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THE LEPIDOPTERA - Their origin and classification
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Paul Smart - The illustrated encyclopedia of the Butterfly world |
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About three-quarters of all animal species are insects. Their present day abundance and diversity of form suggest a long land complex evolutionary history. In some orders of insects, at least part of this history may be traced by the study of fossils, but butterflies are delicate creatures whose bodies are likely to disintegrate after death and be lost rather than preserved as fossils. Consequently our knowledge of the past history of butterflies is very incomplete and details of their ancestry remain conjectural.
Butterfly and moth fossils which show details of wing venation (the structure and pattern of wing veins is an important clue to evolutionary relationships) are usually in the front of impressions in shale deposits. These were laid down about 30 million years ago during the Oligocene period. In evolutionary terms this is comparatively recent, and it is thus hardly surprising that these fossil butterflies are very similar to those alive today. We are left to guess about the more primitive forms which must have preceded it, for the insects as a whole have existed for at least 400 million years. The great 'boom' in insect evolution seems to have occurred in the lush gloomy fern forests of the Carboniferous period (perhaps 300 million years ago) whose curious vegetation later formed our coal deposits. One of the more spectacular Carboniferous insects was the giant dragonfly Meganeura monyi, with a wingspan of over 65 cm. Today large dragonflies hunt butterflies among other insects, but it is unlikely that early butterflies were contemporary with Meganeura monyi. It is more probable that the Lepidoptera (butterflies and moths) evolved much later, perhaps from ancestors of the Mecoptera (scorpion flies) which did not appear until the Lower Permian period 250 million years ago.
The characteristic association of many Lepidoptera with floweringplants might suggest that the two evolved in parallel over a similar period of time. The earliest flowering plants are known from Upper Cretaceous fossils (about 90 million years old), but the wide range of families and genera already present by then strongly suggests that their origins were much earlier. If flowering plants and the Lepidoptera did in fact evolve at the same time the latter group might then be 150-200 million years old. They would thus have originated in the Triassic period as contemporaries of the first mammals.
The nature of these ancestral butterflies is purely speculative. The early butterflies were probably dull in colour, mainly brown and white, with little of the variety and splendour seen in modÑm forms. Perhaps the modem Lepidoptera have some giants among their extinct relatives, just as the reptiles once included huge dinosaurs ? Alternatively and more likely, the Lepidoptera may have always been small and the nearest they have come to evolving giant forms are the birdwing butterflies and atlas moths living in Asia at the present; none of which have a wingspan in excess of 30cm.
THE PROBABLE EVOLUTION OF THE BUTTERFLIES IN RELATION TO OTHERS MAMMALS AND PLANTS
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There are several groups of butterflies alive today which might be considered as primitive, their basic structure having apparently changed little from the presumed ancestral form. Yet these same species may be highly advanced in other respects, for example, having evolved complex breeding patterns. Other modem butterflies would be considered as advanced, having anatomical features which differ in many respects from the primitive plan as well as specializations in other aspects of their biology. However, the fact that both primitive and advanced species are alive today does not mean that the latter are directly descended from the former (any more than human5 have developed directly from modem apes). In fact both primitive and adanced species share a common ancestor; the latter having progressed furthest from the original stock.
The place of butterflies in the Animal Kingdom
Animals are classified into groups called phyla. Each phylum contains creatures which are broadly similar and possess certain diagnostic features in common. For example, the phylum Arthropoda includes all those animals which have jointed limbs and a hard exoskeleton. A phylum is further subdivided into classes: in this case Crustacea (crabs, lobsters, woodlice, etc) Arachnida (spiders, scorpions, ticks, etc) Myriapoda (centipedes and millipedes) and some lesser groups. The largest class of the Arthropoda, and indeed the largest class in the Animal Kingdom, is the Insecta to which the butterflies belong. Insects all have certain things in common; possessing a head, thorax and abdomen, six legs, and one pair of antelae ; further details of their structure and life history on the basis for dividing the class into 29 orders. These orders are divided into two groups, the Apterygota (wingless) and the Pterygota (winged), the latter being the larger group of orders. The Pterygota are further divided into the Exopterûgota and the Endopterygota; the life history of insects in the fonner groups consists' of egg larva (which is similar to adult) adult, and that of the latter is egg larva (which is unlike adult) + pupa adult.
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The Yellow jezebel Delias agostina - Picture: Phùng Mỹ Trung |
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The Lepidoptera is a very distinctive order of insects in the Endopterygota, second only to another endopterygote order, the Coleoptera (beedes) in numbers of species. It is difficult to say exactly how many species of Lepidoptera there are since many new ones are described every year. Many more are certain to be discovered in the future, adding to the 140,000 or so already known.
Butterflies, including recently extinct forms, account for about 20,000 of these species.
Particular attention has been paid to the butterflies, perhaps because most are active during the daytime (therefore easily seen and collected) and many are strikingly coloured. It is therefore likely that fewer species remain to be discovered than for other insect groups
and we might reasonably assume that less than 30,000 species exist.
With so many species in such variety, there is obviously need to devise an orderly system of classifying them into groups. Traditionally we think of butterflies and moths as though they were two equivalent subdivisions of the Lepidoptera (called by early entomologists respectively Rhopalocera - 'club-homed' and Heterocera 'varied-homed'). In fact the numbers of species given above show that this is not the case. The 140,000 Lepidoptera comprise 20,000 butterflies plus six times as many moths in several equally distinct
but quite separate groups. Even the division into butterflies and 'moths' has little scientific validity, though it is still used for convenience. Conventionally, butterflies have clubbed antennae, fly by day and are often brightly coloured ; whereas moths are nocturnal, usually dull coloured and have tapered or feathery antennae. The looseness of such a classification is shown by the abundance of exceptions to it, eg the many colourful day-flying moths, some of which even have clubbed antennae.
An alterative distinction is based upon the way in which the wings are linked during fight. In moths there is usually a lobe in the fore wing or spines on the hind wing which act as a coupling mechanism. In butterflies the coupling is achieved by the large area of overlap of the fore and hind wings ; but this is again subject to exceptions .
Another method of dividing up the Lepidoptera, convenient though not universally accepted, involves grouping them into two suborders and then various superfamilies. Under this obsolete scheme a few primitive moths which have similar fore and 'hind wings were placed in the suborder Homoneura. All butterf1ies and most moths make up the other suborder, Heteroneura, having structural differences between fore and hind wings. Within this huge group there are 20 Superfamilies, only two of which (Hesperioidea and Papilionoidea) are described as butterflies. More recently the subdivisions of Monotrysia and Ditrysia have been used, these are based on characters describing the female genitalia.
The details of classification are controversial and perhaps better left to the specialist. In this book the traditional family divisions have been retained for simplicity, though there are strong arguments in favour of alternative groupings. For instance of the 15 families of butterflies (Hesperiidae, Papilionidae, Pieridae, Lycaenidae, Nemeobiidae, Libytheidae, Heliconiidae, Acraeidae, Nymphalidae, Amathusiidae, Morphidae, Brassolidae, Satyridae, Ithomiidae and Danaidae) some authorities would regard nine as constituting a single complex family, the Nymphalidae, but such an arrangement would be unwieldly and unhelpful.
As a small guide amidst apparent confusion, readers should note that all family names in the Animal Kingdom end in - idae to distinguish them from super family names (ending in - oidea) and sub families which end in - inae. |
