the use of the wings.
The number and arrangement of the wing veins are of great importance in the Lepidoptera, much of the classification in this order being based upon these structures. The main veins are, of course, longitudinal, starting at the point of attachment of the wing to the body and diverging toward its outer margin, some of them branching several times. Cross veins are very few, however, and consequently there are only a few closed cells, and some at least (perhaps all) of these are produced by the fusion of branches of longitudinal veins, rather than by true cross veins.
Various ways of designating the veins and their branches have been offered, but these are best comprehended in connection with laboratory work on the insects themselves and are therefore not given here.
The eggs of Lepidoptera vary greatly in form and also in color. They may be elongate, spherical, flattened, scale-like or of other forms, and the shell or chorion may be smooth or sculptured with ridges and reticulations. The eggs may be laid singly or in clusters and may or may not be covered with hairs from the body of the parent moth or with a secretion which conceals them from view. They may hatch in a few days or after longer periods, in some cases many months. The adults have no ovipositor so the eggs are always laid on the surface of the place of deposition; though if the abdomen of the insect be small, this may be in a small crack or other opening.
The larvae produced by the hatching of the eggs are called caterpillars and have no resemblance whatever to the adults they are to become. They are usually rather worm-like animals, with a generally recognizable head and a body consisting of a series of rather similar segments, the first three of which correspond to the thorax of the adult and almost always bear six legs. Some of the following segments will also have legs but these are totally different in structure from the others and are merely temporary in their nature, designed to support this portion of the body.
The internal structures of the caterpillar do not differ greatly in their arrangement from those of an adult insect, except that the reproductive organs are only slightly developed at this time, and that there is present, along each side of the body, a silk gland, large in those which will later need large quantities of silk, but present in all. A duct from each gland runs forward to the mouth where the two unite and open to the exterior through the spinneret already referred to.
Most caterpillars feed on plants or vegetable material. Their work is noticed chiefly by their stripping plants of their leaves, though some bore in stems, roots, fruit, seeds or other parts. A few attack feathers, silk, etc., but this is not the general habit. The larval stage may last only a few days for some species but is generally a month or more, and some feed during the fall, become quiet during the winter and complete their feeding the following spring.
A large majority of the caterpillars are termed naked, having only a few tiny spines or hairs, not large enough to be noticeable. From this condition every grade of density of covering occurs, to species entirely covered by long, thickly placed hairs which give the animal a hairy or "woolly" appearance. Some have large warts or horns on the thorax or a sort of horn above, near the hinder end of the body.
Their colors also vary greatly, some being brightly colored while others-green-either with or without white streaks, appear to seek concealment by their resemblance to the leaves on which they feed. Those living in protected situations, such as in plant stalks, are nearly white; cutworms which pass the day in the ground are dark as a rule, with rather faint markings.
When the caterpillar has become full-grown, it generally leaves the place where it was feeding and in some satisfactory location spins a cocoon around itself, using for this purpose the silk produced by its silk glands. In some species the cocoon is very complete, thick, tough, and entirely conceals the larva within. On the other hand, there are cocoons where only sufficient silk is used to attach the insect and hold it in place; and between these extremes all degrees of cocoon construction occur. Sometimes leaves, hairs from the body of the caterpillar or dirt when the insect enters the ground at this stage are incorporated in the cocoon.
Within the cocoon the caterpillar molts, leaving its cast-off skin at one end. The result of this molt is a pupa, its form showing through its new skin, which is generally brown. The outlines of the adult body and its appendages including the wings are evident, these last, however, being very small as there would be no room for the full-sized wings of the adult within the cocoon.
Internal changes and the completion of such external ones as are necessary now proceed until the adult insect has been entirely formed and is ready to escape. When this happens, another molt releases the insect from the brown outer pupa skin and, either before or after this, an opening in the cocoon is made and the adult emerges. It then crawls up on something and remains quiet for a while; its wings, being free to expand, increase rapidly till of their full size; the surplus fluids in the body are expelled, and after an hour or two the insect is ready for flight.
While for most Lepidoptera this outline of development is in general correct, in the butterflies we find that cocoon making is limited to attaching the hinder end of the body by silk to the object on which it is to pupate, and the formation of a silken loop around its body to hold it up. Such a pupa, producing a butterfly, is usually given the special name "chrysalis".
Besides the names "butterflies" (Rhopalocera) and "moths" (Heterocera) used to distinguish different sections of the Lepidoptera, we also have the terms "Microlepidoptera," or small moths, and "Macrolepidoptera," or large ones. These are wholly relative and rather indefinite but are, nevertheless, convenient in spite of the fact that it would be doubtful under which head to designate many species of the order.
The latest list of the insects of this order found in North America places them in about 70 families, but there are more of these divisions in other parts of the world. Some of the families include many species and insects of much economic importance, while others have only a very few. Only the more important families, either in size or because of the pests they contain, are included here.
Over 100,000 species of Lepidoptera are known.