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Art. VII.—Notes on Flightless Females in certain Species of Moths, with an Attempted Explanation.
[Read before the Wellington Philosophical Society, 31st July, 1912.]
The existence of semi-apterous females in certain species of moths has long been known to entomologists, and has been fully described in many entomological works, but, as far as I have been able to ascertain, no attempt has yet been made to explain why the semi-apterous condition has been assumed by the female sex, or in what respects such a condition in that sex can benefit the species.
Owing to the limited number of foreign works on entomology, and the restricted nature of the exotic collections at present available for study in the Dominion, I have been obliged to confine my attention to species inhabiting New Zealand and the British Islands. The circumstances in connection with the occurrence of flightless females in both these regions prove, however, to be strikingly similar, and this fact merits careful consideration when seeking to obtain an explanation of this interesting phenomenon. It is perhaps needless to point out that the loss of the power of flight in one sex, whilst fully retained in the other, is a very remarkable and interesting circumstance, and the present requirements of natural history demand not only that a detailed account of the surrounding facts be given, but that a provisional theory at least be set up to account for it. I should perhaps here point out that these semi-apterous females are quite on a different footing to those insects where the power of flight has been lost in both sexes. The general question of apterous insects has very often been dealt with, and many adequate explanations have been given to account for the loss of flight under the most varied conditions. The present paper, however, is solely concerned with those insects in which the wings of the female are so abbreviated as to render that sex incapable of flight, the male retaining his flying-powers quite unimpaired.
Before proceeding further it will be desirable to consider the following list of Lepidoptera in which the females are semi-apterous. In addition to general remarks, this list gives, as far as is known, the food plant of the larva of each species, as well as its distribution, and the time of the year when the perfect insect appears.
In the Psychidae, which are represented in New Zealand by two species—i.e., Oeceticus omnivorous and Orophora unicolor—the females are not only apterous, but have rudimentary legs, and are incapable of walking, being, in fact, mere egg-bags. I have not included these insects in the present paper, as the species here dealt with have the females normally developed except in respect of the wings.
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| Name. | Time of Appearance. | Food Plant. | Distribution of Food Plant. | General Remarks on Habits. |
| Melacrias strategica | Middle of November to end of January | Various grasses | General | Frequents the flat country near Invercargill, and has also been taken on the Richardson Range, South Canterbury. The males fly in warm sunny weather only. (Philpott.) |
| Metacrias erichrysa | January | Senecio bellidioides (Meyrick) | " | A strictly mountain insect, at present only recorded from the tableland of Mount Arthur, at elevations of over 4,000 ft. The males fly with great rapidity in the hottest sunshine. |
| Metacrias huttonii | December and January | Various grasses | " | Another strictly alpine species, frequenting the mountains around Lake Wakatipu, at elevations of about 4,000 ft. The males fly very rapidly in hot sunshine. |
| Hybernia indocilis | July to January | Leptospermum? | " | According to the late Mr. R. W. Fereday, the male is found plentifully at rest on bare ground amongst Leptospermum, and the female on the stems. |
| Atomotricha ommatias | August and September | Unknown | Unknown | Found on fences during cold nights in August and September. The female, if touched, hops 2 in. or 3 in. (Philpott and Sunley). |
| Brachysara sordida | June and July | " | " | Found on fences during cold nights in the depth of winter. (Sunley.) |
| Mallobathra scoriota | Early in September (Invercargill) | " | " | The males of this species were found by Mr. Philpott flying in numbers over ferns in an open space in the forest. A semi-apterous female evidently referable to this species was found at rest on a fern-lesf. |
| Taleporia aphrosticha | December | " | " | Two males and one semi-apterous female were taken by Mr. Philpott on The Hump, Southland, at an elevation of 3,500 ft. above sea-level. It is evidently a mountain insect, and therefore subject to winter conditions. |
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With the few exceptions shortly to be specified, it will be seen that a striking agreement exists between the New Zealand and the British species in the following respects:—
(1.) General Distribution of the Food Plant of the Larva in the Region where the Insect is found.—There is no exception to this rule, which holds good in every case where the female is semi-apterous and the food plant is known. It is, in fact, obvious that the semi-apterous state would be absolutely fatal to an insect feeding on a scarce or local plant, as the females would require to travel over extensive areas in order to deposit their eggs. Any advantage which a species might obtain by possessing a semi-apterous female would, therefore, be wholly neutralized unless its food plant were very common and generally distributed.
(2.) Appearance of the Imago in Winter, or in Very Early Spring.—It is in this circumstance, which is common to all the species having semiapterous females, with the exception of the three species of Metacrias in New Zealand and the two species of Orgyia in the British Isles, that, in my opinion, an explanation of the semi-apterous condition of the female is to be sought. As a matter of fact, M. erichrysa and M. huttonii can hardly be regarded as exceptions, seeing that they are both strictly alpine insects, and therefore practically exist under winter conditions. One of the species of Orgyia also occasionally appears in winter, and may therefore have recently changed its habits. The only important exceptions are, therefore, M. strategica (New Zealand) and O. antiqua (Britain), and of the latter insect there appear to be a succession of broods throughout the entire summer. As corroborative evidence from insects belonging to an order other than the Lepidoptera, two of our common crane-flies (Tipula obscuripennis and T. heterogama) occur to me. They have semi-apterous females, and appear often in considerable numbers late in April and during May, a period which must, of course, be regarded as winter in New Zealand. I think, therefore, there can be no doubt that the semi-apterous condition of the female is in some way connected with the appearance of the species during cold periods.
The Effects of Cold on Insects.
In considering the cause of semi-apterous females amongst moths appearing in winter, the effect of cold on insects generally must be briefly considered. A slight lowering of the temperature below the normal produces torpidity, which is first manifested by inability to fly. Further cold results in inability to walk, then in suspended animation, and, if prolonged, in death. It is a matter of common observation that species appearing late in the autumn or in the winter are frequently so overpowered by the cold as to render them incapable of flight, and it is equally obvious that a female so incapacitated, when away from the food plant of the larva, would fail to leave offspring, owing to her eggs being deposited in a position where the young larvae would be unable to obtain food. In this way the loss of the power of flight would be a distinct advantage to the female, as she would be prevented from straying from the food plant, and, although semitorpid through cold, would in most cases have sufficient vitality to deposit her eggs in a fitting spot before death. On the other hand, the power of flight would still be necessary for the male in order to enable him to seek out the female, and to prevent the evil effects of prolonged interbreeding. The fact that a certain number of males would no doubt perish from the cold without pairing would be of little importance so far as the perpetuation
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of the species is concerned, especially if the males outnumbered the females, which is known to be, the case in many insects. It may, of course, be urged that a far simpler explanation of the semi-apterous condition is to be found in a lowered vitality, induced by cold, preventing the full development of the organs of flight; but the winged condition of the male, in my opinion, negatives this explanation, and the existence of many winter insects with fully developed wings in both sexes is also against it. Briefly stated, then, I believe that the semi-apterous females have been evolved in many insects appearing in the winter or very early spring because such a condition prevents them from leaving the food plant and being afterwards unable to return to it to deposit their eggs when overcome by the effects of cold. Such insects would naturally feed on widely distributed plants, as otherwise the semi-apterous condition of the female would be a fatal disadvantage; and this fact is in complete agreement with actual observation. In view of the restricted data on which this theory is based, it is put forward as a provisional hypothesis only, and may, of course, be subject to modification or rejection in the light of more extended investigations.
As a subsidiary advantage, the ability possessed by semi-apterous females to secrete themselves in crevices in the bark or in the ground may be mentioned; but I do not consider such an advantage would be commensurate with the loss of flight, and it cannot therefore, in my opinion, be regarded as the primary cause of the modification.
In conclusion, I should point out that the extreme abundance of many of the species possessing semi-apterous females indicates that the innovation has proved a most successful one in the struggle for existence, and this is further demonstrated by the fact that almost all the species appear in winter, when the insectivorous birds are often sorely pressed by hunger, and in consequence keenly on the alert for insect food.