Part 4

STRUGGLE FOR EXISTENCE.--All animals and plants produce far more offspring than can possibly survive, thus giving rise to the struggle for existence. For example: The average number of eggs laid by a Lepidopterous insect is certainly over 100, and in many species this number is greatly exceeded. Assuming each female to lay 100 eggs, the progeny from a single pair would amount, after six generations, to over six million individuals.

NATURAL SELECTION, or the SURVIVAL OF THE FITTEST.--In the struggle for existence which necessarily results from such a great increase of individuals, those variations which favoured the possessors would be preserved, whilst those which did not, would be gradually exterminated. This principle of the preservation of the favourable varieties in the struggle for life is called Natural Selection, or the Survival of the Fittest.

DIVERGENCE OF CHARACTER.--As there are so many different places and conditions in the economy of nature which can be occupied by organic beings differently constituted, individuals which diverged most from the original type would be brought into less severe competition, than those which diverged only in a slight degree. For instance, if we represent the original form as A, occupying one place in the economy of nature; a second form as B, occupying a somewhat similar place; a third form as C, occupying a very different place to A although somewhat similar place to B, it is obvious that B would enter into severe competition with both A and C, whilst A and C might not trend to any great extent on one another's place in the natural economy; hence B would be exterminated before either A or C. In other words, natural selection continually tends to increase the slight differences, which we call varieties, into the greater differences, which we call species.

The following phenomena, which have long been observed by students of the Lepidoptera, will serve as excellent examples of the operation of natural selection:--

PROTECTIVE RESEMBLANCE.--This term is applied to those classes of form or colour which enable an animal to so closely resemble its surroundings as to escape the notice of its enemies. Numerous examples of protective resemblance exist in the New Zealand moths and butterflies; in fact, it may safely be asserted that nearly all the colouring we observe in these insects has been acquired for protective purposes. The following species, amongst many others which will be described hereafter, exhibit in a very marked degree the phenomenon of protective resemblance: Epirranthis alectoraria, Selidosema dejectaria, and Drepanodes muriferata resemble dead leaves; Chloroclystis {xv}bilineolata, Tatosoma agrionata, and Erana graminosa resemble, when at rest, patches of moss; Selidosema productata and S. lupinata resemble the bark of trees; Chloroclystis lichenodes, Declana floccosa, and Elvia glaucata resemble variously coloured lichens. It is almost unnecessary to point out that all those variations, which tended to conceal the possessors from their enemies, would be preserved in the struggle for existence, and that these numerous and perfect instances of protective resemblance would inevitably result from the operation of natural selection. The dark colouration of Alpine and Arctic Lepidoptera, which enables them to rapidly absorb heat during the short and fitful gleams of sunshine experienced on mountains or in high latitudes, is also an instance of adaptation to conditions through the influence of natural selection. This was first pointed out by Lord Walsingham in 1885. The almost complete absence of white species in these localities is a good example of the extinction of forms unfitted to their surroundings.

CONTRAST COLOURS.--In this class of colouring the fore-wings only are protectively coloured, the hind-wings being very conspicuous. Contrast colouring is well exemplified by several of the insects included in the genus Notoreas. The sudden exhibition of the hind-wings during flight dazzles the eye of the pursuer. When the insect immediately afterwards closes its wings and the fore-wings alone are visible, it is extremely difficult to see. This form of protective colouring was also first drawn attention to by Lord Walsingham. (See page 75.)

WARNING COLOURS.--Insects, which are unfit for food or nauseous, are not protectively coloured, but on the contrary are rendered as conspicuous as possible. This class of colouring is well illustrated by one of our commonest moths, Nyctemera annulata (Pl. IV., figs. 1 and 2). The principle of warning colours was first discovered by Mr. A. R. Wallace, and is graphically described in Professor Poulton's entertaining work, 'The Colours of Animals.' The possession of nauseous qualities would be of little value to an insect, unless it could be at once recognised by insectivorous animals and avoided as food. If a nauseous insect were not easily identified it would speedily be destroyed by what Professor Poulton ingeniously terms "experimental tasting"; hence, through the process of natural selection, all nauseous species have become very conspicuously coloured. It may be remarked that warning colours are extremely rare amongst the New Zealand species, and I am not aware of any other example than that already given.

MIMICRY.--This term is applied to those remarkable cases where a harmless or edible species imitates in form and colouring a highly armed or nauseous species. No instances of this extremely interesting class of protection are yet known amongst the New Zealand Lepidoptera, but a very perfect example of mimicry exists between two common introduced species of Hymenoptera and Diptera, the well-known honey-bee and the drone-fly. The superficial resemblance between these two insects is very close. The bee, as every one knows, is armed with a powerful sting, whilst the drone-fly is unarmed. In this case it can be seen that if a harmless insect varied in the direction of resembling a formidable or objectionable species it would be a decided advantage to it, and such varieties would tend to be continually preserved and improved, through the operation of natural selection. The subject of mimicry has been alluded to here as it is not impossible that some instances of it may yet be discovered in connection with our native Lepidoptera.

{xvi}ORNAMENTAL COLOURING.--This class of colouring occurs in many species, especially amongst the butterflies, and is not apparently connected in any way with protection. Darwin supposes that it has arisen through the females of each species always selecting the most beautiful males as mates, hence these alone would leave progeny, and the females themselves would afterwards become beautiful through the effects of inheritance. This principle Darwin has termed Sexual Selection, and has discussed it in great detail in his work on the 'Descent of Man.' The fact, that amongst birds and butterflies the males are nearly always the most brilliantly coloured and the most beautiful, together with an immense mass of other evidence, tends, I think, to entirely support Darwin's theory, although it should be mentioned that several eminent naturalists, including Mr. Wallace, do not admit the principle of Sexual Selection.

IV.--CLASSIFICATION.

From a further consideration of the foregoing principles it will be seen that all existing species are held to be descended by true generation from pre-existing species, and that, consequently, all the relationships we observe between species are explained by community of origin. The most natural system of classification is, therefore, that which best reveals the scheme of descent, or, as it is termed, the phylogeny, of the group of organisms classified. To construct a perfect system of classification on these principles a knowledge of not only all the existing species of Lepidoptera would be essential, but also of all the extinct species, and it is needless to say that such knowledge is quite unattainable. Nevertheless large numbers of species are now known from many parts of the world, and a very extensive collection has recently been employed by Mr. Meyrick in framing a classification of the Lepidoptera, which is, to the best of my belief, the first constructed on strictly Darwinian principles. Although adopting Mr. Meyrick's system in the present work I do not agree unreservedly with all his conclusions; but I have not attempted to alter his system in accordance with my own views, as I conceive that the conclusions of a naturalist, who has only had the opportunity of studying a restricted fauna, would necessarily be liable to considerable error.

The general principles on which Mr. Meyrick has founded his system are practically those laid down by Darwin in his 'Origin of Species,' and may be thus summarised:--

A. Resemblances between all organisms are explained by community of origin, the amount of difference representing the amount of modification and expressible in the classification as varieties, species, genera, families, groups, orders, &c. The amount of difference does not necessarily bear any direct relation to time, many forms remaining almost stationary whilst others are undergoing development.

B. By a consideration of the following laws the age of a division can be approximately arrived at; that is to say, its position in the great genealogical tree of the Lepidoptera can be, to some extent, determined:--

"(1) No new organ can be produced except as a modification of some previously existing structure.

"(2) A lost organ cannot be regained.

"(3) A rudimentary organ is rarely redeveloped."--(Meyrick.)

{xvii}C. The greatest care is necessary to avoid being misled by adaptive characters, i.e., characters which are very important to the welfare of the species, and hence much modified through the agency of natural selection. A familiar instance of superficial resemblance, due to the presence of similar adaptive characters, may be observed in fishes and whales, where two groups of animals with but little real relationship have, through living under similar conditions, become extremely like each other in external appearance. Other examples might be given amongst exotic Lepidoptera. Thus, many noxious species are closely mimicked by harmless forms which are often far removed from them in real affinity. These cases of adaptive resemblances abound amongst all organisms, and have often deceived experienced naturalists. It is in consequence of the illusive nature of these external resemblances amongst different members of the Lepidoptera, that the structure of the neuration of the wings is now considered of such great importance as a character for purposes of classification. The numerous modifications in the position of the veins and their presence or absence in certain groups can, so far as we are able to see, have had very little effect on the well-being of those insects possessing such modifications. Hence it may fairly be assumed, that these structures have been free from the influence of natural selection for a very lengthened period. It is thus contended that the neuration of a Lepidopterous insect probably reveals more plainly than any other character its true relationship with other species.

The descent of all the Lepidoptera from some ancient member of the Trichoptera (or caddis-flies) is thus proved, according to Mr. Meyrick:--

"From a consideration of the laws enunciated above, there can be no doubt that the Micropterygina are the ancestral group of the Lepidoptera, from which all others have descended; this is sufficiently proved by the existence of the four or more additional veins in the hind-wings of that group, for these veins, if not originally present, could not have been afterwards produced. Of the two families of that group, the Micropterygidæ, which possess an additional vein (or veins) in the fore-wings, and fully developed six-jointed maxillary palpi, must be more primitive than the Hepialidæ. Now if the neuration of the whole of the Lepidoptera is compared with that of all other insects, it will be found that in no instance is there any close resemblance, except in the case of the Micropterygidæ; but the neuration of these so closely approaches that of certain Trichoptera (caddis-flies) as to be practically identical. The conclusion is clear, that the Lepidoptera are descended from the Trichoptera, and that the Micropterygidæ are the true connecting link. If the other marked structural characters of the Micropterygidæ are taken into consideration, viz., the possession of the jugum, the large development of the maxillary palpi as compared with the labial, and the sometimes functionally active mandibles, they will be all found commonly in the Trichoptera, affording additional confirmation. It may be added that in one New Zealand species of Micropterygidæ (Palæomicra chalcophanes) vein 1b is basally trifurcate, a character frequent in the Trichoptera, but not yet discovered in any other Lepidopteron. In most Trichoptera the veins of the hindwings are much more numerous than those of the fore-wings, in the Micropterygina they are usually equal in number, in other Lepidoptera they are less numerous; in the course of descent there has therefore been a greater progressive diminution in the number of veins of the hind-wings as compared with those of the fore-wings, though these also have diminished.

{xviii}"It is unnecessary to trace back the descent of the Lepidoptera further; but it may be worth while to point out that we may assume as the primitive type of Trichopterous neuration, a system of numerous longitudinal veins gradually diverging from the base, mostly furcate terminally, and connected by a series of irregularly placed cross-bars near base, and another series beyond middle."

The following is Mr. Meyrick's method of arrangement, which has been adopted in this book:--

"The natural order of arrangement, which is that of a much-branched tree, cannot be adequately expressed by a simple linear succession, such as is alone practicable in a book. It is, however, possible to devise a linear succession which shall be consistent with the natural genealogical order, if some additional explanation can be given. The method here adopted is as follows:--

"Suppose the accompanying diagram represents a portion of the genealogical tree; then the order will begin at M and descend to K, recommence at L and descend to K, and thence to G, recommence at H and descend to G, and thence to B, recommence at F and descend to D, recommence at E and descend to D and thence to B, recommence at C and descend to B and thence to A, and so on. Thus the order begins with the most recently developed forms and descends gradually to the earliest or most ancestral, which are the last in the book. To understand the order in practice, it may be assumed that each genus is descended from that which immediately follows it in the book, unless its actual descent is expressly stated otherwise; such statement will, of course, require to be made before every recommencement of a fresh branch. This system has been adhered to throughout, and after a little use will not be found unintelligible. If adopted in the arrangement of a collection in the cabinet, it would be a good plan to indicate the recommencement of a fresh branch by a special mark, such as a red bar drawn above the first (or highest) species."

PHYLOGENY OF LEPIDOPTERA. (After Meyrick.)

Notodontina Papilionina | | Caradrinina Lasiocampina Pyralidina | | | +--------------------+---------------------+ | Psychina Tortricina | | +-------+------+ | Tineina | Micropterygina

{xix}V.--GEOGRAPHICAL DISTRIBUTION.

The details of geographical distribution are given under the headings of the respective species, so far as I have been able to ascertain them; but our knowledge in this direction is necessarily limited, and I have found much difficulty in obtaining reliable information, on account of the obstacles which exist in regard to the correct identification of species in other countries.