The Strange Way in which the Vishnu Moth (Trabala vishnu) deposits her eggs in the Shape of Larvae.
Psyche 33(1):6-7, 1926.
This article at Hindawi Publishing: https://doi.org/10.1155/1926/85951
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6 . Psyche [February
THE STRANGE WAY IN WHICH THE VISHNU-MOTH (TRABALA VISHNU) DEPOSITS HER EGGS IN THE SHAPE OF LARV.E.1
In the small residential town of Kuching, Borneo, an invasion of voracious caterpillars in the garden and house of one of the residents was reported to me in October 1924. Certain plants were entirely defoliated and a large number of large hairy cater- pillars of the usual Lasi~ca~mpid type were seen crawling about everywhere, even entering the rooms of the house and causing much inconvenience and annoyance. Obviously a mass propaga- tion due to favorable circumstances had taken place. Ordinarily Trabala uishnu is a moth of rare occurrence; even in the rich collections of the Sarawak Museum only some few specimens were to be found.
As the larva has already been described and figured, I shall not consider it in the present note.
A fertilized female was temporarily placed by me in an empty paper box and left over night. On opening the box on the following day I was very much surprised to see beside the moth, a considerable number of larva on the sides. I thought it very strange to begin with, but a somewhat closer examination soon revealed the fact that what I had taken for larva consisted of eggs laid in two parallel rows with an odd egg at the top. Furthermore, the mother had arranged it so that a darkly pig- mented spot on each egg was always directed outwards with the effect that two dark longitudinal lines were formed, reminding one of the condition so often found in caterpillars. When I add that the mother had covered the egg strings with hairs from her own body, it is easy to understand that these completely con- veyed the impression of being small hairy caterpillars. On plate 1, A, a photograph of the eggs found in the box is reproduced. There are altogether 13 string, 8 laid separately and 5 more or less joined together. In nearly every string an Contribution from the Entomological Laboratory of the Bussey Insti- tution, Harvard University, No. 258.
19261 Strange way in which the Vishnu-Moth deposits her eggs 7
odd egg is found at the top, representing a head. All are equally covered with short hairs and on every string there are two longitudinal rows of dark spots along the outer margin. That the mother really has displayed a tendency to imitate the head of a caterpillar by placing one single egg on the top seems to be evidenced not only by the fact that in all strings there is an odd top egg but also that in strings no. 2, 5, 6, 7 and 10 (from left) there are two top eggs, one on top of the other one, whereas at the lower end of each string there seems to be no marked tendency to place an odd egg (except no. 3). The longest egg string is no. 13 with 22 eggs. It is a strange fact t'hat most of the other strings show two parallel rows of 7 eggs in each with 1-2 single top eggs.
In order to find out whether this peculiar way of depositing the eggs in the shape of larvee was only a queer habit in this ,in-. dividual case or really a specific habit, I isolated six more fer- tilized females and awaited the result. In all cases the females laid their eggs in exactly the same way. Two of the females under observation were kept in very large cases, but the eggs were found to be laid in a strikingly singular way. One or two top eggs were always to be seen and the same arrangement with the dark spots forming two longitudinal lines was also observed. We may therefore safely conclude that this peculiar way of depositing the eggs in larva-like shape is a characteristic of Trabala vishnu.
The advantage of thisform of "mimicry," if we may use this term, is obvious. It is a well-known fact that hairy cater- pillars are distasteful and discarded by most birds, the insects' greatest enemies. Only the cuckoos seem to form an exception to the rule. Undoubtedly the eggs laid in this way are better protected than if laid separately or in disorderly heaps as is the case with most Lasiocampids, and it seems to be fairly clear that the mother in this case by making the unprotected earliest state of the development so strangely similar to hairy distasteful larvae, has solved the problem of protecting her offspring ab ovo in a more effective way.
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