Art. XXI.—Notes on Protective Resemblance in New Zealand Moths.
[Read before the Philosophical Institute of Canterbury, 1st November, 1905.]
These notes have been put together with a view of forming a base for future work. There is, I am afraid, but little that is new in them, but I have preferred to risk repetition in order to give them some measure of completeness.
By far the greater number of our native moths are protectively coloured; one can easily gauge from this fact the severity of their struggle for existence. Many of our birds live entirely or in part on insect food. The fantails (Rhipidura) and
bell-birds (Anthornis) take them on the wing; creepers (Mohua and Finschia), warblers (Gerygone), and wrens (Xenicus) search for them while at rest on the tree-trunks or branches; while the parrakeets (Cyanorhamphus) and kakas (Nestor) prey on the larval forms of those species which inhabit decayed wood. It is therefore not a matter for surprise that our Lepidoptera should have developed the principle of protective colouring to such an extent and to such perfection as is exhibited by several forms.
Though I have dealt with the Lepidoptera only, I feel sure that other groups of our insects would well repay study in this direction; the Coleoptera especially would yield some interesting examples.
This species is a very striking example of “warning colours.” The larvÆ and pupÆ are as conspicuous as the perfect insect, and I know of no instance of either being eaten by birds or preyed upon by other insects. It was at one time thought that a disease which affects cattle grazing in districts overrun by Senecio jacobœa and its allies—the food plants of the larvÆ—had its origin in the animals inadvertently eating the larva along with its food plant. It is now known that the Senecio itself contains a poisonous substance. Still, there can be no doubt of the nauseous, if not poisonous, qualities of the insect. The larvÆ are avoided by poultry, and if picked up with other food are at once rejected. The moths may be often seen entangled in spiders' webs, but I do not remember ever having seen one that had been attacked by the spider.
This moth, in common with erichrysa and huttoni, is probably also decked in “warning colours.” If it were edible it would fall an easy prey to birds, particularly when fluttering in the grass in search of the apterous female. A hidden female will soon attract dozens of males, and such a gathering would form a fortunate chance for any passing insectivorous bird. The females, which are covered with a thick coat of yellowish hair-scales, do not leave their pupal birthplaces under logs, &c., and are therefore not in much need of protective colouring. They are, however, very inconspicuous, and might easily be passed over for a fragment of dry earth. They move about very little, and this also would be in their favour if inconspicuousness were aimed at. It would be of great interest to find one sex protected by “warning colours” and the other by
protective resemblance, but in the absence of adequate observation and experiment nothing definite can be said on the point with regard to the present species. Some apterous females are considered to mimic spiders, but, except in the case of immunity from attack by other spiders, it is difficult to see in what manner such mimicry would benefit them.
Mr. Meyrick states that the members of this genus are almost all autumnal, and that “their yellow and ferruginous colouring is doubtless adapted to the autumn tints of falling leaves.” This is strikingly true of O. immunis, but O. comma appears in November. Its variegated and speckled appearance would harmonize well with the dead and decaying leaves which are to be found in the bush at all seasons.
This moth resembles in colour a much darker dead leaf than Orthosia immunis imitates, and the greyish dots and strigÆ resemble those minute patches of mould which are often found on decaying leaves in damp situations.
It is difficult to see how the grey colouring of moderata and its allies can be of value, but the ochreous and brownish hues of atristriga, propria, unica, &c., bear considerable resemblance to faded leaves of grass, and as the larvÆ all feed on Gramineœ this must be of some importance to the perfect insect. In some species, as in toroneura and neurœ, the resemblance is further enhanced by the veins being outlined in blackish; in others, as propria and acontistis, the same end is gained by brownish or black basal and discal streaks. The light-yellow forms, such as sulcana, semivittata, and others appear to be rather conspicuous insects; on dead, dry herbage, however, they would be far from noticeable, and in this connection the black dots which are scattered over the wings have probably some use in assisting to render the resemblance still more natural, dried herbage being often covered with such spots.
The markings of the species of this genus are, as Meyrick observes, “usually very similar, and the colouring dull and adapted to conceal insects which are accustomed to hide amongst dead leaves or refuse.” Several forms, however, seem to have a tendency to rest on trees; for instance, mutans is often found
thus situated, and on rough brownish bark is extremely inconspicuous. Vitiosa is also hard to detect under similar circumstances, and on moss-grown trunks plena and insignis are alike unnoticeable. Such species as lignana, pelistis, composita, and steropastis would be well protected about the roots of grass, while the uniform brown colour of infensa blends admirably with the underside of dead bark, and in this position I have several times found it. The strikingly contrasted black and green colouring of exquisita might be quite inconspicuous on a lichen-grown trunk, but the insect is so rare as almost to lead one to think that its striking appearance has had an adverse effect on the species in the struggle for life. Octans is also exceedingly rare, but two examples having been taken so far. In this instance, however, the rarity of the species in collections is probably due in great measure to the perfection of its resemblance to its environment. The type specimen was found on a rough limestone rock, and its presence was only revealed by the closest scrutiny.
In common with Melanchra infensa, this species has a liking for resting under pieces of dead bark; in fact, I have several times found small colonies hibernating under one flake of bark. Its uniform dark-brown colour is very suitable for such a resting-place; and B. sericea would also be protected in a like situation.
This is a dead-leaf-mimicking species. The light and dark points on the forewings help to carry out the resemblance. The moth is slow and feeble of flight, and if pursued often drops to the ground and remains motionless.
Tatosoma and Chloroclystis.
Nearly all the species of these two genera are more or less greenish in colour. They are spring and summer insects, and frequent bush. It is worth noticing that in Tatosoma, where the hindwings are small and covered by the forewings when the insect is at rest, the hindwings exhibit no protective colouring, but are pale dull-yellowish or grey. In Chloroclystis, however, where the hindwings are exposed in repose they partake, in a marked degree of the colour and markings of the forewings. This interesting fact is noticed by Mr. Hudson* when speaking of Elvia glaucata, the beautiful white-and-green lichen-mimicking species. The tendency of some forms of the genera
[Footnote] * “New Zealand Moths and Butterflies,” p. 46.
under notice—such as T. agrionata and Topea and C. plinthina—to have patches of white amid the green of their wings may perhaps be explained by supposing that such patches serve to represent the effect of the rays of light which glance through apertures in the foliage. C. lichenodes, according to Mr. G. V. Hudson,* “frequents forests, resting with outspread wings on lichen-covered tree-trunks, where its wonderfully protective colouring may be seen to great advantage. The remarkable brown patches on the wings have undoubtedly been acquired for this protective purpose.” I should think it probable that maculata, and inductata also frequent lichens, but both are rare, and their habits little known.
This large genus exhibits considerable diversity of colour and markings, and several of the species, viewed apart from their natural environment, appear to be very conspicuous insects. H. purpurifera is one of the most striking, but the conspicuous white fasciÆ of the forewings become quite inconspicuous when the insect is resting amongst foliage. H. siria is a most peculiar form. It frequents grassy bush tracks, and flits about somewhat after the manner of a butterfly, calling to mind, with its bright-orange colouring, a small specimen of Chrysophanus sallustius. It is improbable that there is any significance in this resemblance, but the fact is worth noticing.
Asthena schistaria, Venusia verriculata, and Xanthorhoe gobiata.
These represent a peculiar and interesting style of marking. In each form both fore- and hind-wings are crossed by numerous fine lines. Of course, each species rests with the hindwings exposed. In V. verriculata so perfectly do the lines of the forewings correspond with those of the hindwings that it is difficult to notice the overlapping edge. The abdomen also is crossed with fine lines: thus there is no break from the costa of one forewing to the costa of the other. According to Mr. Fereday as quoted by Mr. Hudson,† verriculata frequents the cabbage-tree (Cordyline australis), resting on the dead leaves which always hang in numbers from this tree, and always sitting across the leaf, so that the lines across the wing are continuous with the veins of the leaf. I do not know if A. schistaria and X. gobiata are also attached to the cabbage-tree, but faded leaves of the flax-plant (Phormium) or the toitoi (Arundo) would answer the purpose equally well.
[Footnote] * “New Zealand Moths and Butterflies,” p. 44.
[Footnote] †“New Zealand Moths and Butterflies,” p. 53.
This beautiful moth is attached to the pepper-tree (Drimys colorata); the lemon-coloured forewings of the typical form with their dark-brown costal borderings harmonize in a remarkable manner with the leaves of this plant. There are, however, a great many striking variations of this insect, one in particular having developed a most inconspicuous drab colour.
In this genus a noticeable case of colour - development to suit environment occurs. X. orophyla is almost exactly similar in markings to X. semifissata, but while semifissata is pinkish-brown, orophyla is bluish-grey. While semifissata frequents open forest districts, orophyla is found on the mountain-ranges, and its colour is well adapted to the rocky nature of its habitat. It is, I think, probable that X. orophyla has been developed from X. semifissata, and this is the more likely as some forms of semifissata vary in the direction of the grey colour of orophyla. X. clarata presents a case of the protective colouring being chiefly developed on the undersides of the wings. This form is very conspicuous and striking in the cabinet, but when resting with folded wings amongst the tussock and rough herbage of its natural surroundings it is far from easy to see. The insect folds its wings over its back, and the undersides are covered with dark elongate dots on an ochreous ground, a type of marking very suitable to its ordinary environment. In X. bulbulata we have an instance of “contrast colours.” The bright - orange hindwings, conspicuous in flight, are in strong contrast to the dull-grey forewings, and when the insect suddenly drops into a tussock and closes its wings it is hidden at once; that which on the wing appears to be a bright-yellow insect is instantaneously transformed into a dull and inconspicuous grey one.
The members of this genus, with Lythria and Dasyuris, are for the most part protected by “contrast colours.” The genera, however, contain forms, such as N. perornata, in which the fore- and hind-wings are alike brilliant; and others, as N. omichlias, in which almost all trace of bright colours has been lost, and the insect is admirably protected by its resemblance to the general hue of its environment. There is still, however, much to be learned of the habits of these mountain forms, and future investigations may throw considerable light on their economy.
This family exhibits some of the most striking “dead leaf” resemblances to be found. Foremost among these stands Drepanodes muriferata. Mr. Hudson has observed that when disturbed this insect accentuates its resemblance to a dead leaf by keeping its wings extended and motionless, and allowing itself to fall to the ground as a dry leaf falls. I have not had much opportunity of observing muriferata, but can state that similar tactics are often adopted by its ally Sestra humeraria—in fact, one can never be quite certain until after close examination whether moth or falling leaf has been observed. In the genus Epirranthis all stages of dead and decaying leaves are represented, the irregular margins of the wings aiding the resemblance. The same remark applies to Selidosema panagrata and dejectaria, but Selidosema productata seems to be particularly protected when resting on tree-trunks, its blotched black-and-white colouring being eminently suitable to such trees as the matai (Podocarpus spicata). Gonophyla azelina, though one of the handsomest of New Zealand moths, is one of the most inconspicuous when at rest amongst the stems and dead leaves of forest ferns, a position in which it is most often found.
Turning to the group Pyralidina, we find the members of the genus Crambus admirably protected both in form and colour. As a general rule the forewings are of some ochreous shade, with one or more longitudinal white lines. In repose the wings are folded back upon the body, and the position assumed is almost invariably in line with the stalks and linear leaves of the rough herbage which these moths commonly frequent. Thus when siriellus or simplex has been observed to alight in a patch of tussock-grass, it often requires much patient search before the moth can be discovered. The system of marking by longitudinal lines is also continued in the smaller and darker forms, such as Œthonellus and corruptus, and is still in evidence in Orocrambus. In Scoparia several forms are attached to tree-trunks as resting - places; the coloration of such forms as philerga and submarginalis is well suited to such a situation. Another section containing such forms as octophora, sabulosella, and their allies frequents red tussock and vegetation of a like character; these, as might have been expected from their habitat, are dull-greyish ochreous forms. Yet another section has developed the linear arrangement of marking, as in Crambus, but here the ground-colour is light and the lines dark. S. trivirgata is a good example of this group, and the effect of the colour-arrangement seems to be equally successful, trivirgata being a very abundant insect and widely distributed.
The members of this group find their chief protection in their resemblance to dead leaves. Cacoœcia, Ctenopseustis, Pyrgotis, Adoxophyes, and other genera mimic the tints of the faded leaves of many of our small-leaved shrubs. The genus Strepsicrates, however, seem to rely more on their resemblance to twigs; and some species of Heterocrossa, notably gonosemana, are well protected on rough mossy bark. The lovely white wings of Nymphostola galactina, with their delicate tracery of green veins, might be mistaken for the petals of some bush flower, and as the moth appears in midsummer this is probably the end aimed at.
In the Tineina several species of Trachypepla exhibit a curious form of colouring. The head, thorax, and anterior portion of the wings are white, the remaining portion being darkcoloured. T. euryleucota I have several times taken from Leptospermum bushes when the white buds were just unfolding, and have been struck with the resemblance between such buds and the moth as it sits at rest with closed wings. Mr. Meyrick, however,* inclines to the opinion that “euryleucota, with leucoplanetis and conspicuella, mimics the droppings of birds.” The remaining forms of Trachypepla evidently mimic moss and lichens, and the illusion is strengthened by the surface of the forewings having a rough, irregular appearance, caused by a number of patches of raised scales. This method has also been adopted in Lysiphragma, the object in this case being to create a resemblance to the rough bark of the broadleaf (Griselina littoralis), under which the larvÆ feed, and on which the mature insect is often found. Megacraspedus calamogonus frequents the seed-heads of Arundo conspicua, on which its larva feeds. It is well protected both in form and colour.
Several genera are remarkable for the position assumed in repose. In Stathmopoda the posterior legs are erected over the back with the tarsi directed more sideways; Thylocoseles holds the posterior legs out behind, but bent, after the manner of a grasshopper. Other genera have adopted like unusual positions; and Mr. Meyrick suggests that the attitudes are assumed in order to deceive enemies by their unnatural appearance.
[Footnote] * Trans N.Z Inst., xvi, 13.