External morphology of Lepidoptera
The external morphology of Lepidoptera is the physiological structure of the bodies of insects belonging to the order Lepidoptera, also known as butterflies and moths. Lepidoptera are distinguished from other orders by the presence of scales on the external parts of the body and appendages, especially the wings. Butterflies and moths vary in size from microlepidoptera only a few millimetres long, to a wingspan of many inches such as the Atlas moth. Comprising over 160,000 described species, the Lepidoptera possess variations of the basic body structure which has evolved to gain advantages in adaptation and distribution.
Lepidopterans undergo complete metamorphosis, going through a four-stage life cycle: egg, larva or caterpillar, pupa or chrysalis, and imago / adult. The larvae – caterpillars – have a toughened head capsule, chewing mouthparts, and a soft body, that may have hair-like or other projections, three pairs of true legs, and up to five pairs of prolegs. Most caterpillars are herbivores, but a few are carnivores or detritivores. Larvae are the feeding and growing stages and periodically undergo hormone-induced ecdysis, developing further with each instar, until they undergo the final larval–pupal moult. The larvae of many lepidopteran species will either make a spun casing of silk called a cocoon and pupate inside it, or will pupate in a cell under the ground. In many butterflies, the pupa is suspended from a cremaster and is called a chrysalis.
The adult body has a hardened exoskeleton, except for the abdomen which is less sclerotised. The head is shaped like a capsule with appendages arising from it. Adult mouthparts include a prominent proboscis formed from maxillary galeae, and are adapted for sucking nectar. Some species do not feed as adults, and may have reduced mouthparts, while others have them modified for piercing and suck blood or fruit juices. Mandibles are absent in all except the Micropterigidae which have chewing mouthparts. Adult Lepidoptera have two immobile, multi-faceted compound eyes, and only two simple eyes or ocelli, which may be reduced. The three segments of the thorax are fused together. Antennae are prominent and besides the faculty of smell, also aid navigation, orientation, and balance during flight. In moths, males frequently have more feathery antennae than females, for detecting the female pheromones at a distance. There are two pairs of membranous wings which arise from the mesothoracic and metathoracic segments; they are usually completely covered by minute scales. The two wings on each side act as one by virtue of wing-locking mechanisms. In some groups, the females are flightless and have reduced wings. The abdomen has ten segments connected with movable inter-segmental membranes. The last segments of the abdomen form the external genitalia. The genitalia are complex and provide the basis for family identification and species discrimination.
The wings, head parts of thorax, and abdomen of Lepidoptera are covered with minute scales, from which feature the order Lepidoptera derives its names, the word lepidos in Ancient Greek meaning "scale". Most scales are lamellar and attached with a pedicel, while other forms may be hair like or specialised as secondary sexual characteristics. The lumen, or surface of the lamella, has a complex structure. It gives colour either due to the pigments contained within it or through its three-dimensional structure. Scales provide a number of functions, which include insulation, thermoregulation, and aiding flight, amongst others, the most important of which is the large diversity of vivid or indistinct patterns they provide which help the organism protect itself by camouflage, mimicry, and to seek mates.
External morphology
In common with other members of the superorder Holometabola, Lepidoptera undergo complete metamorphosis, going through a four-stage life cycle: egg, larva / caterpillar, pupa / chrysalis, and imago / adult.Lepidopterans range in size from a few millimetres in length, such as in the case of microlepidoptera, to a wingspan of many inches, such as the Atlas moth and the world's largest butterfly Queen Alexandra's birdwing.
General body plan
The body of an adult butterfly or moth has three distinct divisions, called tagmata, connected at constrictions; these tagmata are the head, thorax, and abdomen. Adult lepidopterans have four wings – a forewing and a hindwing on both the left and the right side of the thorax – and, like all insects, three pairs of legs.The morphological characteristics which distinguish the order Lepidoptera from other insect orders are:
- Head: The head has large compound eyes and, if mouthparts are present, they are almost always a drinking straw-like proboscis.
- Scales: Scales cover the external surface of the body and appendages.
- Thorax: The prothorax is usually reduced.
- Wings: Two pairs of wings are present in almost all taxa. The wings have very few cross veins.
- Abdomen: The posterior abdominal segments are extensively modified for reproduction. Cerci are absent.
- Larva: Lepidoptera larvae are known as caterpillars, and have a well-developed head and mandibles. They can have from zero to five pairs of prolegs, usually four.
- Pupa: The pupae in most species are adecticous and obtect, while others are decticous and exarate.
Distinguishing taxonomic features
- the mouthparts
- the shape and venation of the wings
- * whether the wings are homoneurous or heteroneurous
- * whether the wings are aculeate or nonaculeate
- * the type of wing coupling
- the anatomy of the reproductive organs
- the structure of larva and position of primary setae
- whether the pupa is exarate or obtect
Head
Like all animal heads, the head of a butterfly or moth contains the feeding organs and the major sense organs. The head typically consists of two antennae, two compound eyes, two palpi, and a proboscis. Lepidoptera have ocelli which may or may not be visible. They also have sensory structures called chaetosemata, the functions of which are largely unknown. The head is filled largely by the brain, the sucking pump, and its associated muscle bundles. Unlike the adults, the larvae have one-segmented mandibles.The head capsule is well sclerotised and has a number of sclerites or plates, separated by sutures. The sclerites are difficult to distinguish from sulci which are secondary thickenings. The regions of the head have been divided into a number of areas which act as a topographical guide for description by lepidopterists but cannot be discriminated in terms of their development. The head is covered by hair-like or lamellar scales and found either as tufts on the frons or vertex or pressed close to the head.
The sensory organs and structures on the head show great variety, and the shape and form of these structures, as also their presence or absence, are important taxonomic indicators for classifying taxa into families.
Antennae
Antennae are prominent paired appendages that project forwards between the animal's eyes and consist of a number of segments. In the case of butterflies, their length varies from half the length of the forewing to three-quarters of the length of the forewing. The antennae of butterflies are either slender and knobbed at the tip and, in the case of the Hesperiidae, are hooked at the tip. In some butterfly genera such as Libythea and Taractrocera the knob is hollowed underneath. Moth antennae are either filiform, unipectinate, bipectinate, hooked, clubbed, or thickened. Bombyx mandarina is an example with bipectinate antennae. Some moths have knobbed antennae akin to those of butterflies, such as the family Castniidae.Antennae are the primary organs of olfaction in Lepidoptera. The antenna surface is covered with large numbers of olfactory scales, hairs, or pits; as many as 1,370,000 are found on the antennae of a monarch. Antennae are extremely sensitive; the feathered antennae of male moths from the Saturniidae, Lasiocampidae, and many other families are so sensitive that they can detect the pheromones of female moths from distances of up to away. Lepidoptera antennae can be angled in many positions. They help the insect in locating the scent and can be considered to act as a kind of "olfactory radar". In moths, males frequently have antennae which are more feathery than those of the females, for detecting the female pheromones at a distance. Since females do not need to detect the males, they have simpler antennae. Antennae have also been found to play a role in the time-compensated sun compass orientation in migratory monarch butterflies.
Eyes
Lepidoptera has two large, immovable compound eyes, which consist of a large number of facets or lenses, each connected to a lens-like cylinder that is attached to a nerve leading to the brain. Each eye may have up to 17,000 individual light receptors, which in combination provide a broad mosaic view of the surrounding area. One tropical Asian family, the Amphitheridae, has compound eyes divided into two distinct segments. The eyes are usually smooth but may be covered by minute hairs. The eyes of butterflies are usually brown, golden brown, or even red as in the case of some species of skippers.While most insects have three simple eyes, or ocelli, only two ocelli are present in all species of Lepidoptera, except a few moths, one on each side of the head near the edge of the compound eye. On some species, sense organs called chaetosemata are found near the ocelli. The ocelli are not homologous to the simple eyes of caterpillars which are differently named as stemmata. The ocelli of Lepidoptera are reduced externally in some families; where present, they are unfocussed, unlike stemmata of larvae which are fully focussed. The utility of ocelli is not understood at present.
Butterflies and moths can see ultraviolet light and wing colors and patterns are principally observed by Lepidoptera in these wavelengths of light. The patterns seen on their wing under UV light differ considerably from those seen in normal light. The UV patterns act as visual cues that help differentiate between species for mating. Studies have been carried out on Lepidoptera wing patterns illuminated by UV light.